Vol 61, No 3 August 2008 International Journal of Dairy Technology ORIGINAL RESEARCH *Author for correspondence. E-mail: hnm@agfe.iitkgp.ernet.in © 2008 Society of Dairy Technology 284 Blackwell Publishing Ltd Oxford, UK IDT International Journal of Dairy Technology 1364-727X 1364-0307 Society of Dairy Technology 2008 XXX ORIGINAL RESEARCH ORIGINAL RESEARCH Modelling of acidification kinetics and textural properties in dahi (Indian yogurt) made from buffalo milk using response surface methodology SHIBY VARGHESE K. and H N MISHRA* Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur-721302, India The simultaneous effects of fermentation temperature (20.8 –29.2°C), total solids (TS) level (13 –16% w/v) and total inoculum level (1–4% v/v) on the acidification process in buffalo milk fermented with dahi culture were explored by means of response surface methodology. The changes in lactose content and lactic acid in buffalo milk (14.5% TS) inoculated with 2.5% culture were investigated during fermentation at 27.5°C. The utilization of lactose followed first order kinetics during dahi fermentation. The syneresis increased with increase in fermentation temperatures. A firmer gel was achieved with a medium acidification rate in the range of fermentation temperatures from 27°C to 29.2°C, medium TS of 14.5 – 15%, and at a total inoculum level of 2.5%. Keywords Acidification kinetics, dahi, Fermentation, Lactic acid, Lactose, Yogurt. *Author for correspondence. E-mail: hnm@agfe.iitkgp.ernet.in INTRODUCTION Dahi (curd) is a yogurt-like product made from cow, buffalo or mixed milk (Aneja 1995). It is widely consumed all over India and the neighbouring countries, either plain, sugared or salted. It has been the most important fermented milk used in India from time immemorial. According to the Prevention of Food Adulteration (PFA) rules (Ministry of Health and Family Welfare 1976), dahi or curd is the product obtained from pasteurized or boiled milk by souring, natural or otherwise, by harmless lactic acid or other bacteria. Dahi may contain additional cane sugar. It should have the same percentage of fat and solids-non-fat as the milk from which it is prepared. Lactic acid bacteria (LAB) have a major role in the fermentation process, and the key metabolite is lactic acid. Homofermentative LAB convert the available energy source (lactose) almost completely to lactic acid via pyruvate to produce energy and to equilibriate the redox balance. The fermentation action of specific LAB strains may lead to reduction of components such as lactose and galactose from fermented milk and thus help to prevent lactose intolerance and accumulation of galactose. Mathe- matical analysis of the biokinetics of functional starter cultures may yield precious information about the relationship between the food environment and the bacterial functionality. This may result in better process control, enhanced food safety and quality, and in the reduction of economic losses. Total solids (TS) in milk have been found to affect the firmness and viscosity of yogurt (Tamime and Robinson 1985; Becker and Puhan 1989; Biliaderis et al. 1992; Gastaldi et al. 1997). The susceptibility to syneresis (Harwalker and Kalab 1983; Hess et al. 1997) and the development of starter culture (Tamime and Deeth 1980; Radhke-Mitchell and Sandine 1986) are also significant quality determi- nants of the fermented product. Fermentation temperature (FT) primarily affects the growth of yogurt bacteria (Radhke-Mitchell and Sandine 1986). The amount of inoculum is important for a normal acidification process (Tamime and Robinson 1985). Torriani et al. (1996) studied the influence of the variation of parameters such as FT, fat and solids content, inoculum size and initial cocci/rods ratio on the fermentation process of yogurt by applying Response Surface Methodology (RSM).The method was useful in the modelling of the acidification of starter bacteria as a function of the above parameters and in considering the optimal combination of the factors for a better fermentation process. Kristo et al. (2003) accom- plished the modelling of the acidification process and the rheological properties of yogurt using RSM. The present work on fermentation kinetics of dahi aimed to model and to evaluate the simultaneous effects of FT, milk solids level and total inoculum (TI) concentration on the acidification process. The changes in lactose and lactic acid contents during the fermentation of standardized buffalo milk were also studied. The effect of TS level on syneresis and the effect of inoculum level on hardness of dahi were also studied.