Yogurt, made from fortified skim milk by conventional methods using Streptococcus thermophilus and Lactobacillus bulgaricus, was used in studies of the effect of fermentation on nutritional value of milk. In all experiments, the product was compared with the uninoculated base milk. The concentration of most vitamins was less in yogurt than in milk and was most noticeably so for biotin which was 60 % less. The effect on folic acid content was inconsistent. In nutritional experiments with rats, high values for true digestibility, biological value and net protein utilization were obtained for both yogurt and its base milk, only minor differences being apparent between the two materials. In growth tests with rats, yogurt was not found to be consistently superior to the base milk when the milk was subjected to a double heat treatment to reduce bacterial contamination. Yogurt did not confer a nutritional advantage on fresh milk in this respect.Fermented dairy products are widely regarded as being health-promoting and this is especially true of yogurt. Fermentation may improve the nutritional value of milk per se, or the consumption of live organisms may affect the bacterial flora of the gastrointestinal tract to the benefit of the host. Support for the special properties of yogurt is evident from the work of Hargrove & Alford (1978), who found in studies with young rats fed on a number of fermented dairy products that yogurt increased growth rate and food conversion efficiency (FCE; weight gain/weight of food eaten on a dry matter basis). So far this effect has not been explained satisfactorily. An attempt has been made, therefore, to confirm these findings as part of a larger study of the value of fermented dairy products from nutritional and bacteriological viewpoints. The effects of such fermentation on protein quality and on the concentrations of water-soluble vitamins were also investigated.A part of the present study which dealt with the effect of the consumption of yogurt on the gut flora has already been reported (Garvie et al. 1984).
MATERIALS AND METHODS
Yogurt and base milkFor expt 1, base milk and yogurt which were made from the same batch of raw milk were supplied weekly by a commercial dairy (Unigate, London).