CHAPTER ONE

1.0 INTRODUCTION

Interest in functional foods has recently increased among consumers due to a greater consciousness of health and nutrition; as well as the need to cure diseases and also the increasing scientific evidence of their effectiveness. Fermented products are a significant part of many indigenous diets.

Yoghurt is a Turkish name for a fermented milk product. It is originated by early normadic herdsman, especially in Asia, Southern and Eastern Europe. Yoghurt is made by adding a culture of acid forming bacteria to milk that is usually homogenized, pasteurized and fermented. Yoghurt is defined as a fermented milk product that evolved empirically some centuries ago by allowing naturally contaminated milk to sour at a warm temperature, in the range of 40-50 °C (Kwon et al., 2011). The micro-organisms which are used conventionally in this process are referred to as “Starter Culture”. They include Lactobacillus delbrueckii subsp. Bulgaricus and Streptococcus thermophilus. The average size of Lactobacillus bulgaricus ranges from 0.8 to 1.0μm in diameter (Rakis .,1978 , Sanfu. , 2009).

During the fermentation, hydrolysis of the milk proteins occurs, the pH drops, the viscosity increases and bacterial metabolites are produced that contribute to the taste and possibly to the health promoting properties of yoghurt. The sugars are fermented by the bacteria into lactic acid, which causes the formation of the characteristic curd. The acid lowers the pH of the yoghurt and restricts the growth of food poisoning bacteria (putrefactive or pathogenic).

Not only is yoghurt a wonderful quick, easy and nutritious snack, but also research evidence point to the fact that milk and yoghurt may actually add years to life as found in some countries where fermented dairy products are a dietary (Wolf.,1978) Several health benefits have been reported for traditional yoghurt and this healthy image is enhanced by supplementation with probiotic bacteria ( ; Magenis, et al 2006).

Soymilk is an aqueous extract of soya beans (Glycine max) and is quiet similar in appearance to cow milk (Agure-Dam, 1997). It is commonly characterized as having a beany, grassy or soy flavor, which reportedly can be improved by lactic acid fermentation, as in yoghurt-like products Microorganisms possess endogenous β-glucosidases which can be utilized to hydrolyze predominant isoflavone glucosides in soymilk to improve biological activity.

 It has been reported that probiotic organisms including Bifidobacteria and some other lactic acid　bacteria hydrolyze isoflavone glycosides into corresponding aglycones (Cheng., 1998).

Recent reports indicate that some probiotic bacteria could better compete with yoghurt cultures in a soy-based substrate. Soy has been examined as a substrate for the Lactobacillus species such as L. Casei, L. helveticus, L. fermenti, L. fermentum and L. reuteri (Garro et al., 1999; Murti et al., 1993b;( Chumchuere and Robinson, 1999; Garro et al., 2004; Tzortzis et al., 2004).

The problems of which is beany flavor can be improved by lactic fermentation, so production of fermented soymilks such as soy yoghurt is important (Nsofor et al., 1992; AbdEl Khair, , 2009).

Due to continuous increase in the population and inadequate supply of animal milk protein leading to malnutrition in Nigeria many research works have been geared finding alternative protein source from legume.

Soymilk can serve as a very good alternative to the expensive cow milk as it contains all the essential amino acid even though some are in a low concentration

It is well known from experiment that diets containing containing casein or other animal protein could induce elevation of plasma total LDL cholesterol concentration but this can be prevented by vegetable protein such as soy protein (caroll and kurowska 1998)

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