Most of the studies indicate that yeast or natural fermentation results in slight improvement in protein content, which is mainly attributed to the loss in dry matter content mainly carbohydrates (Kazanas and Fields, 1981). Fermentation of corn meal with yeasts, (S. cerevisiae and C. tropicalis) resulted as increment in protein content from 7.6 to 8.9 and 8.4%, respectively (Wang and Fields, 1978). However, the increase in protein content was attributed to losses in dry matter signifinicantly. Chavan et al. (1988) reported that the enhancement in protein content of sorghum; however, some researchers observed no change in protein content due to fermentation in cereals (Hamad and Fields, 1979). Although the quantitative changes are less, a significant qualitative improvements occur in proteins due to natural fermentation. The relative nutritional value (RNV) of maize increased from 65% to 81% as a result of germination and fermentation respectively, further fermentation of germinated maize flour resulted an increase in RNV to 87% (Lay and Fields, 1981). A significant increment in essential amino acids, particularly lysine, methionine, and tryptophan has been observed during fermentation (Hamad and Fields, 1979; Au and Fields, 1981). The available lysine content increased considerably in rice, wheat, oats, millet and corn during fermentation (Hamad and Fields, 1979). Further, in all cereals the increment in lysine content was higher at fermentation temperature of 37°C as compared to at 22 to 25°C. Rice meal, when subjected to natural lactic fermentation with or without hulls resulted a substantial increase in nutritive value by enhancing isoleucine and lysine content (Tongnual and Fields, 1979). Fermentation of corn meal with food yeasts, improved the lysine content but decreased the methionine content in fermented meals. The percent RNV increased significantly due to this fermentation (Wang and Fields, 1978).

Bacterial fermentation of cereals and cereal- legume blends resulted the highest enhancement in methionine content, whereas yeast fermentation caused a decrease in methionine. The main reason behind different changes in amino acids composition during bacterial and yeasty fermentation is the bacterial fermentations involve proteolytic activity, while yeasts mainly degrade carbohydrates. Both types of fermentation reportedly increased bioavailability of essential amino acids significantly, particularly lysine and methionine. The increment in percent RNV value is attributed to increase in availability of limiting amino acids of cereals and legumes (Hamad and Fields, 1979; Au and Fields, 1981). In animal feeding trials it was found that natural fermentation of pearl millet, sorghum, or finger millet did not enhance the PER significantly (Aliya and Geervani, 1981). The biological value (BV) and net protein utilization (NPU) of products of sorghum and finger millet improved significantly after fermentation but not those of pearl millet. It was further established that fermentation had minimal effect on overall protein quality of millets. Several researchers reported changes in PER of various foods as a result of fermentation as shown in Table 2.

Carbohydrates mainly starch and soluble sugars are the principle fermenting component for lactic acid bacteria. Hence, degradation subsequent resulting a decrease in starch as well as total carbohydrate content are expected to occur during natural fermentation of cereals. Kazanas and Field (1981) and Chavan (1988) reported a decrease in the starch and fibre contents and increase in the reducing sugars content during natural fermentation of sorghum. The availability of starch increased significant level during natural fermentation (Kazanas and Fields, 1981). The decrease in fibre content during fermentation might be due to partial solubilisation of cellulose and hemicellulosic by enzymes from microbes (Beuchat et al. 1975).

The report depicting quantitative changes in lipids of cereals during natural lactic or fungal fermentations are limited. Most studies regarding natural fermentation of cereals and legumes with bacteria or yeasts have described changes in proteins, amino acids and B-group vitamins. This is because the fact that cereals are low in fats. But this type of investigations are important, particularly relatively oil-rich cereals like corn and pearl millet. Kazanas and Fields (1981) did not observe a significant change in crude fat content of sorghum during natural lactic fermentation. However, Chavan (1988) reported a slight increase in crude fat content of sorghum and sorghum plus green gram blend due to fermentation process. Actually, fat content is pretty small in most cereals and legumes and their availability is not a nutritional concern in these food grains. The changes in lipid portion in cereals during fermentation may impact the functional and sensory qualities of fermented products.

The vitamin content, especially B-group vitamins has been reported to increase during natural fermentation of cereals, although considerable variation were found in different reports. The thiamine, riboflavin, and niacin contents increased significantly during natural fermentation of pearl millet (Stott et al. 1963), rice (Tongnual and Fields, 1979) and sorghum (Kazanas and Fields, 1981). Murdock and Fields (1984) reported a significant increase in the vitamin B₁₂, folacin, riboflavin, and pantothenic acid contents during natural fermentation of corn meal. No appreciable changes in biotin, pyridoxine, thiamine, choline and niacin contents were observed. A significant increase in riboflavin content and a significant decrease in niacin and thiamin content was observed during fermentation rice meal (Tongnual and Fields, 1979). A slight increase in thiamine content, a greater increase in niacin content, whereas no appreciable change in riboflavin content was observed during fermentation of sorghum meal for kisra preparation (El-Tinay et al. 1979). Most of the studies reported changes of B-group vitamins during natural fermentation of cereals and cereals-legumes blends, which are influenced by several factors like raw materials nature, type and level of microflora, temperature, fermentation duration, and methods of determination of these vitamins. The mechanism of such changes in vitamin content is not fully explained and needs to be further investigated.

Cereals are good source of dietary minerals, but availability of minerals are important nutritional issue. Kazans and Fields (1981) did not find any change in total ash content of sorghum during fermentation. The changes in individual mineral elements during fermentation of rice, black gram, and rice plus black gram blend were studied by Reddy and Salunkhe (1980). No apparent changes were noted in the concentrations of minerals (calcium, magnesium, total phosphorus, zinc and iron). Several workers observed enhancement in availability of minerals due to fermentation. Fermentation was observed more effective in increasing Ca, P and Fe bioavailability in finger millet (Sripriya et al. 1996). Similarly rabadi fermentation of barley flour at different temperature and time resulted a significant increase in extractability of minerals. Longer periods and higher temperature of fermentation resulted in an increased HCl-extractability of minerals (Gupta et al. 1992). Similar increment in mineral extractability was observed due to natural fermentation of precooked pearl millet flour (Khetarpaul and Chauhan, 1990) and rabadi fermentation of freshly ground wheat flour-buttermilk mixture (Gupta et al. 1991). Similarly during rabadi preparation from pearl millet flour/buttermilk mixture at different temperature and periods of fermentation, fermentation at all temperatures decreased phytate P, increased nonphytate P and HCl extractability of minerals; the increased extractability was observed with an increasing periods of fermentation (Dhankher and Chauhan, 1989). Probiotic fermentation of food blends consisting of germinated pearl millet flour, whey powder and tomato pulp caused a significant (P&aaalt; 0.05) improvement in availability of Ca, Fe and Zn (Arora et al. 2011). The probable reason for increment in mineral availability as a result of fermentation may be decrease in antinutrients, mainly phytic acid content, as a negative correlation have been detected with HCl-extractability of minerals (Gupta et al. 1992; Gupta et al. 1991).

Idli is apopular breakfast item in India. It is a low calorie, starch enriched nutritious food. It comprises about 3.4% protein, 20.3% carbohydrate and 70% moisture (Teniola and Odunfa, 2001). Idli is prepared from a fermented thick suspension made of a blend of rice and dehauled black gram by application of steaming (Nagaraju and Manohar, 2000). The lactic acid bacteria Lactobacillus fermenti, Leuconostoc mesenteroids, Streptococcus faecalis, Pediococcus cerevisiae and Lactobacillus lactis have been found to be liable for the fermentation process, although L. mesenteroids and S. faecalis are the major flora considered for acid production in idli batter. The yeasts like Geotrichum candidum and Torulopsis candida have also been found in fermented idli batter (Chavan and Kadam, 1989). Fermentation of idli batter has been reported to have a enhancing effect in essential amino acids and in the reduction of antinutrients (Steinkraus et al. 1993).

The dosa batter is very similar to idli batter and the microbiological, physical, biochemical and nutritive changes of dosa batter during fermentation are quite similar to idli (Purushothamman et al. 1993). The product is prepared by spreading of fermented suspension on a flat heated plate with a little oil or fat (Battacharya and Bhat, 1997).

Kenkey is a fermented maize product consumed in Ghana. Kenkey is made by using two methods, firstly, the maize grains are water-soaked at room temperature for 1–2 days, followed by draining of water and wet-milling of grains and the meal is kept for a spontaneous or solid state fermentation to produce a stiff dough. In the second method, the maize meal is combined with a large volume of water and strained followed by the overnight fermentation of mixture. The wet mash after removing the water is cooked to make porridge (McKay and Baldwin, 1990). The fermentation is mainly governed by array of lactic acid bacteria, like Lactobacillus fermentum and L. reuteri followed by presence of yeasts and moulds, that contribute towards flavour development.

Pozol, mainly consumed in South-eastern Mexico, is a fermented maize product in the form of balls of various sizes and shapes. Maize grains are boiled in limewater and grounded coarsely, the resulting dough is subjected to kneading to form a ball of compact mass which is wrapped in banana leaves and left for fermentation at ambient temperature. A complex microbial community (Lactococcus lactis, Lactobacillus plantarum, Lactobacillus casei, Lactobacillus delbruekii and Clostridium sp., etc.) incorporated mainly during the grinding process takes part in the fermentation of the dough (Nanson and Field, 1984).

Kishk is an important food of Middle East and mainly prepared by fermentation of milk-wheat mixtures. Kishk is usually made by addition of strained yoghurt to cracked and bran-free parboiled wheat termed as bulgur followed by fermenting the mix at ambient temperature for different time intervals. The wheat grains are boiled to make soft, dried, milled and sieved to remove the bran. Milk is separately fermented by lactic bacteria in a container, concentrated and mixed with the moistened wheat flour. The resultant paste is dried to around 10-13% moisture content and then made to powder by grinding and the product is stored in the form of dried balls having brown colour with a rough surface and hard texture. The microorganisms associated with fermentation includes Lactobacillus casei, Lactobacillus plantarum, Lactobacillus brevis, Bacillus subtilis and yeasts (Beuchat, 1983; Chavan and Kadam, 1989). Kishk is a nutritious food rich in B vitamins having good preservation quality..

Tarhana (Trahanas) is produced by blending wheat flour, yoghurt, yeast and several cooked vegetables and spices (tomatoes, onions, salt, mint, paprika) followed by fermentation of the substrate for 1-7 days. The fermentation process and type of product is very similar to kishk. The major fermenting organisms are Streptococcus thermophilus and Lactobacillus bulgaricus. The fermented matter is dried and stored in the form of biscuits (Campbell-platt, 1994). Tarhana has an acidic taste and yeasty flavour and is a good source of nutrients like protein and vitamins. The addition of cereal protein with dairy protein leads to improvement in nutritional quality. The low pH (3.8-4.2) and low moisture content (6-9%) make the product less suitable for pathogens and spoilage organisms. The tarhana powder can be stored for long time (1-2 years) without any sign of deterioration.

Ogi is considered as the most important weaning food for infants in West. Ogi is a fermented cereal guel processed from maize, although sorghum and millet are also sometimes used for fermentation. Lactic acid bacteria, yeasts and molds are liable for the fermentation. The predominant species involved is Lactobacillus plantarum and other organism like Corynebacterium sp., sometimes yeasts of Saccharomyces and Candida species are responsible for flavour development (Caplice and Fitzgerald, 1999). However, about 20-50% of the nutrients available in the original cereals are lost during production of ogi. Substantial losses of lysine and tryptophan have been reported. The colour of ogi varies on the basis of cereal used for its preparation: cream-white for maize, reddish brown for sorghum and dirty grey for millet.

Injera is undisputed national food of Ethiopia (Oda et al. 1983). It can be produced from different cereals like sorghum, tef, corn, finger millet and barley etc. Although, tef is the major cereal and sorghum second most preferred ingredient for making injera. The grains are dehulled and milled into flour, the flour is mixed with water to form a dough followed by addition of starter. The dough is subjected to fermentation for 2 to 3 days. After fermentation the dough is converted into a thick batter and poured onto a lightly oiled pan, which is then covered and steamed (Parker et al. 1989). The microbiota involved in fermentation are mainly yeasts (Beuchat, 1983). The major quality attribute of a good product is its slightly sour flavour. The product contains over 50% moisture, 4.8% protein, and is a good source of calcium and iron (Zegeye, 1997).

Mahewu, a non-alcoholic beverage consumed in Africa and Arabian Gulf countries, is sour in nature and generally prepared from corn meal (Chavan & Kadam 1989). It is made from maize porridge, which is mixed with water and sorghum, millet malt or wheat flour and subjected to fermentation (Odunfa et al. 2001). The fermentation is held by natural flora of the malt at ambient temperature (Gadaga et al. 1999). The predominant flora of the fermentation belongs to Lactococcus lactis sub sp. Lactis (Steinkraus et al. 1993).

Boza is a non-alcoholic beverage consumed in Albania, Bulgaria, Romania and Turkey and characterized by a light to dark colour, sweet, slightly sharp and slightly sour in taste. It is made of wheat rye, millet, maize and other cereals combined with sugar and sweetner (Hancioglu and Karapinar, 1997). Boza mainly consists of lactic acid bacteria and yeasts in the ratio of 2.4 (Gotcheva et al. 2000). The lactic culture identified mainly of Lactobacillus plantarum, Lb acidophilus, Lb fermentum, Leuconostoc raffinolactis, Ln mesenteroids and Ln brevis and the yeasts isolated consist of Saccharomyces cerevisiae, Candida tropicalis, Geotricum candidum etc. (Gotcheva et al. 2000). A significant increase in glucose level is observed during Boza formation, with a decrease in pH, viscosity, free amino nitrogen content and dry matter.

Yosa is a snack type food prepared from oat bran which cooked in water and fermented with lactic organism and Bifidobacteria. After fermentation, the matter is subjected to flavouring with sucrose or fructose and fruit jam. The product is mainly consumed in Finland and other Scandinavian countries. Though, the product has a similar texture and flavour like yoghurt but it is completely free from milk (Toufeili et al. 1997). It is a vegetarian diet which is lactose-free, low in fat and having β-glucan in it. Yasa is a healthy food as it contains oat fibre and probiotic which is beneficial for intestinal health (Toufeili et al. 1997). Oat fibre is also a good source of β-glucan, which has been reported to reduce cholesterol and helpful in heart disease.

Most probiotic food products in the markets worldwide are milk-based, and very limited attempts have been made to utilize other substrates such as cereals and combination of milk and cereals for probiotic product formulation. Traditionally fermented milks like yogurt was considered as major matrix for probiotic organisms, but in recently probiotic is being incorporated into a range of food items like beverages, powder, ice cream etc. (Shah, 2001). Increasingly, whole grain is turning out to be one of the chosen substrate as delivery vehicle of probiotic organisms. This is primarily because the formulation of cereal based probiotic products offers consumers a dual benefits of bioactive components of whole grain and probiotic (Marquart and Cohen, 2005). This enhances the nutritive value of the product and also appeals to the consumers. The possible application of cereal components in probiotic food formulations could be as a fermentable substrates for starter cultures or encapsulating material. Numerous cereal-based prebiotics and probiotic foods have been launched recently, particularly in the European market. Some of the examples can be indicated as cereal bars (CornyActiv® )in Germany, probiotic flakes (Muesli®) in Portugal, whole wheat breakfast cereals (Weetaflakes®) in France and snack bar (Goodness®) in UK (Dornblaser, 2007).