BY-PRODUCT PROCESSING OF CEREALS

INTRODUCTION

During the milling process, bran, germ, and the endospermic tissue aleurone layer are removed from cereal grain, and comprise the dry milling by-products. These grain fractions are a rich source of bio functional molecules, fibre, minerals, vitamins, lignans, phytoestrogens, and phenolic compounds. Depending on the milling process (dry or wet milling) the final nutritional value of the by-products varies greatly. There- fore the fractions obtained during dry or wet milling can find many applications in food and non-food products apart from use only as feed. Malting, brewing, and distilling of cereals are processes that result in various by-products that are becoming very attractive raw materials for other industries.

CORN BY-PRODUCTS

The most common by-products of corn milling are normally used for animal feeding. The feed by-products diverge according to dry matter, protein, fat, fibre, and energy content: dry milling produces distiller’s grains and distiller’s soluble, whereas wet milling produces corn gluten feed. These by-products are marketed in wet or dry form. Dry milling fractions such as pericarp, germ cake, standard meal, and broken kernels are usually combined and hammer milled to produce hominy feed. Hominy feed accounts for nearly 35% of the starting corn quantity and competes with other corn by-products such as corn gluten feed and brewer’s spent grain.

Although produced and used for animal feed, the hominy feed can also be the start- ing material to produce ethanol due to its high starch content and can return more profit than by selling it as low-protein feed ingredient.

The germ from dry and wet milling processes is directed to industry for oil extraction. Oil is recovered from the germ fraction obtained after dry milling of corn by utilizing mechanical screw presses or a combination of screw presses and solvent extraction. Although the oil content of the germ fraction varies from 15% to 25%, its recovery is dependent on the oil content of the corn, the efficiency of germ fraction recovery from the dry or wet milling process, and the efficiency of the extraction method. The resulting cake from the oil recovery process contains residual oil and proteins and can be sold as is or added to gluten feed.

The common feed by-products of corn wet milling are starch molasses, liquefied corn product, gluten feed, gluten meal, germ meal, condensed fermented corn extractives, and hydrolysed corn protein.

These by-products are classified as either liquid feeds or protein supplements. The solids obtained from wet milling are rich in nutrients and are generally exploited by pharmaceutical industries as a growth media for molds and other microorganisms or to produce antibiotics and related products. The recovered protein meal and bran from the wet milling process are mainly used in the feed industry.

Generally, the bran from the corn milling industry has also found other applications for human food supplements.Dried distiller’s grains represent the main by-product of the production of distilled alcohol. After fermentation of cereals, the rest of the nutrients other than starch present in the kernel (i.e., protein, non-starch polysaccharides, fat, vitamins, and minerals) remain in the dried distiller’s grains making it a valuable source for use in, e.g., bread production and the bakery industry

RICE BY-PRODUCTS

After dehusking and pearling of paddy rice the obtained rice bran consists mostly of pericarp, seed coat, nucellus, aleurone layer, partial endosperm portions, and embryo. Rice by-products represent almost 30% of paddy grains with husk amounting to 20%, bran 8%, and germ 2% of the by-products. Generally,

rice milling by-products are used alone or mixed with other feeds as livestock feeds. Now a- days, rice by-products are becoming an attracting source for food and pharmaceutical applications.Lipids are mainly found in the bran layer of rice grain and as a consequence their state is affected by the milling process. Lipids deteriorate rapidly, thus the taste and colour changes of the bran during storage contribute greatly to the final quality of this by-product if used for food and pharmaceutical applications. It is reported that lipases and lipoxygenases present in rough rice increase their activity during storage). Milling of rice causes the production of phospholipase physical injury and the high emperature destroys the lipid membrane of the cells, thus initiating lipid hydrolysis in the presence of lipases. Lipid hydrolysis promotes the oxidation of lipids since the fatty acids are easily oxidized.

Stabilization of rice bran immediately after milling comprises a necessary step to avoid the oxidation of lipids present in the bran. Different methods are used to inactivate the endogenous enzymes responsible for oxidation as well as to decrease the moisture content of the bran. Generally, heat treatment is used to inactivate or decrease the activity of the enzymes responsible for lipid oxidation.

Other methods used to perform enzyme inactivation are hot-air fluidized drying, superheated-steam fluidized drying, and infrared vibrated drying.Rice bran and the derived products are considered potentially useful in the prevention of cardiovascular diseases. Micronutrients such as oryzanols, tocopherols, tocotrienols, and phytosterols are found in rice bran in considerable amounts making it an attractive substrate for their isolation as nutraceuticals or functional food ingredients. The lipids present in the rice bran can be extracted by different methods (i.e., pressing, solvent extraction, ohmic heating, or supercritical fluid technology) to obtain rice bran oil.

Rice bran oil contains a significantly higher level of bioactive minor components (i.e., g-oryzanol, tocotrienols, and phytosterols) when compared to common vegetable oils. These bioactive com- pounds have the capacity to lower blood cholesterol and decrease cholesterol absorption, prevent cardiovascular diseases and some cancers, and they are also recognized as powerful antioxidants.

The protein of rice bran has a very high digestibility (greater than 90%) and it is considered

hypoallergenic. In general, by-products obtained during rice milling could be used in gluten-free products since they do not contain gluten. Rice bran contains biologically active peptides known to manage hypertension, oxidative stress, and type 2 diabetes mellitus. The hydrolyzates containing these peptides could be used for functional food products that improve human health.

ice bran represents also a valuable source of dietary fibre since it contains w24% total dietary fibre,

mostly insoluble rather than soluble. Rice bran major polysaccharides are cellulose, hemicellulose, or

pentosans. Nowadays, cell wall-degrading enzymes are used for the hydrolysis of the bran layer.

Enzymes such as xylanase and cellulase are used to break down polysaccharides present in rice bran into

their constituent sugars. Rice husk is used as fuel to generate heat for the parboiling of paddy rice.

Moreover, it is used as biomass to produce energy by using biochemical (bio-methanization of biomass)

and thermochemical (combustion, pyrolysis, and gasification) processes.

WHEAT BY-PRODUCTS

Wheat milling by-products according to AAFCO (1996) involve wheat bran, wheat middlings, wheat shorts, wheat red dog, and wheat feed flour, representing almost 25%e30% of the total wheat. The wheat milling by-products are used as animal feed because they provide a source of energy, amino acids, and phosphorus. The chemical composition of wheat by-products varies due to differences in the variety of the wheat being processed, environmental factors, and differences in the processing techniques.

The principal by-products of the dry milling industry are bran and shorts and some- times germ. Bran represents a very rich source of dietary fibre that has a low content of lipids. Wheat bran contains non-starch polysaccharides (w38%), starch (w19%), protein (w18%), and lignin (w6%).). In addition, it contains phytochemicals such as phenolic compounds and vitamins. Although the main phenolic compound is ferulic acid attached to arabinoxylans, other phenolics such as p-hydroxybenzoic acid, vanillic acid, p-hydroxybenzaldehyde, vanillin, and trans-cumaric acid can be also present in the wheat bran.

Wheat bran is used to replace some of the flour in bread, muffins, and cookies to increase their dietary fibre content. It is observed that incorporation of wheat bran in cereal-based products negatively affects dough rheology and handling ability as well as impairs the quality and organoleptic parameters of the obtained products. To counteract these detrimental effects on cereal-based products, different strategies are evaluated. One way to compensate for the loss of quality in baked products with high bran content is to add vital gluten to increase the gluten content of whole- wheat flours. On the other hand, processing of bran (i.e., mechanical, thermal, or enzymatic) modifies the functional properties of wheat bran and consequently the impact it has on food products.Shorts represent a mixture of bran endosperm and germ that remains after grinding and sifting, being an important source of dietary fibre, proteins, oils, vitamins, and phytochemicals. They are mixed with bran and used regularly as animal feed. Never- the less, shorts, if utilized immediately after production, could also be used in human food. If stored, they are subject to a rapid increase in rancidity due to the enzymatic and nonenzymatic oxidation of lipids.

Conventional milling removes all bran layers together, whereas the debranning process takes off each individual bran layer in sequence. Debranning leads to the production of added-value by-products. Each bran layer has distinct physicochemical and nutritional properties, giving debranning by-products great potential as novel food commodities and food ingredients. Fractions obtained af- ter debranning of wheat could be used for the production of new types of bread and cereal-based fermented foods orcould be used as a source of arabinoxylans since the 70% of non-starch polysaccharides present in bran are Arabinoxylans It is reported that debranning of wheat was effective in producing bran fractions for functional food or nutraceutical purposes, highly enriched in dietary fibre and antioxidant activity.

Wheat bran that contains both the starchy and hemicellulose/cellulose parts would greatly increase potential applications in a biorefinery for the production of bioethanol. A range of physical or chemical pre- treatments is required since enzymatic treatments of wheat bran alone may be not be sufficient to produce simple sugar.

Wheat germ accounts for 2%e3% of the total weight of wheat kernel. Globally, the annual amount of germ separated from the grain during the milling process has been estimated to amount to 25 million tons Wheat germ is reported as a rich source of bioactive substances that have a viable potential to be

used in different foods.

OTHER CEREALS

Generally, almost all the harvested barley is used in animal feeding and the brewing industry, whereas only a very low amount (w2%) is intended for human food production.Pearlings obtained from the pearling process of barley are a rich source of bioactive components such as phytate, vitamin E, phenolics, and insoluble dietary fibre. Pearling by-products of hull-less barley are 2.7e4.4 times more enriched in vitamin E than whole barley grain. Barley pearling by-products were incorporated in functional pasta formulations. Moreover, the application of pearling technology facilitates the separation of b-glucan-rich fractions present in the pericarp, aleurone, and sub- aleurone layers of oat kernels. Different health benefits are attributed to b-glucans.Barley middling from barley milling for flour have an increased fibre content and in particular a high b-glucan content. Therefore, the incorporation of barley middling into bread results in both health benefits to the consumer and economic benefits to the food industry. Barley by-products obtained after starch isolation during wet milling could be used for food and non-food applications. These by-products (i.e., hulls, fiber such as b-glucan and arabinoxylan, and protein) are used for animal feeding but they also have a potential for ethanol production.

The oat bran fraction is a rich source of b-glucan and dietary fibre since it contains at least 5.5% d.w. b-glucan and 16.0% d.w. dietary fibre from which at least one-third is soluble. The bran fraction from industrial oat dry milling procedures contains between 6% and 9% b-glucans and about 40%e52% of starch.

MALTING BY-PRODUCTS

Floating kernels in the steeping vessel during the steeping process represent a by- product and are collected in an overflow and sold as low-value animal feedstuff. According to Kunze (2004) the loss due to rootlet growth can be about 4% of the malt dry weight.

Another by-product of malting is barley malt sprouts that are separated from kilned malt after the kilning process. Malt sprouts consist of roots, sprouts, and malt hulls and are classified as a protein source.

Brewer’s spent grain represents approximately 85% of the total by-products obtained during beer brewing. It is mainly made of the barley husks that remain after wort production but is also rich in nonextracted sugars and proteins. The main use of this by-product in wet and/or dry form is as animal feed.

However, in the last decade, due to its valuable chemical composition and its low-cost, brewer’s spent grain has attracted increased interest for applications in different areas such as food, energy production, and in chemical and biotechnological processes.