Vol 59, No 1 February 2006 International Journal of Dairy Technology ORIGINAL RESEARCH *Author for correspondence. E-mail: idrgalic@pbf.hr © 2006 Society of Dairy Technology 40 Blackwell Publishing Ltd The quality of plain and supplemented kefir from goat’s and cow’s milk LJUBICA TRATNIK, RAJKA BO Z ANI C , ZORAN HERCEG and IDA DRGALI C * Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia Goat’s or cow’s milk was fortified with 2 g/100 g skimmed milk powder (SMP), whey protein concentrate (WPC) or inulin. All the milks were fermented with kefir grains at 25°C for 19 h and stored for 10 days at 5°C. All the kefir samples were analysed on the 1st, 5th and 10th day of storage. The acidity level remained very stable in all the samples during the storage period. Goat’s samples have significantly lower viscosity and slightly lower sensory profiles, mostly due to softer consistency. Concentration of ethanol was low, regardless of milk type used, especially for control samples. Keywords Acidity, Cow’s and goat’s milk, Kefir, Sensory, Storage period, Viscosity. INTRODUCTION Kefir is a fermented dairy product originating in the Caucasian mountains and has become popular in many European countries. This beverage differs from other fermented milk products because it is not only the result of the metabolic activity of lactic acid bacteria. The unique flavour and aroma of traditional kefir are mainly due to the associa- tive metabolic activity of different species of bacteria and yeast present in the kefir grains. Kefir is a slightly carbonated beverage that owes its distinctive flavour to the ongoing lactic acid and alcoholic fermentation (Beshkova et al. 2003). Depending on the milk type used, different char- acteristics of fermented milk beverage products are obtained. It is well known that goat’s milk has better digestibility than cow’s milk but, although goat’s milk is used for therapeutic purposes such as antiallergenic effect (Haenlein 2004), it is still not thoroughly investigated. Goat’s milk has unique sensory properties characterized by spe- cific and typical ‘goaty’ flavour. Some consumers strongly dislike the aroma and taste of goat’s milk and it is therefore manufactured into different fermented milk products (e.g. kefir and yoghurt) and cheeses. The presence of different supplements in milk used for fermentation can, however, improve the sensory characteristics and nutritive quality of such products. Besides the addition of skimmed milk powder (SMP), whey protein concentrates (WPC) have been used to fortify the solid content of the milk base. However, WPC has a higher biological value than SMP, and it also stimulates the growth of several strains of lactic acid bacteria (Bury et al. 1998; Bozanic and Tratnik 2001; Martin-Diana et al. 2003; Bozanic et al. 2003b). At present, prebiotics components, such as inulin and fructo-oligosaccharides, are added to the milk base of probiotic fermented milk products. Such prebiotic components stimulate the growth of bifidobacteria, which are considered to be beneficial for human health (Simmering and Blaut 2001). Inulin is very popular, and it has already been successfully applied in several well-known dairy products in the market, but relatively, little research has been carried out on fermented goat’s milk. Data on the rheological properties of foodstuffs and the influence of various ingredients and additives on those properties will facilitate the defining of process quality control, the predicting of prod- uct stability during storage, and the modification of the texture of the product (Hegedutic et al. 2000; Herceg et al. 2000). The aim of this study was to produce a kefir from goat’s milk and to evaluate the effect of SMP, WPC and inulin addition on the acidity, sensory quality and viscosity of kefir during 10 days of stor- age in comparison with the cow’s milk product. MATERIALS AND METHODS The raw goat’s and cow’s milk were supplemented with 2 g / 100 g of inulin (Raftiline, Orafti, Belgium), SMP (0.05 g/100 g fat, 4.0 g/100 g moisture and pH 6.7), and WPC (60.0–60.5 g/100 g pro- teins, 3.1 g/100 g moisture, 4.7 g/100 g ash and pH 6.5). The latter two types of powders were obtained from LURA d.d., Zagreb, Croatia. Kefir grains were obtained from Croatian household sources. Fermented milk production Goat’s and cow’s milk were divided into four parts. The first part served as a control sample, and the