INFLUENCE OF STABILIZERS ON THE FERMENTATION RATE AND NUTRITIVE VALUE OF SET YOGHURT

ABSTRACT

Stabilized yoghurt samples were produced by reconstituting  powdered milk in water along  with sugar and 0, 0.5 and 1.0 % concentrations of carboxyl methyl cellulose (CMC), Corn starch and gum acacia before fermentation. The yoghurt samples were either produced as short set yoghurt by incubating at 42 ^oC for 5 hours or incubating at 30 ± 2 ^oC (room conditions) for 24 hours as long set yoghurt. Samples were analyzed at intervals of 1 h for 5 h for the short set yoghurt and at intervals of 4 h for 24 h for long set yoghurt. The proximate, physicochemical, niacin content, microbial and sensory evaluation were carried out in the yoghurt samples as fermentation progressed for both short set and long set yoghurt. Results showed that the addition of stabilizers reduced moisture content from 88.54 ± 0.02 % (no stabilizer) to 84.59 ± 0.02 % (CMC), 85.59 ± 0.02 % (corn starch) and 86.70 ± 0.02 % (gum acacia). Due to dilution effect, addition of stabilizers depressed protein contents from 3.37 ± 0.04 % (no stabilizer) to 3.28 ± 0.51 % (CMC), 3.08 ± 0.03 % (corn starch) and 3.32 ± 0.03 % (gum acacia). Total solids increased on addition of stabilizers from 11.64 ± 0.02 % (no stabilizer) to 15.41 ± 0.02 % (CMC), 14.47 ± 0.02 % (corn starch) and 13.30 ± 0.02 % (gum acacia). Increase in stabilizer concentration and fermentation time decreased moisture content but increased total solids, protein, fat, ash and sugars. The viscosity of the yoghurt samples significantly (p < 0.05) increased with addition  of stabilizers from 1.48 ± 0.03 Cp (no stabilizer) to 78.03 ± 1.14 Cp (CMC), 2.74 ± 0.04 Cp (corn starch) and 1.99 ± 0.03 Cp (gum acacia), with CMC having the highest  increase (p < 0.05) and gum acacia the least. Viscosity significantly increased (p < 0.05) as stabilizer concentration and fermentation time increased. Although CMC increased the pH from 6.19 ± 0.03 (no stabilizer) to 6.26 ± 0.02, corn starch did not affect pH (6.18 ± 0.02) while gum acacia reduced pH to 5.94 ± 0.03. Increase in stabilizer concentration and fermentation time increased pH. Addition of stabilizers increased the titratable acidity from 0.29 ± 0.01 % (no stabilizer) to 0.39 ± 0.01 %( CMC), 0.32 ± 0.01 % (corn starch) and 0.45 ± 0.00 % (gum acacia). Lactic acid production reduced as concentration of stabilizers increased, but increased as fermentation time increased. Vitamin B3 (niacin) increased with the addition of stabilizers from 0.15 ± 0.00 (no stabilizer) to 0.175 ± 0.02 mg/ml (CMC), 0.185 ± 0.01 mg/ml (corn starch) and 0.185 ± 0.00 mg/ml (gum acacia). Niacin production increased as stabilizer concentration and fermentation time increased. The total viable bacteria significantly (p < 0.05) reduced with increase in concentration of stabilizer and fermentation time, while lactic acid bacteria decreased with increase in stabilizer concentration it increased as the fermentation time increased. Significant (p < 0.05) interactions were observed between stabilizers and their concentrations for all parameters studied. Sensory results indicated that yoghurt produced with 1.0 % concentration of CMC gave the best mouth feel while yoghurt produced with corn starch produced the most desirable taste and flavor. Results showed that reaction rates were higher for the short set yoghurt for all parameters studied, resulting to achieving an equivalent effect in 5 h in short set yoghurt compared to the same effect achieved in 24 h in the long set yoghurt.

CHAPTER ONE

INTRODUCTION

1. BACKGROUND OF THE STUDY

Dairy products are generally defined as foods produced from commercially domesticated cows, goats or buffalo’s milk (United Kingdom food Standard Agency, 2009). They are usually high energy-yielding food products. Raw milk for processing of dairy products comes mainly from cows and to a lesser extent from other mammals such as goats, sheep, yaks, camel or horses. Dairy products are commonly found in the European, middle-Eastern and Indian cuisines, whereas they are almost unknown in Eastern cuisines. The United Kingdom Food Standards Agency defined Dairy as “foodstuffs” made from mammalian milk (Bandler and Singh, 2009). Most dairy products contain large amounts of saturated fat and most of them are usually fermented. Examples of dairy products include Cheese, Kefir, yoghurt, etc.

            Fermented dairy products, also known as cultured dairy foods or cultured milk products, are dairy foods that have been fermented with lactic acid bacteria such as Lactobacillus, Lactococcus and Leuconostoc. The fermentation process increases the shelf-life of the product as well as adds to the taste and improves the digestibility of milk (Canadian dairy Commission, 2007). There is evidence that fermented milk products have been produced since around 10,000 B.C, and a range of different Lactobacilli strains has been grown in laboratories for a wide range of cultured milk products with different tastes.

            Fermented milk products are sour tasting milk products which have been made by either fermenting the milk naturally or by the use of starter culture to produce the desirable milk product. Examples of fermented milk in Africa, Syria, Asia and America are Cheese, nono, buttermilk, yoghurt, irgo, kadam, laban, shenineh, dahi, shirkand, mahi, etc (Ajayi, 2006).

            Yoghurt, as a fermented dairy product, is a semi-solid milk product and the best known of all fermented milk products. It is obtained by souring of milk using a pure culture of Lactobacillus bulgaricus and Streptococcus thermophilus (Chandan and Shahani, 1993). It can be manufactured from liquid cow milk, powdered milk and vegetable milk (Soy milk) as base material (Adolfsson et al., 2004). Lactic acid and the other molecules that are formed during fermentation of milk make yoghurt a food product that is both acidic and creamy, appreciated for its taste and nutritional qualities notably for its calcium content (Buttriss, 1997). Yoghurt is thus a very convenient food as compared to milk which is very fragile. Due to the health benefits and taste, it is known to constitute an appreciable proportion of total daily food consumption or even just as a refreshing beverage in several countries (Khan et al., 2008). It is regarded as a nutritionally balanced food, containing almost all the nutrients present in milk and in a more assimilable form (Younus et al., 2002). Yoghurt is a source of highly nutritive protein, energy from added cane sugar, milk fat and unfermented lactose as well as vitamins (Ihekoronye and Ngoddy, 1985). It is actually considered to be more nutritive than milk in terms of vitamins content, digestibility and as a source of calcium and phosphorus (Foissy, 1983). It is believed that yoghurt has valuable “therapeutic properties” and helps in curing gastro intestinal disorders (Adolfsson et al., 2004). It also prevents and controls diarrhoea, capable of modulating the inflammatory response produced by carcinogens, helps in reducing the inflammatory response through an increase in apoptosis.

            Yoghurt is characterized as a smooth viscous gel with specific taste of sharp acid and green apple flavor (Bodyfelt et al., 1988). Some yoghurts exhibit a heavy consistency that closely resembles custard of milk pudding, in contrast to others that are purposely soft boiled and are essentially drinkable (Connolly et al.,1984). The most important textural characteristics of yoghurt are firmness and the ability to retain water which is a factor of the type and concentrations of stabilizers used. The type of culture used is also an important factor affecting microstructure and the textural properties of yoghurt (Hussan et al., 1999).

            Stabilizers and thickeners are important in several manufactured products and dairy products such as chocolate dressing, milk drinks, ice-cream and yoghurt. These substances prevent separation of various ingredients, increase the viscosity and inhibit the formation of large crystals. Substances used as stabilizers and thickeners include vegetable or tree gums such a gum tragacanth and gum Arabic, agar, cornstarch, gelatin and pectin. Cellulose compounds like methylcellulose and CMC (sodium carboxyl methyl cellulose) are also used (Awan, 1995).

            Yogurt is mainly classified based on its chemical composition (full-fat, reduced fat and low-fat), manufacturing method (set and stirred yogurt), flavour type or post incubation process. Yoghurt on the basis of method of production prior to incubation, cooling and final packaging exists as set and stirred yoghurt. Set yoghurt is a type of yoghurt which when produced is incubated and cooled in the final retail package and it is characterized by a firm jelly-like texture (White, 1995). On the other hand, stirred yoghurt is a type of yoghurt that is produced and incubated in a tank and the final coagulum is “broken” by stirring prior to cooling, addition of flavours and packaging (Skriver et al., 1993; White, 1995).

1.1      STATEMENT OF RESEARCH PROBLEM

From previous researches, it is noteworthy to say that fermentation increases the vitamin content of products, more especially some B-complex vitamins due to microbial activities during fermentation where synthesis and breakdown of substances occur (Ochanda et al., 2010). Yoghurt starter cultures utilize some vitamins present in milk during fermentation for their growth. However, this increment depends on the rate of the inoculation, the strain of yoghurt starter cultures and the fermentation conditions (Tamime and Robinson, 1999). Stabilizers or hydrophilic colloids bind water, prevent separation of various ingredients, increase the viscosity and inhibit the formation of large crystals which are attributes for consumer acceptability. It is, therefore, necessary to rebuild yoghurt with stabilizers and thickeners at such concentrations that will give the desired body to the final product. This goal will be achieved by optimum selection of stabilizers with protective colloid properties, by assessing how the activities of the fermenting organisms will be enhanced or inhibited by the hydrocolloids used with respect to vitamin synthesis by evaluating the chemical,   microbiological, nutritional and sensory properties of yoghurt produced under controlled incubation fermented at 42 ^oC for 5 hours and 24 hours at 30 ^oC respectively.

1.2   JUSTIFICATION

Yoghurt is a fermented product obtained through anaerobic fermentation of lactose in milk by relevant micro-organism, most of which are classified as probiotic. It is a very nutritious product in terms of vitamin content, digestibility and has valuable “therapeutic properties”. Much research has been carried out in yoghurt in terms of final product. Stabilizer used in yoghurt production are many and varied but there is little information on how some of the stabilizers locally used in Nigeria influence the fermentation rate of yoghurt and consequently the nutritive value for which yoghurt is consumed. 

1.3     OBJECTIVES

The main objective of this study is to produce yoghurt with different stabilizers and evaluate their nutritional characteristics.

Specific Objectives

1. To evaluate the effect of the stabilizers on the physicochemical characteristics of the yoghurt during fermentation.
2. To compare the effect of the different types and concentrations of stabilizers with respect to their influence on the changes in the vitamin B₃ content during fermentation. 
3. To evaluate the changes in the microbial population during fermentation of yoghurt produced with different stabilizers.
4. To evaluate the effects of different types and concentrations of stabilizers on their sensory characteristics of yoghurt.