Runyang Zhang,Sen Ma,Li Li,Minghui Zhang,Shuangqi Tian,Dongying Wang,Kunlun Liu,Huamin Liu,Wenxue Zhu*,Xuede Wang*

College of Food Science and Engineering,Henan University of Technology,Zhengzhou 450001,China

ABSTRACT Corn is a high starchy cereal crop with the highest production and provides over 85%of the starch produced worldwide.Various by-products,differentiated by technological process features such as steep liquor, corn germ, corn bran, gluten, are created largely during corn starch processing.They are inexpensive,nutrient-rich,and vary widely in chemical composition such as proteins,oils,carbohydrates,and minerals.In an increasingly resource-constrained modern world,the utilization approach of these by-products for non-starch industrial processing is attractive widely considering both nutritive value and economic aspects.In fact,at present,applications of these by-products can often be found in feed,fermentation,nutrient extraction and other industries.For example,protein-rich corn gluten can be used as a good animal feed,and corn germ can be used as a raw material for the high-quality edible oil industry.Undoubtedly,increasing utilization means that these by-products will no longer be treated as waste but will be transformed into high value-added products.In this work,the separation process and chemical composition of several main by-products of the corn starch industry is briefly described,and the application in many industrial fields of these by-products over the last ten years are discussed in particular.This review attempts to summarize all aspects of the application and research of these by-products.For the by-products of the corn starch industry,the most promising way is to be utilized in high value and used to produce high value-added products.According to the characteristics of their chemical composition,they have a better application prospect and research significance in the industries directly related to human beings,such as medicine,green food and health care products.In fact,in recent years,some researchers have recognized this and carried out the research.It is clear from these studies that the main issues to be faced now and in the future are how to produce efficiently while maintaining the quality of the product and using it effectively.The retrospective discussions also provide some ideas for other grain and oilseed crops to be fully utilized.

Keywords:Corn starch processing By-products Utilization

1.Introduction

Corn, or maize (Zeamays L.), is a monocotyledonous annual herb cereal crop belonging to the grass family.It is thought to have originated in Mesoamerica and is now cultivated globally in tropical and temperate regions,such as the Americas, East Asia, South Asia, Southeast Asia,European Union,etc.[1,2].

Corn kernels consist of 61%–78%starch on a dry basis(d.b.), non-starch polysaccharides (about 10%, d.b.),protein(6%–12%,d.b.),and lipids(3%–6%,d.b.)[3,4].Besides,it is easily dried and readily transportable.Therefore,corn is a near-perfect starch grain crop for the food industry.As a result,it's the leading and important cereal crop in the world.It is estimated that world corn production is more than 1136.3 MMT (million metric tons) in 2020–2021(Sep–Aug), and is much higher than the production of wheat(776.8 MMT)and rice(504.4 MMT).Furthermore,about 31.7% (360.3 MMT) was produced in the United States, followed by China (260.6 MMT) and Brazil(109.0 MMT)[5].The year before,1150.5 MMT was consumed worldwide, with the United States (305.4 MMT),China(289.0 MMT)as the major consumers of corn.About 30%of total corn production is consumed as industrial materials,and the consumption of the United States accounts for 40%of those raw materials[6].

Corn,as the major starch source,provides over 85%of the starch produced all over the world[6].The starch amylose and amylopectin molecules have distinctive characteristics that provide a wide array of properties.The functional diversity provided by amylose and amylopectin molecules with a wide array of distinctive properties makes native and modified corn starch an adaptable and valuable industry resource.Therefore,these starches are widely applied as the thickener,gelling agent,bulking agent and water retention agent in the food industry[7].Meanwhile,about 40%of corn starch from wet-milled processing is applied for the non-food industry,such as paper-making,textiles,laundry,gypsum wallboard,adhesives,bio-plastics,glass-fibre sizing[6, 8].Besides, with the development of biotechnology,corn starch is now the main material used in bioethanol production,because it can be easily converted into simple sugars by hydrolysis and then fed to Saccharomyces cerevisiae before fermentation to produce ethanol.The ethanol manufacturing industry provides a huge demand for starch,and more than 90% of the ethanol produced material comes from corn in the United States[8].

The corn starch granule is abundant in the endosperm of corn kernel,and is developed by successive layering of amorphous and crystalline starch molecules,but surrounded with protein matrixes and protein bodies[9].While,the whole intact corn kernel consists of endosperm (82%–84%, d.b.),germ(10%–12%,d.b.),pericarp(5%–6%,d.b.),and tip cap(1%)[10],see Fig.1.All ingredients except starch must be removed for the production of corn starch.The seed coat would be removed during corn starch processing and named bran but is rich in polysaccharides,such as cellulose and hemicellulose.During this de-branning processing,the tip cap, namely the connection point between kernel and cob,was also largely removed.Similarly,the germ/embryo,containing almost all the oil of the corn kernel,is also separated from the kernel[11].Therefore,during the process of starch production,many by-products rich in organic ingredients and nutrients are also produced,such as corn bran,corn germ,corn steep liquor,corn gluten,etc.

Fig.1.A schematic of mature corn kernel with gross structure.

In the current century,energy conservation,emission reduction and environmental protection are among the biggest challenges facing industrial production and human beings.As revealed by numerous researches, all the by-products from crops have unthinkable potential value and benefits[12,13].Thus,the effective transformation,cyclic utilization of these by-products resources, is of great significance to human beings [14–16].The starch processing industry represents an important role in the food or non-food industrial field.About 100 million tons of organic by-products were produced from the starch industry every year [16].These by-products are currently used for non-starch industrial processing in different ways.The corn germ can be used to extract edible oil and to process foodstuffs such as bread and biscuits.It can also be used as a protein feed for animals in the same way as corn gluten.Corn bran can be used to extract edible oils,and is more commonly used in the production of dietary fibre,sterol and zeaxanthin.Corn steep liquor has been used extensively in the fermentation industry as an ingredient in the preparation of culture media for antibiotics,glutamate and nucleotides,and also for the production of phytic acid [6, 7, 9].These by-products were processed into higher-value products instead of being used wastefully or inefficiently.Indeed, sustainable processing means,which are based on cost reduction,environmentally friendly production,product acceptability,have become the greatest challenges.Therefore, the by-products were designedand developed innovative products is a very important development approach for food and other industries[17].

This article aims to summarize and discuss the current utilization approach and research of the by-products of the corn starch industry for animal feed,a fermentation substrate,polysaccharides,oil,protein,etc.This information could help improve the understanding of recent findings and the development of further research in this field.

2.By-products of corn starch processing

Milling schemes represent the principal procedure to yield corn starch and are conveniently classified as wet or dry(Fig.2),while each has its characteristics[18].Dry milling is a gradually abrasive technique separating the bran and germ to obtain dry-milled cornflour[19].Its products mostly serve as feed,food,industrial material and as starch sources for ethanol production,fermentation processes and related starch conversions.Dry milling production of corn has increased in the last decade years due to the increasing demand for corn flour used for fermentation ethanol.Moreover,90%of corn bioethanol is processed from dry-milled corn[20–22].

Fig.2.The wet and dry milling processes.

Wet milling aims to fractionate different components of the kernel and release starch granules with minimal mechanical damage.High purity starch can be yielded by this technique, which is the distinct advantage over dry milling.Wet milling is used to produce high purity starches(99.95%),and more than 85%of corn starch is produced by wet milling.Corn kernel was firstly steeping by adding the sulfurous acid solution to increase the permeability of corn hull,to swell the kernel,reduce mechanical strength,and disrupt the protein wrapping around the starch granules.Then the germ,bran and protein can be easily separated in subsequent processing,such as grind screen and washing [23].This industrial process involves chemical,biochemical,and mechanical operations to separate pure starch fractions from corn kernels.Various products were isolated, including starch (about 68.0%), corn bran(12.0%), corn germ (7.5%), steep liquor (6.5%), gluten(5.6%), and others (0.04%) [24].Therefore, several byproducts were also obtained during this processing except for starch[25].Redundant nutrient substances and microelements containing these by-products cannot be ignored,and are now generally exploited and used by other industries.For example,the recovered protein meal and bran can be applied in feed factories, and the corn germ has also found lipid applications for edible oil supplements[26,27].

In brief, some by-products were obtained from starch processing(Table 1),and their differences in producing process and chemical composition between these by-products are briefly described below.

Table 1 Compositional values(%)for corn starch processing by-products.

2.1.Corn steep liquor

During steeping,the corn was softened by steep water to produce optimum efficiency of grinding kernel and separating corn components.It involves maintaining the correct balance of sulfur dioxide concentration, temperature,water flow, and Pondus Hydrogenii value (pH) during soaking.Corn kernels are generally soaked in steep water for 30–40 h at 48–52°C.The germ cell membranes become porous and organic compounds are easily liberated intactly and separated from hull or endosperm.At this point, approximately 6.0%–6.5% (dry matter content) of the corn kernel is released into the condensed steep water, and mainly in the organic compounds of living cells,such as proteins (40%–50%), minerals, soluble sugars, amino acids,etc.[28,29].The previous study indicated that about 50%of steep water-soluble matter belonging to the corn germ,because of the high proportion(95%)of naturally soluble substances in the germ,such as germ protein(60%).In contrast,only 9%of the endosperm protein can be solubilized during steeping[30,31].

2.2.Corn germ

After steeping the corn kernel undergoes the first milling operation by an attrition mill,and more than half of starch and germ is released from the kernel.Then the separation process of the heavier kernel and oil-rich germ is proceeded by continuous flow-through liquid cyclones or hydro clones taking advantage of the large density difference between them.The isolated germ was washed and dewatered to 50%–55% water content to be used for other products(wet weight basis)[32].Typically,the crude oil of corn kernel(4%)is almost stored in corn germ and the entire germ mainly contains oil (48%), protein (13%), and starch(12%).However, the recovery germ composition in oil(18%–41%),protein(12%–21%)and starch(6%–21%)is also influenced by growing conditions,genetic factors,and isolation methods[33].Corn germ oil can reduce the risk of chronic diseases and exhibits a long shelf life,becauseof its high content of unsaturated fatty acids and tocopherols[41–43].Regarding proteins,albumins,globulins,watersoluble and saline-soluble proteins,represent over 60%of germ protein which accounts for 29%of total kernel protein[44,45].

2.3.Corn bran

Corn bran mainly comes from the pericarp, which is another by-product derived from the production of corn starch[46].Corn bran is lignocellulosic material containing complex carbohydrate polymers,such as cellulose(20%),hemicellulose (30%–50%), starch (9%–23%), protein(10%–13%),crude oil(2%–3%),and phenolic acids(4%,mainly ferulic and diferulic acid)[34].It is usually removed as the by-product during dry milling processing,also known as dehulling or de-branning[47].During the second milling step,the endosperm particles are milled into the fine slurry to release the starch granules from the protein matrix,and the pericarp also is released and separated by a metal sieve[46].Isolated bran was recognized to produce dietary fibre which is consumed by humans,such as bread,biscuits,pasta and snacks.It is an attractive alternative for the preparation of bread,biscuits,pasta and extruded snacks.

2.4.Corn gluten

Corn gluten mainly comes from steep liquor and corn bran and may also contain few germs and damaged corn kernels.Dry corn gluten typically contains 90%dry matter,including considerable crude protein(19%–24%).This content of proteins is much higher than corn seed,beans,and sunflower seeds.Therefore, it is an attractive source for the protein product processing industry[48].Gluten and starch released by milling must be separated from the fibre,which is usually accomplished by taking advantage of the difference between the fine particle sizes of starch granules and gluten particles, and the larger endosperm and pericarp particles.The underflow from the germ cyclones containing fibre and pieces of horny endosperm is more thoroughly milled to recover the maximum yield of starch.Typically,finished fibre contains 15%–20%starch and 65%–75% moisture content [49].Then, these fibres are blended with concentrated steep liquor,corn cleaning,spent(de-oiled)germ flakes and starch hydrolysate residue,and finally dried to produce corn gluten[50].Therefore,dietary fibre is rich in gluten and also regarded as an energy fibre source.

3.Application of corn starch processing by-products

3.1.Livestock and poultry feed

The downstream processing of corn mainly focuses on the utilization of corn starch,and the non-starch parts are not fully utilized.Most of them are used as a feed or directly discharged in the form of wastewater, with low added value.It also exacerbates environmental pollution.Corn starch processing by-products contain large amounts of protein and carbohydrates,which are the high-quality raw material for livestock and poultry feed[51].Corn bran,germ meal, corn gluten and gluten meal are feed commodities traded worldwide,while corn pulp and corn fibre usually need to be treated or fermented before they can be added to animal diets[51,52].Due to the redistribution of nutrients in the processing,the chemical components and nutritional value of different corn starch processing by-products are differing greatly,and the digestion characteristics in different animals are also different(Tables 1,2).

Table 2 Nutritional value of by-products as feed.

Corn bran is usually used as an energy source for ruminants due to its lower price than grain and better nutritional value depending on chemical composition and content.Also,because of the considerable content of dietary fibre,adding bran in the feed of livestock and poultry can effectively regulate the structure of digestive tract flora and promote the health status of livestock and poultry[52].Corn bran can be used to fully replace corn grain in the concentrate fed to cows without lowering milk quality[53,54].However, a protein supplement is necessary for corn bran as feed.In Tanzania, sunflower meals were added to corn bran as protein sources and the mixture was effectively to increase the yield of milk[55].The noticeable advantage of corn bran is it hardly causes negative digestive interactions with other ingredients.In Kenya,supplementation with 1 kg of dry corn bran in elephant grass did not significantly reduce the rumen degradation of forage[55].For non-ruminants, nutrient digestibility, growth performance,gut hormones,or fermentation products of weaned pigs cannot be dietarily affected by corn bran.While 5%corn bran was added in the post-weaning diet,intestinal microbial diversity was increased.This also leads to more fibrolytic bacteria and promotes the anti-inflammatory of piglets[56].

Corn gluten meal is rich in protein(60%–75%),residual starch(15%–20%)and has a small amount of crude fibre(1%),fat(3%)and minerals(2%).Due to its high protein content, corn gluten meal is mostly used as a potential alternative to other plant or animal-based proteins [57].In recent years,there has been more research on the application of corn gluten meals in the diets of broilers,beef cattle and dairy cows, but less in pig production.Compared with traditional technology,at present,corn gluten meal is mostly fermented to produce soluble peptides,which improves its utilization rate in livestock feed[58].Supplementation with a 60:40 blend of ground corn gluten meal significantly increased cow's milk production when the cows are restricted grazing on Italian ryegrass[59].In another similar study,by-pass protein intake milk yield,milk protein content and body condition score of the cow were increased, while body weight losses were reduced [60].However, the low lysine and tryptophan content of corn gluten meal is the main factor limiting its use in pig diets,and it is necessary to supplement an appropriate amount of lysine in practical application.When 5% corn gluten meal and lysine were added to the diets of growingfinishing pigs,there was no significant difference in growth performance between corn gluten meal added and control groups[61].The application of corn gluten mainly focused on ruminants,but less on monogastric animal feed.Plenty of wet gluten feed was added to the diet of dairy cows in the feeding factory.These cows fed 20%to 35%wet corn gluten feed was more efficiently produced energycorrected milk than controls.Moreover,yields of milk protein and lactose were increased,with a lower fat percentage, while fat yield was unaffected [62].The animal(rowing ewes) performance improved when dehydrated corn gluten feed(10%or 20%dietary levels)was blended in a rice straw/concentrate diet, and daily gain and feed was significantly increased[63].

The dietary fibre content of corn gluten is also much higher than corn kernel.For growing pigs,the Net Energyof corn gluten is about 60%that of corn grain[73].After 15%corn gluten feed was added to the pig diet,the growth performance was significantly improved.But when the proportion of corn gluten increased to 30%, the growth performance and nutrient digestibility were relatively decreased.Therefore,corn gluten feed can be added 15%–30%to the pig diet[74].

Corn germ was usually firstly extracted oil and the defat germ(corn germ meal)was also considered a good ingredient for all livestock species[75].It has good palatability,rich in digestible amino acids and hemicellulose,and can be used as protein feed for monogastric animals,as well as protein and energy feed for ruminants[76].Due to the variety and processing technology of corn,the effective energy of different corn germ meals has a great difference [77].Corn germ meals have been widely used as a substitute for corn grain in Brazil, China and Ethiopia.However, the corn germ meal was detrimental to the performance of lamb[78].Furthermore,it is not recommended to extrapolate the results of those trials to all types of corn germ meal.In the USA,diet consumption,net yield,weight gain,feed coefficient,survival,fillet yield,and fillet protein,fat and moisture concentrations were not affected by adding corn germ meals included at up to 35%in the diets of channel catfish fingerlings.However,it has been shown that adding corn germ meal to the diet of coloured broilers could significantly reduce the digestibility and metabolism of dry matter and organic matter,but 0–25%addition level had no effect on Carcass Performance of coloured broilers,and 0.05% phytase had no significant effect on growth performance[79].The content of omega-3 polyunsaturated fatty acids in egg yolk increased with the increase of corn germ meal in the diet of laying hens,and the feeding effect was the best when 0.5%was added[80].

Supplementation with a 60:40 blend of ground corn gluten meal significantly increased cow's milk production when the cows are restricted grazing on Italian ryegrass[59].In another similar study,by-pass protein intake milk yield,milk protein content and body condition score of the cow were increased,while body weight losses were reduced[60].

In brief, by-products from corn starch processing had good application effects and prospects in livestock and poultry production.However,because of the higher fibre content,and also varied content of other nutrients such as crude protein and energy due to different types of raw materials and processing techniques,its application in the production of some monogastric animals was limited.At present,with fermentation or combining with enzyme and other feed materials,the feed value and application range of corn starch processing by-products can be improved greatly.To improve the animal's growth performance and health level,corn pulp can be made into enzyme-hydrolyzed protein feed after enzymolysis treatment.10% corn pulp enzymehydrolyzed protein feed was proven to increase the average daily gain of meat rabbits in an experiment[81].

3.2.Food ingredients

The use of corn starch by-products for animal feed is a very easy application approach to process them.This is because animals do not have a high demand for quality feed compared to humans.But this application route also significantly reduces the added value of the by-products.To improve the application value of by-products,many researchers consider using them as food ingredients directly in human food.By-products derived from corn starch processing,which are rich in fibre, protein and antioxidants can be added as low-cost and low-calorie agents in food products to partially substitute fat or sugar[82].The corn bran contains a high content of dietary fibre(up to 90%),which can be used to increase the total fibre content of foods at low levels of inclusion.In a study,used corn bran was added at 10%–80% inclusion in extruded snacks which were made with different starch (10%–80%) and Bambara nut flour (10%–80%).The total dietary fibre in all treatments was above 7%, therefore the extruder can be considered to be high fibre[83].But no sensory analyses were carried out as part of this study.Sousa et al.[84]evaluated the application of corn bran to low-calorie snack bars.Formulations with up to 40% corn bran were well accepted by the judges, by comparison with the formulation without corn bran.In another study, maize milling by-product was also contained in the short-dough biscuit,and the supplement of wheat flour at a level not exceeding 20% was necessary.This product was equally well received by the panellists that took part in the sensory evaluation [85].The defatted germ flour can be added to wheat flour and then the protein content significantly increases in the blends.Moreover,the textural properties increased with the defatted germ flour incorporation level up to 30%, such as fracture force, hardness, breaking strength,breaking energy,cutting strength and cutting energy [86].

Enrichment of bread and bun with dietary fibres from corn starch processing by-products is also an interesting way.It can increase consumers'fibre intake and decreases by-products [87].When the corn bran was developed to partially replace flour at a level of 18%,20%,and 22%in white bread,the product was a much smaller loaf volume than control bread and had worse microstructure and textural properties.To resolve the issue effectively,the authors have to increase its standard values from 38.3%, 38.6%,38.8% to 40.8%, 41.9%, and 44.0%, respectively.Then,the product presented similar microstructure,volume,and textural properties compared to the control bread[88].In another recent study, the bun was produced by blending wheat flour with different proportions of defatted germ flour(5%–25%).With the increase in proportion,the fibre content, yield and hardness increased, but volume decreased.Overall,the bun prepared from 10%incorporation of defatted germ flour scored highest for its comprehensive appearance [89].The fortification of bread with maize germ protein hydrolysate (1%–4%) would improve the bread texture with reductions in hardness and chewiness during storage[90].

Recent study reported that emulsion-based meat products can be added dietary fibre from corn bran without reducing its sensory and textural quality.Panthera et al.[91]added the corn bran(5–15 g)in chicken nuggets and the firmness and toughness scores of products increased significantly(P<0.01)with the increase of bran proportion.A study showed the chicken sausages were developed with good acceptability,higher dietary fibre content and storability by replacing lean meat with corn bran at a 3%level[92].

These reports revealed that by-products of corn starch processing can be used as fortification of biscuits, bread,meat products for human consumption to improve their nutritional quality.Certainly,by-products as additives in the food chain entail the final products must comply with the safety regulations of the food industry.

3.3.Substrate resource of biological fermentation

Corn starch processing by-products cannot only be used as food sources for humans and animals, but also as substrates for biological fermentation and energy sources for microorganisms.The field of biological fermentation has been expanding, including organic acids, amino acids,starch sugar, enzyme preparations, yeast, polyols, multifunctional biological products,biological medicine,paper making, new energy, etc.It is an important way to solve the major problems of mankind[93].Corn contains a variety of rich nutrients,and various fermentation products can be obtained through fermentation.The alcohol industry was the first to use corn fermentation to produce bioethanol.Later,with the simultaneous development of various technologies and the increasing demand of people,some fermented products began to be produced on a large scale and industrialized, including biosurfactants, amino acids,antibiotics,organic acid,etc.(Fig.3).

Fig.3.Application of corn starch by-products in biological fermentation.

Biosurfactants are compounds derived from microorganisms,which have surface activity,low toxicity and high biodegradability.The use of industrial waste to produce high value-added biomolecules, such as biosurfactants, is a promising method to reduce the total cost of production.It has recently been discovered that there are biosurfactants produced by Bacillus strains in corn steep liquor which is a liquid waste obtained from the corn milling process.The Bacillus strain was subjected to 16S rRNA amplification and sequence analysis and then was identified as A.aneurinilyticus.The biosurfactant is composed of a mixture of lipopeptides,containing C16 and C18 fatty acids and amino acids,including valine,phenylalanine,proline,cysteine, histidine, aspartic acid/asparagine, alanine,glycine,leucine/isoleucine[93].In another similar study,biosurfactant production by Mucor circinelloides using apple peel,vegetable oil and corn steep liquor(2%)as substrate was considered as a promise emulsifier agent[94].One study aims to use Rhizopus UCP 1607 to produce biosurfactants with crude glycerol(3%)and corn steep liquor(5%)as substrates.These biosurfactants are composed of protein (38.0%), carbohydrates (35.4%) and lipids(5.5%),and can reduce the surface tension of water from 72.0 to 28.8 mN/m,and the yield was 1.74 g/L[95].

Corn steep liquor is also commonly used as a media supplement for the fermentation industry of penicillin,erythromycin,and gentian.Because it is widely considered as a cheap N-,C-,vitamin or trace element source and as an alternative to expensive media ingredients.The most prominent example is the production process of Penicillin,which is a typical secondary metabolite of microorganisms.The productivity has increased about 20%, owing the corn steep liquor was added since 1940s[96].

The numerous applications of citric acid have led to continuous research on improving production systems and reducing the cost of substrates.In one study,corn steep liquor was used as a low-cost and non-seasonal nutrient source for the growth of Yarrowia lipolytica to produce citric acid.The citric acid yield of up to 38 g/L in flask culture after proper manipulation.In addition,the scale-up of the bioreactor increased productivity by 130%and greatly reduced the fermentation time.Overall,the combination of these ingredients from the corn starch processing industry and low-cost nutritional sources that can be stored at room temperature seems attractive for citric acid production [97].Rivas et al.[98] studied glucose consumption and lactic acid generation using a variety of media made with different nutrient supplementation.The results showed media containing glucose, the biomass of D.hansenii and corn steep liquor as unique components led to product yields similar to those obtained in a fully supplemented medium under selected conditions.

The utilization of corn bran for inulinase production by Penicillium oxalicum has been studied.The optimized conditions are moisture(80%),incubation time(6.0 days)and pH (6.5) and substantial inulinase production(77.95 IU/GDS)was obtained.Hence,corn bran is a good substrate for inulinase production by P.oxalicum at laboratory scale[99].

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Therefore,corn starch by-product is a commonly applied raw material in biological fermentation processes.However, its mechanistic impact on process performance is still not completely unravelled.

3.4.Extraction of polysaccharide

In the previous sections,it was described that corn starch by-products can be used as food ingredients for humans,or animal feeds,or as fermentation substrates for microorganisms.These utilization approaches mainly apply the byproduct as a whole,rather than extract and apply a specific component of the by-product.There are many nutrients and functional substances worth separating and extracting from the by-products of corn starch processing.The soluble polysaccharide from the corn hull is valuable for development.Polysaccharide is the main component of corn hull,which is a good edible plant compound.It has the effect of reducing blood sugar,blood lipids and blood pressure,promoting bowel movement and treating colon cancer [100].The polysaccharide extracted from corn hull with functional properties and biological activities is shown in Fig.4.Generally,the following extraction methods are used:solvent extraction,enzyme extraction,ultrasonic and microwaveassisted methods and so on.The solvent extraction method uses water,acid or alkali solvent for extraction.The method is mild and easy to operate,but the extraction cycle is long,the yield of the product is low,acid and alkali will destroy the stereo structure and activity of polysaccharides.Compound enzymes are often used in enzymatic extraction.The enzymatic extraction conditions are mild,the extraction rate is high, and it has a wide range of industrial applications.Compared with traditional solvent extraction methods,ultrasonic and microwave-assisted methods have higher extraction efficiency and less damage to the structure of polysaccharides.Microbial fermentation has also been studied.Research on submerged fermentation using corn bran acid hydrolysate (CBAH) as a nutrient source and utilizing lasiodiplodan to produce β-glucan exopolysaccharide [101].Steam explosion with periodic peristalsis is also exploited to separate corn pericarp and can improve the extraction rate[102].The structure and purity of polysaccharides are usually identified by NMR and IR.Corn bran hemicellulose was purified to obtain corn fibre glue.The molecular structure information of PCFG and its acid hydrolysates was provided by NMR and methylation analysis[103].

Recently, chemical modifications of polysaccharides,such as sulfation,carboxymethylation,acetylation,and oxidation have been extensively researched.Ultrasoundassisted extraction of polysaccharides from corn bran and then carboxymethylation could make the surface of polysaccharides particles loose.Carboxymethylated polysaccharides can inhibit the proliferation of A549 and HepG-2 cells,and improve its antioxidant activity[104].Corn bran arabinoxylan (CAX) was esterified by glucuric acid (SA) to obtain glucuric acid corn bran arabinoxylan ester (SA-CAX) with different degrees of substitution.It has shown that SA-CAX has higher antioxidant activity and gel behaviour[105].

Corn bran dietary fibre has attracted much attention because of its anti-cancer and hypolipidemic activities.Dietary fibre is prepared by physical,chemical and biological methods.Physical methods include high temperature and high-pressure method,ultrasonic method,ultra-micro pulverization method,etc.However,these methods have the problem of high energy consumption and are usually used together with other methods.The chemical method uses chemical reagents such as acids and alkalis to process raw materials.Its extraction process is simple,and it can remove starch and protein well.Fermentation can also be used to extract dietary fibre,but its application is limited due to the long fermentation period.The enzymatic method is currently the main method of extracting dietary fibre,and the enzymatic reaction conditions are mild.Xylanase has been studied extensively.Appropriate xylanase has been shown to affect the binding of corn bran dietary fibre to bile salt[106].Chemical and enzymatic methods can also be combined to extract dietary fibre from corn.Firstly,enzymatic decomposition was adopted and followed by alkali modification at high temperature and high pressure.It was an effective method for glycosidic bond cracking.Through this approach,corn dietary fibre with a larger specific surface area and more hydroxyl groups could be obtained,which could combine a large amount of water,nitrite and blood lipids.The physiological activity assay shows that dietary fibre extracted by enzymatic chemistry is an ideal dietary component to reduce cancer risk by adsorbing dietary nitrites and to prevent hyperlipidemia by lowering lipid concentrations[107].

Fig.4.The polysaccharide extracted from corn bran with its functional properties.

Corn bran fibre is mainly composed of cellulose,hemicellulose and lignin.Lignin is the most difficult impurity to remove in the process of cellulose extraction, and the content of lignin in corn hull is less than other by-product raw materials such as straw,rice husk and bagasse.Therefore,corn husk can be used as a high-quality raw material for cellulose extraction.The main processing methods of lignin are physical, chemical and microbial.Because of environmental protection,convenience and quickness,cellulase hydrolysis is the most suitable method to produce corn husk fibre.Cellulose is the main component of the corn hull.Cellulase can easily remove the loose tissue cellulose around the fibre.However,if the treatment time is too long,the strength of the fibre will be destroyed.Moreover,Ultrasonic-assisted extraction [108], lye separation and enzymatic extraction are also available methods[109].A previous study revealed the influence of acid hydrolysis reaction time and fibre/acid ratio on the crystallinity,particle size and thermal stability of nanocrystalline cellulose.The results show that all these factors have a certain influence,and the reaction time has the most influence [110].The okra bast fibre and corn bran fibres were treated by different degumming methods, such as alkalization, bleaching and vinyl acetate.Corn hull fibres were found to be more susceptible to chemical treatment, and corn husk fibres treated with vinyl acetate had the highest mechanical strength.Therefore,corn husk fibre may have applications in non-load-bearing applications, such as insulation or noise control[111].Corn bran and oat hulls were concentrated and then rotated or spray-dried to produce crude bio-based fibrous gum,which has functional characteristics,such as water-in-oil and oil-in-water emulsifier,binder and antioxidant[112].Further treatment of the extracted corn fibre can increase its application in industry,such as improving the emulsification stability,increasing the mechanical properties of the membrane, etc.However, the modification method of corn fibre extraction still needs to be further studied to expand its application in various products.Fibres are widely used in industry.To improve their properties,they are modified as shown in Table 3.

Table 3 Methods for modification of corn fibre.

Antioxidant properties,gel properties,immunostimulatory and anti-metastatic activity are the functional properties of corn fibre polysaccharides.Corn fibre polysaccharide has antioxidant activity in vitro.Arabinoxylan is the main structural polysaccharide of corn fibre polysaccharide,which contains ferulic acid and other components,and is believed to provide antioxidant capacity.Sulfated corn bran polysaccharide was obtained from corn bran by sulfation and showed strong scavenging ability to free radical (DPPH and ABTS) and iron-zchelating ability,and it can inhibit the proliferation of two kinds of cancer cells[113].Water-extractable AXs are well-known owing to their ability to form viscous solutions and their gelling capacity after being treated with the oxidizing agent.As is known to all,gel formation occurs through oxidative crosslinking of FA (erulic acid) moieties.The arabinoxylan was extracted from corn bran by alkali treatment.It was found that the axons extracted by mild alkali treatment were cross-linked by FA under the action of laccase to form a strong gel.The higher the strength of alkali treatment,the lower the average FA content of axial fibres and the lower the gel elasticity[114].According to its gel properties,it can be used to make products such as gels and drinks with low sugar and pH[115].There is a kind of immunestimulating polysaccharide in corn polysaccharide,which is an important functional compound.The main mechanism by which corn polysaccharides enhance the immune system is their ability to activate macrophages and NK cells as well as the complement system.Polysaccharides were isolated from corn steep liquor,which was divided into supernatant and sediment.The effects of polysaccharides on innate immune stimulation and anti-metastasis were evaluated in vitro and in vivo.The results show that the supernatant is mainly composed of hyperbranched polysaccharides rich in arabinose glycans, and has strong innate immune system stimulation activity.It can be used as an immunostimulatory by stimulating the innate immune system to promote the inhibition of lung metastasis[116].

At present,cellulose extracted from corn husks can be used to produce CMC.Compared with oat husk,the oxidized corn husk fibre has a higher carboxylic acid content,so the oxidized corn husk fibre has a greater tendency to nano fibrosis [119].In previous research, Pickering emulsions were stabilized by zeatin/corn fibre gum(CFG)composite particles(ZCPS)which were prepared by electrostatic interaction.The adsorption of ZCPS on the oil-water interface was uniform, which was beneficial to the formation of dense interfacial deposits and the gel network structure of emulsion droplets [120].Hemicellulose can also be used to prepare pentose,and sodium hydroxide was used to extract corn husk and corn cob to obtain hemicellulose.The hemicellulose was hydrolyzed by xylanase to obtain pentose and xylooligosaccharides[121].The preparation of cellulosic ethanol is also a research hotspot.

In another research,the glucose and lipid concentrations of ageing mice could be effectively regulated by three differently processed corn bran(corn bran soluble hemicellulose (HEM), hemicellulose hydrolyzed by oxalic acid(HOA),Amberlite XAD-2 eluate(XE).The high degree of esterification of feruloylated oligosaccharides has a significant effect on reducing glucose and lipid concentrations of the human body[122,123].

In the future,in the research and development of corn by-products,it is necessary to strengthen the research on the industrialization extraction technology of corn husk active polysaccharide, the functional and physicochemical properties of the product,to obtain polysaccharide products with high yield,high activity and high application quality.

3.5.Lipids and their derivatives

Table 4 Fat acid composition in corn germ and bran.

Table 5 The contents of sterol,tocopherol and carotene from the oils of corn germ and bran.

As a kind of high-quality vegetable oil,corn germ oil has been studied for many years by using lots of extraction methods including pressing extraction,Soxhlet extraction,organic solvent extraction,supercritical CO2and aqueous enzymatic [124,125,132–134].In most industries, corn germ oil is extracted by pre-pressing and solvent to improve the yield of corn germ oil[132].Recent study demonstrated that the corn germ oil extracted by supercritical CO2could improve the antioxidant capacity of oil without lowering the profile of fatty acids[124].The longer extraction timeand the larger amount of solvent may reduce the selectivity of organic solvents,though the organic solvents presented the highest values of oil yield.In comparison with the traditional approach,supercritical CO2extraction could obtain more functional compounds due to its lower operating temperature.Furthermore,to reduce the chemical contamination for the extraction of vegetable oils,aqueous enzymatic oil extraction attracted much more attention among the researchers and corn germ oil was also explored by using this technology.Although many advantages prove that aqueous enzymatic oil extraction has a great future,a variety of problems such as the high price of enzymes,long extraction time and low extraction yield are needed to be solved.Therefore,many studies aimed to improve the technology for the extraction of corn germ oil[135,136].

Originally,liquid corn germ oil was commonly produced as an expensive novel edible oil for domestic uses,such as dressing, cooking, frying and roasting.Later, low-cost germ oil became available because of mature product technology,and it is widely used in the formulation of many industrialized products, such as shortenings, margarine,mayonnaise, sauces mixtures and technical fats [137].Corn germ oil is well received by consumers because of its preferable appearance,mild aroma,high levels of polyunsaturated fatty acids and high oxidative stability [138].Corn germ oil is also applied as an ingredient in the formulation of snacks,baking mixes and bakery products.Meanwhile, the application of corn germ oil as frying oil has expanded dramatically due to its comparatively better oxidative stability and the restriction of hydrogenated fat[139].In addition to the application in food,corn germ oil can be used in medical treatment because of its high content of linoleic acid, tocopherols, phytosterols and lecithin.Linoleic acid can prevent the deposition of cholesterol in the arterial wall by combining with cholesterol in human serum and reducing the absorption of cholesterol [140].Therefore,linoleic acid could effectively restrain the formation of atherosclerosis.Furthermore, in comparison with other vegetable oils, corn germ oil has abundant phytosterols which can significantly prevent cardiovascular diseases [141,142].Moreover, corn germ oil was rich in tocopherols,which is demonstrated to have effective antioxidant effects and can be used to scavenge free radicals,soften blood vessels, improve blood circulation and enhance the immunity of the human body.During the production and refining process of corn oil,the triglyceride fatty acid composition of the oil itself did not change significantly,but the content of elements(Na,Mg,K,Ca,Fe,Pb,Cd,Ni,Mn,Zn,Co,Cr,P,Cu)decreased significantly.Cd,Co and Zn were not detected in the refined corn oil.Therefore, the chemical refining process affected some of the quality properties of corn oil [143].In addition to corn germ, corn bran can also be used as the raw material to extract oil.Many researchers extracted corn bran oil by using a solvent method because of its low content of lipid.According to Ma's study [142], the lipid content of corn barn was 6%.

However, corn bran oil has great application perspectives owing to its abundant unsaturated fatty acids(80%,oleic acid and linoleic acid)and phytosterols[142].In addition to traditional solvent extraction(petroleum ether),the methods of hexane extraction and subcritical butane extraction were used to extract corn bran oil which contains high content of unsaturated fatty acids and phytosterols[144].These two approaches could get high-phytosterols corn bran oil with phytosterols concentration of 6.4%(hexane)and 5.2% (subcritical butane).The main phytosterols in corn bran oil were campesterol, sitostanol, stigmastol,β-sitosterol,and stigmastanol.Therefore,it could be developed as a sort of health care edible vegetable oil due to its physicochemical and nutritional properties.The development of corn bran oil could improve the valorization of corn by-products, reduce the cost of production and increase the overall profit for the corn processing industry.

Generally speaking,the oil from corn germ or bran is a type of functional vegetable oil and have a great perspective to be used as the raw material in pharmaceutical manufacturing and applied in food to develop functional foods.Other industrial uses include formulations of insecticides,paints,resins,plastics,varnishes,soaps and textiles[141,142].Furthermore,another attractive application of corn germ oil is biodiesel,which is a renewable and biodegradable fuel converted by fatty acids esters and alkyl esters of long-chain carboxylic acids.

The so-called biodiesel,that is,diesel produced from vegetable oil,is a monoalkyl ester of long-chain fatty acids.Natural oils and fats are mostly composed of triglycerides of straight-chain fatty acids.After transesterification with methanol,the molecular weight is reduced to close to that of diesel, and it has performance close to that of diesel[145].It is a kind of renewable and environmentally friendly fuel.Biodiesel is a promising alternative to fossil diesel fuel and is recognized as one of the power sources for long-chain fatty acid alkyl esters that meet the prescribed and conventional standards[146].Recently,many biodiesel production companies and researchers are developing technologies for recovering corn oil from the germ and converting it into biodiesel.For biodiesel,the higher the content of unsaturated fatty acids,the better the lowtemperature fluidity of the corresponding fatty acid methyl ester.Therefore,the existence of a large number of unsaturated fatty acids in corn oil can effectively improve the low-temperature flow of biodiesel.And the cold filter point,turbidity point,pour point and condensation point of corn oil biodiesel are −3, −2, −3 and −4 °C respectively[147].At present,transesterification method using acid,base or enzyme as catalyst is the most widely used.But there are problems of high cost or pollution.Therefore,low cost and energy-saving production technologies have received continued attention.Waste oil from corn oil refining is more suitable for making biodiesel when production costs are taken into account[148].With the massive consumption of earth resources and the serious pollution of the environment, green energy which can replace petroleum resources has been paid more and more attention by researchers and consumers.In the future,the production of fuel from vegetable oil is likely to get more research and wide application.

3.6.Protein production

The protein content of corn accounts for about 7.8%.About 30% of the by-products such as corn gluten, corn barn,corn germ is produced during the deep processing of corn,which can be used as high-quality feed resources.The protein content of corn endosperm is about 8%,the most important of which is zein,accounting for 35%–60%of the protein in the endosperm.Hydrophobic amino acids are predominant in zein(about 50%),but lack charged acidic and basic amino acids, especially tryptophan and lysine.Therefore,the composition of amino acids in zein is not balanced,resulting in low nutritional value.However,the hydrophilic and hydrophobic parts of zein are distinguished,so it has unique advantages in biologically active substances,drug delivery carrier systems and edible materials,and is of great significance to the preparation of micro- and nanomaterials [149].Prietto et al.[150] added red cabbage anthocyanins to zein ultrafinefibres to obtain ultrafine fibres sensitive to pH changes through electrospinning.Curcuminloaded zein nanofibres were prepared and used as a coating material on the surface of apples to understand the degree of decay of apples after harvest[151].

Protein in corn germ accounts for 18%–22%of the mass of corn germ and 29%of total protein of corn.It has a good amino acid balance.However, corn germ is usually subjected to solvent extraction and high-temperature treatment to extract oil,resulting in severe denaturation of residual protein.These denatured proteins almost lose their functional properties and are mainly used as a protein and energy supplement in animal feed[152].Study by Widmer et al.[153]has shown that adding 5%and 10%of corn germ to the daily feed of pigs has no adverse effects on the growth performance,carcass quality,and meat flavour of pigs.

The defatted corn germ(CGM)has a protein content of about 60%–71%.Corn germ protein is a whole protein,and its essential amino acid composition is close to the FAO/WHO model [149].Soluble dried corn distillers'grains(DDGS)are another by-product of dry milling and have a protein content of about 27%–35% [154].The main protein component of CGM is composed of approximately 65%zein and 30%gluten[155].However,the low water solubility of CGM and DDGS limits their performance and application in food.Corn bran accounts for about 12%–14% of the dry quality of corn.Corn bran is dried after adding corn syrup,which is called pulp fibre,which can replace part of gluten.At present,corn bran is mainly used in the production of the feed industry.For instance,Galeana-Lopez et al.[156]evaluated the influence of dietary supplementation of corn bran on growth performance,liver antioxidant status and intestinal microbiota of Nile tilapia(Oreochromis niloticus) fingerlings.The results revealed that corn bran is safe to improve crude protein and enhance SOD activity.The deep processing of corn bran is mainly to extract dietary fibre, corn yellow pigment, ferulic acid,L-arabinose, sterol, etc.There are few reports on protein extraction and hydrolysis.

To achieve the ideal food functional properties of corn gluten, better develop corn gluten products, and expand the application in medicine and health and food processing,we can degrade the macromolecular protein in corn to make soluble peptides, and increase the added value.There have been many reports on the biological activity of corn peptides,such as antioxidant[157],antihypertensive[158],anti-inflammatory[159],and antiproliferative activity[160].In particular,Yu et al.[161]extracted albumin from corn germ meal,and studied the in vitro antioxidant activity of corn germ meal albumin peptides,and revealed that protein-peptide components less than 1 kDa after proteolysis (APF4) when the dosage is higher than 800 mg/kg body weight,it can significantly reverse the adverse changes of alcoholic liver injury in mice,and the possible effect of APF4 on the liver from alcoholic injury is proposed.Ruan et al.[162]used corn polypeptide particles and polypeptide-modified citrus soluble fibre as a stabilizer to prepare a citrus fibre-based high internal phase emulsion under acidic conditions, and its rheological properties,thermal stability,freeze-thaw recovery rate and other properties were evaluated.Research has been carried out and the potential application prospects of this product in the development of plant-based egg substitutes have been proposed.Study has shown the corn protein hydrolysates or peptides had attractive antioxidant activity by in vitro chemical methods[154].However,there is still a need for more researches on the performance of food and non-food systems which are crucial for commercialization.

The protein extracted from the byproducts of corn can be used in the production of feed, medicine and bioactive peptides,as well as in food preservation.Due to its special molecular structure and shape and unique solubility,corn protein can form a uniform, transparent and soft film,with good oil retention and water retention.Corn protein can also play an important role in the gelling stage,coating stage of paper manufacturing process to produce lustrous and oil-proof paper.

3.7.Micro-functional component extraction

Corn is also reported to contain non-nutrient phytochemicals such as carotenoids,phytosterol phenolic acids,and anthocyanins.The content of these phytochemicals in corn varies according to genetic and environmental factors.Plant sterols (phytosterols) were generally found to be a minor unsaponifiable chemical component of plant seed oil in two forms including free and esterified with fatty acids.Corn oil usually comes from corn germ and contains total sterols of more than 8 mg/g oil(Table 5),which represents a higher content than other common vegetable oils[141].The several main phytosterols types of corn germ oil were shown in Fig.5.Furthermore, the content of phytosterols can be affected by the varieties of corn and the extraction parts.Following Harrabi's report[163],the crude oil content of corn germ (24.2%–30.7%) is much higher than that of endosperm and pericarp fractions(0.4%–1.2%).While endosperm oil contained the greatest amount of phytosterols and 4,4-dimethylsterols;pericarp oil had the highest levels of 4-desmethylsterols and 4-monomethylsterols.Study[164]have shown that phytosterols can affect the absorption of cholesterol in the intracellular and intestinal lumen, and reduce serum cholesterol levels.Moreover,the cholesterol-lowering effect of phytosterols occurs even when had a high-fat diet.Reducing plasmatic cholesterol levels is the major factor for the prevention of cardiovascular diseases and some chronic diseases.

Plant pigments,a kind of bioactive substance and functional effective ingredients in the plant can also be found in corn, including carotenoids and flavonoids.However,these plant pigments are mostly extracted from corn bran.It is reported that high levels of carotenoids (zeaxanthin etc.) were contained in corn bran [125].As a mixture of carotenoids,maize yellow pigment consists of β-carotene,zeaxanthin,cryptoxanthin and lutein.The carotenoid content of corn is 0.6–57.9 mg/kg(depending on the cultivar),with lutein and zeaxanthin as the major chemical components[165].Maize yellow pigment,a category of isoprene pigments, has important physiological functions which coexert effects on the protection of vision and epithelial tissue and enhance the body's immunity.Furthermore,many studies[166–168]confirmed that corn bran had a variety of flavonoids,especially ferulic acid and anthocyanins.Generally, ferulic acid is linked to macromolecular polymers by ester bond in the plant cell wall,hence corn bran is the ideal raw material to extract ferulic acid with its highest content (3%) in comparison with other plant by-products[167].Ferulic acid could provide plenty of beneficial effects including anti-inflammatory, antioxidant,antimicrobial and anticancer[168–170].It can also lower the risk of coronary disease,reducing the liver cholesterol level and increase sperm viability[171,172].Purple corn is an important source of anthocyanins.Like the other type of flavonoid pigment,anthocyanins have lots of physiological functions,such as anti-inflammatory,antioxidant,anti-cancer, anti-obesity, and anti-diabetic properties[173,174].

Additionally,some minerals,phenols and flavan-3-ol can also be found in a corn hull.According toDuru's study[175],some essential mineral elements in human and animal nutrition including calcium,sulfur and potassium were the most abundant minerals in corn hull.Besides,some phenols and flavan-3-ol were in concentrations of 12.35 mg/g and 10.74 mg/g, respectively.Phenolic compounds can be used as antioxidants to protect cell components from damage and extend food shelf-life by mediating stress responses due to their capacity to scavenge free radicals produced in stress-induced oxidative reactions[176].

Fig.5.The chemical structure of β-sitosterol(A),campesterol(B),stigmasterol(C)and Δ-5-avenasterol(D).

These micro-functional components have been regarded as being associated with the health effects of preventing cancer,obesity,hypertension,and inflammation.However,the content and availability of these phytochemicals are not satisfactory.Some processing,such as heating,will lead to the loss of vitamins and phytochemicals because of the poor heat stability,hence more study needs to be conducted.

4.Conclusion

Corn starch processing comprises a major part of the grain processing chain.The increasing quantity of corn cereal and starch, especially in developing countries, have produced a mass of by-products that have received wide attention.Proverbially, sustainable methods for utilizing these by-products are very necessary for food industrial waste management.Nowadays, many by-products have been used as raw materials for value-added products in many industrialized countries.Research on the utilization of corn starch processing by-products has also advanced significantly over the last few decades,particularly the development and optimization of the production of feed,functional foods,lipids,protein,bioactive compounds,fuel and so on.Commercialization applications always need to be considered in terms of security,environmental protection and economic cost aspects.Because of the potential health hazards,the high cost of the value-added products and the high pollution of the production process may limit its development.At present,much high-value research of corn deep processing by-products has not been invested in commercial products, and most of them are still in the research stage.For instance,corn steep liquor can be used in the fermentation industry to replace antibiotics and prepare microbial fuel cells.It can also be used in the production of poly(L-lactic acid),a green and degradable medicinal repair material.Corn germ and corn gluten are raw materials for producing high-quality protein food and bioactive peptide.Corn bran is used in some of the latest research to prepare dietary fibre, ferulic acid and active polysaccharides that have applications in medicine and human health.Predictably,the innovations in processing technology will likely contribute to the comprehensive utilization of starch processing byproducts.The availability and high-value application approach of these by-products will largely increase in the future.

Author Contributions

Runyang Zhang:Writing-original draft(Sections 1,2,and 3.2),visualization,formal analysis;Sen Ma:Writingoriginal draft(Section 3.4);Li Li and Minghui Zhang:Writing-original draft(Section 3.1),data curation;Shuangqi Tian:Writing-original draft(Sections 3.3 and 3.5),data curation;Dongying Wang:Writing-original draft(Sections 3.5 and 3.7); Kunlun Liu: Writing - original draft (Section 3.6);Huamin Liu:Conceptualization,writing-original draft(Abstract and Section 4);Wenxue Zhu:Project administration,funding acquisition;Xuede Wang:Writing-review&editing.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Acknowledgement

The authors gratefully acknowledge the financial support provided by the Doctor Research Fund of Henan University of Technology(2020BS009)and Science,Technology and Innovation in the Soybean and its Alternative Crops Chain(SQ2019YFD100114).