Yogurt Milk Production

1. INTRODUCTION

Yogurt and similar fermented milk products have been very popular for a long time all over the world. The demand for yogurt saw a large increase during the world war and since then it became an everyday consumed product. Compared to the olden days when yogurt was produced in traditional way an industrial revolution has taken place with its manufacturing processes. The bio-chemical processes involved in the manufacturing of yogurt have been studied and well understood over the decades and thus this would be a good starting point in understanding the working of a bio-chemical industrial plant. In this review paper we would focus on the

of lactic acid production in the broth, which in turn is dependent on the consumption of dissolved

ln addition to carbonyl substances, many volatile compounds have also been recognised in yogurt mainly volatile fatty acids and several compounds derived from the thermal degradation of lipids, lactose and proteins during the heat treatment of milk before yogurt manufacture. Acetaldehyde is considered as the major flavour component of yogurt even when in comparison high concentrations of this compound in other dairy products (cheese or cream) lead to flavour defects described as "green" or "yogurt-Iike". Diacetyl contributes to the delicate, full flavour of yogurt and seems to be important when the acetaldehyde content is low. According to studies S thermophilus is the only microorganism responsible for diacetyl production in yogurt, whereas diacetyl is also largely produced by L bulgaricus. Even though the importance of diacetyl, acetone and acetaldehyde for yogurt flavour is well established, it is extremely difficult to evaluate the contribution of several identified volatile

The "ropy" character is often required for the manufacture of many fermented milk products. Ropy strains of lactococci have been isolated from Scandinavian fermented dairy products. In stirred yogurt, yogurt beverages and low milk solids yogurts, production of polysaccharides can improve viscosity and texture, increase resistance to mechanical handling and decrease susceptibility to syneresis. The use of ropy strains is particularly important in France and in the Netherlands where the addition of stabilizers in yogurt is prohibited. In fact, some strains of S thermophilus and L bulgaricus produce neutral exopolysaccharides. Only few studies are available on the characterization and production of these polymers. Researchers isolated ropy strains of some thermophilic lactic acid bacteria. She observed higher exopolysaccharide production in milk at suboptimal growth temperatures, as measured by an increase of relative viscosity. The polymer material obtained contained galactose and glucose in the ratio 2:1. The slime secreted by a strain of L bulgaricus studied by Groux contained mainly galactose and some glucose, mannose and arabinose[11]. Doco studied an exopolysaccharide produced by S thermophilus after 3-4 h of incubation and obtained a polymer composed of galactose, glucose and N-acetylgalactosamine in a ratio of 2:1:1. The molecular mass of the polymer is