530 JOURNAL OF FOOD SCIENCE—Volume 64, No. 3, 1999 MICROBIOLOGY JOURNAL OF FOOD SCIENCE MICROBIOLOGY Production and Evaluation of Yogurt with Concentrated Grape Juice B. A. Öztürk and M. D. Öner Authors Öztürk and Öner are affiliated with the Food Engineering Dept., Gaziantep Univ., 27310 Gaziantep, Turkey. ABSTRACT Fruit yogurt was prepared by adding concentrated grape juice (pekmez) CGJ, to milk. Optimum CGJ concentration and its influence on quality and fermentation process of yogurt were evaluated. The pH, titratable acidity, protein content, viscos- ity, whey syneresis, starter bacteria, mold and yeast counts were determined weekly at 4 C for 1 month. Addition of 10% CGJ provided desired sweetness. After 4h incubation of 5- 10–15% CGJ-added yogurts the pH was 4.44, 4.98 and 5.90, respectively, and the control was pH 4.26. CGJ addition in- creased fermentation time and decreased viscosity. During storage, acidity of 10% CGJ-added yogurt remained lower (P 0.05) than controls. CGJ did not affect (P 0.05) protein content and molds or yeasts were not detected. Key Words: pekmez, yogurt, storage stability, grape juice concentrate INTRODUCTION CONCENTRATED GRAPE JUICE (CGJ) IS ONE OF THE MOST COMMON grape products in eastern and central Turkey. From all grapes pro- duced, 37% are processed into CGJ by traditional methods without technological or scientific considerations. It is obtained by boiling grape juice without fortification (Batu and Aktan, 1992) and is called “concentrated grape juice” or sometimes “grape juice molasses.” CGJ is sold in liquid or solid form, and the solid CGJ is obtained by crystallization of liquid CGJ (Güven, 1989). CGJ has a high mineral content, especially calcium and iron. It is brown or dark yellow due to nonenzymatic browning reactions. The pH is 5.05 and solids of CGJ are around 82%. The protein content is 0.63% and sugar is 83% of the total solids (Özkök, 1989; Anon, 1989). The high sugar content provides the product with a long shelf- life (2 hr). Due to its lower protein CGJ can be used for treatment of protein metabolism disorders. Relatively high iron content (5-10 mg/ 100g CGJ) makes CGJ useful for patients that suffer from anaemia (Birer, 1983; Ataç et al., 1988; Topcu and Besler, 1997). CGJ is used for preparation of desserts such as Turkish asure and helva, or can be mixed with tahin and consumed at breakfast. Also CGJ, either in solid or liquid form, may be mixed with yogurt and frequently consumed at home. This mixture is called “fakibeyni” in southern Turkey (Özkök, 1989). Flavored yogurts are made by adding fruit concentrates or flavored syrups to cultured milk before or after incubation (Keating and White, 1990). The production and consumption of fruit yogurt is low in Turkey compared to plain yogurt, but fakibeyni is widely made and consumed in homes. There is no commercial production of either CGJ or CGJ yogurt in turkey or elsewhere. The objective of our investigation was to develop a new type of fruit-flavored set-yogurt by adding CGJ with high iron content and good nutritional value and to study the effects of CGJ addition on overall product quality and effects on fermentation. MATERIALS & METHODS CGJ WAS SUPPLIED BY GAZIANTEP UNIVERSITY. THE TYPICAL pH was 4.96 and solids content about 72%. The CGJ was stored at room temperature in the dark until used in production trials. Com- mercial freeze-dried yogurt culture, Jogurt series 500 (VISBYVAC Jogurt-V, Laboratorium Wiesby, Niebull, Germany) was provided by Wiesby Starter culture and media laboratory. This culture con- sisted of a 1:1 ratio of Lactobacillus bulgaricus and Streptococcus thermophilus. Yogurt making Nonfat dry milk (Pinar A. , dairy company) was used to prepare reconstituted milk with 16% (w/v) total solids. Milk was sterilized at 121°C for 5 min and cooled to 42–45°C. After cooling, CGJ was added to the milk at 5, 10, and 15% (v/v). Milk was inoculated with 3% (v/v) active yogurt culture. Fermentation was conducted at 43°C for 4h. Both the control and CGJ-added yogurts were stored over- night at 4°C for further analysis. Viscosity and whey syneresis measurement Syneresis was determined as follows: 5 mL of yogurt were centri- fuged in a Hettich ABA III model centrifuge (Germany) 5000 rpm for 20 min) and the whey that accumulated after 1 min was measured. Syneresis (%) was expressed as volume of drained whey per 100 mL yogurt (Rodarte et al., 1993). Apparent viscosity was determined by using a RV Brookfield viscometer (Stoughton, USA) on 100 mL yogurt samples at room temperature. Samples were stirred for 40 sec before measurement. Readings were converted to centipoise units. All viscosity values were measured at 10 rpm with spindle #5 (Gassem and Frank, 1991). Acidity development and protein determination The pH was measured at weekly intervals using a Jenway 3010 model pH meter (Essex, UK). Titratable acidity was determined as % lactic acid by titrating with 0.1N NaOH; using phenolphthalein as an indicator (Karleskind et al., 1993; White, 1995). The protein content of samples was measured by a dye-binding method (Eagan et al., 1981). Microbiological analysis Microbiological analysis of yogurts included determination of the numbers of total starter culture, yeast and molds. Samples were homogenized by using an Uve model vortex (type NM 110, Anka- ra, Turkey) stirrer and diluted up to 1/10 6 with sterile peptonized water. Mann, Rogosa, Sharpe (MRS) agar (Oxoid Co.) was used for assaying total starter culture (Anon, 1993). Potato dextrose agar (PDA) (Acumedia Chemical Co.) was used for determining yeasts and molds (Sharf, 1966). Yogurt samples were analyzed at weekly intervals up to 1 month; experiments were conducted in duplicate. Statistical analysis To estimate the effect of CGJ addition on each product parameter, ANOVA was conducted at the P0.05 level. When significance was observed, separation of means was undertaken by using the Duncan Multiple Range test. Experiments were replicated twice.