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SORGHUM-BASED FERMENTATION
The term fermentation is generally used to describe the desirable biochemical changes brought about by microorganisms and/or enzymes on primary food products (Nout and Motarjemi, 1997; Blandino et al., 2003). Enzymatic fermentations involve chemical reactions whereby the enzyme acts as a catalyst, as occurs when starch is converted to dextrin by – amylase. When fermentation involves microorganisms, these are either naturally present on the substrate or they may be deliberately added in the form of starter cultures (Nout and Motarjemi, 1997). In microbiological terms, fermentation is a type of energy-yielding microbial metabolism whereby organic substrates are incompletely oxidized and organic carbohydrates act as electron acceptors (Adams, 1990).
The four main fermentation types are alcoholic, lactic acid, acetic acid and alkali based. Yeasts usually predominate in alcoholic fermentations, resulting in the production of alcohol, whereas bacteria such as Acetobacter species are responsible for fermentations where alcohol is converted to acetic acid in the presence of oxygen (Blandino et al., 2003). The production of foods such as Japanese natto from cooked soybeans, dawadawa from African locust beans and ogiri from melon seeds often involve some form of alkaline fermentation, with Bacillus subtilis being the dominant species (Wang and Fung, 1996). LAB mainly carry out lactic acid fermentation.
LAB are essential for fermentation of most plant-based products where they usually co-exist with yeasts. In this regard, LAB play various roles that include the production of safe products through biopreservation as a result of bacterial antagonism (Oyewole, 1997; Soomro et al., 2002), enhancement of sensorial and nutritive value of the foods, and saving of energy through reduced cooking time (Simango, 1997). A plethora of LAB predominate in plantbased foods such as pozol, a Mexican maize dough (Escalante et al., 2001), Colombian chicha (Steinkraus, 1996), and idli, a fermented Asian food made from rice blended with black gram (Soni and Sandhu, 1991). LAB also play a central role during the production process of most fermented sorghum-based foods such as Ethiopian injera (Vogel et al., 1993), Sudanese kisra (Mohammed et al., 1991), Ugandan bushera (Muyanja et al., 2003) and Tanzanian togwa (Mugula et al., 2003).
Anti-nutritional compounds in sorghum
Sorghum contains significant levels of anti-nutritional compounds that include protease inhibitors, phytic acid (myoinositol hexakis) and oligosaccharides (Serna-Saldivar and Rooney, 1995). The protease inhibitors include non-specific tannins and Kunitz trypsin inhibitor. Although tannins have been shown to reduce serum lipids, increase blood clotting and reduce blood pressure (Chung et al., 1998), their anti-nutrititive attributes in foods far outweigh their medical value. For example, impaired utilization of minerals and vitamins and inhibition of digestive enzymes are associated with these compounds (Chung et al., 1998).
Together, tannins and phytic acid inhibit digestive enzymes and reduce protein availability via different modes of action. Tannins typically form complexes with proteins (Hagerman, 1989), whilst phytic acid, a highly charged molecule (Serna-Saldivar and Rooney, 1995), chelates cations such as potassium, calcium, magnesium, iron and zinc, resulting in the formation of phytate (Wodzinski and Ullah, 1996; Dvorakova, 1998; Stodolak et al., 2007).
Oligosaccharides such as raffinose, stachyose and verbascose are responsible for causing flatulence (FAO, 1998). This is due to their indigestion by enzymes of the human gastrointestinal tract in the small intestine. Consequently, these oligosaccharides are passed to the large intestine where fermention by resident microorganisms takes place, resulting in the production of gas (FAO, 1998).
Nutritional and health advantages of lactic acid fermentation
Lactic acid fermentation of sorghum foods by LAB prior to cooking and/or consumption significantly alters their biochemical properties, rendering a food product with enhanced nutritional value and flavour (Towo et al., 2006). For instance, lactic acid fermentation improves the in vitro digestibility of starch (Hassan and El Tinay, 1995) and sorghum protein (Kazanas and Fields, 1981; Mertz et al., 1984; Taylor and Taylor, 2002; Ibrahim et al., 2005).
The increase in in vitro protein digestibility may be due to a rapid decrease in pH that affects the structure of insoluble sorghum proteins such as prolamines and glutelins in such a way that they become more accessible to pepsin digestion (Taylor and Taylor, 2002). Lactic acid fermentation has been shown to reduce the level of anti-nutritive oligosaccharides, phytate and tannins. The amount of poly- and oligosaccharides in sorghum, e.g., raffinose, stachyose and verbascose, are significantly reduced due to the action of -galactosidases produced by some LAB that include Leuconostoc mesenteroides subsp. mesenteroides, Weissella paramesenteroides, Lactobacillus fermentum, Lb. brevis and Lb. buchneri (Mital et al., 1973; Milliere et al., 1989). The -galactosidases disrupt the -D-galactosidic bonds of oligosaccharides, resulting in lowered abdominal distention and flatulence (Nout and Motarjemi, 1997). Tannin levels are also reduced by lactic acid fermentation (Lorri, 1993; Osman, 2004). The reduction may be due to metabolic processes of the microorganisms. These processes include oxidation, reduction or dissociation of the tannins as a defense mechanism against toxicity by endogenous sorghum microflora (Bvochora et al., 2005). Reduced levels of phytate, as a result of lactic acid fermentation, may increase B vitamins (Kazanas and Fields, 1981; Nout and Motarjemi, 1997) and improve the availability of minerals (Marfo et al., 1990).
CHAPTER ONE
LITERATURE REVIEW
CHAPTER TWO
POLYPHASIC TAXONOMIC CHARACTERIZATION OF LACTIC ACID BACTERIA ISOLATED FROM SPONTANEOUS SORGHUM FERMENTATIONS USED TO PRODUCE TING, A TRADITIONAL SOUTH AFRICAN FOOD
CHAPTER THREE
DIVERSITY AND DYNAMICS OF BACTERIAL POPULATIONS DURING SPONATNEOUS FERMENTATIONS USED TO PRODUCE TING, A SOUTH AFRICAN FOOD
CHAPTER FOUR
USE OF STARTER CULTURES OF LACTIC ACID BACTERIA IN THE PRODUCTION OF TING, A SOUTH AFRICAN FERMENTED FOOD
CHAPTER FIVE
GENERAL DISCUSSION AND CONCLUSIONS
APPENDICES
