Evaluation of volatile compounds and free fatty acids in set types yogurts made of ewes’, goats’ milk and their mixture using two different commercial starter cultures during refrigerated storage Zehra Güler ⇑ , Alev Canan Gürsoy-Balcı Department of Food Engineering, Mustafa Kemal University, 31034-Antakya-Hatay, Turkey article info Article history: Received 16 September 2010 Received in revised form 10 December 2010 Accepted 21 January 2011 Available online 28 January 2011 Keywords: Yogurt Goat milk Ewe milk Mixture Free fatty acids Volatile compounds abstract Six different types of yogurt were manufactured from Damascus goat milk, Awassi ewe milk and a mix- ture of equal portions of the 2 species of milk using 2 types of commercial yogurt cultures (CH-1 and YF- 3331). Yogurts were chemically analysed at 1, 7, 14 and 21 days of storage. Results showed that cultures significantly affected acetaldehyde (P < 0.05), acetone (P < 0.05) and diacetyl (P < 0.001) contents. Type of milk significantly influenced acetaldehyde (P < 0.05), diacetyl (P < 0.001), acetoin (P < 0.001) and ethanol (P < 0.05) levels. Significant variations occurred in acetaldehyde (P < 0.001) and acetoin (P < 0.05) con- tents during the storage. Short-chain free fatty acids were the highest in ewes’ milk yogurt made with culture YF-3331, and increased during storage, while the levels of medium-chain free fatty acids, except for decanoic acid, were unchanged and the amount of long-chain free fatty acids decreased during stor- age. Cultures used and types of milk had no effect on long-chain free fatty acids in yogurts. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The production of high-quality yogurt requires control of sev- eral factors such as the chemical composition of milk base, type of milk, processing conditions and types of starter culture used to produce aroma compounds during incubation period for the manufacture of yogurt (Kneifel, Ulbert, Erhard, & Jaros, 1992; Tamime & Robinson, 2001; Ulbert & Kneifel, 1992). The final flavour of yogurt is associated with the presence of several com- pounds, namely: non-volatile acids (lactic or pyruvic), volatile acids (butyric to acetic) carbonyl compounds (acetaldehyde to diacetyl) and miscellaneous compounds (amino acids to products formed by thermal degradation) (Tamime & Robinson, 2001). Lactic acid as well as acetaldehyde and diacetyl concentrations, and their relative proportions, seem to be essential for the final aroma of the product (Beshkova, Simova, Frengova, & Simov, 1998; Zourari, Accolas, & Desmazeaud, 1992). Acetaldehyde is known to be a key compound for aroma. Both carbonyl compounds and free fatty acids in yogurt are influenced by the type of starter culture, type and quality of raw milk, incubation, cooling and stor- age (Ott, Germond, & Chaintreau, 2000). Even though Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus are lactic acid bacteria used for yoghurt production, variations in the strains affect the synthesis of carbonyl compounds (Tamime & Robinson, 2001; Zourari et al., 1992). Commercial CH-1 and YF- 3331 starters are used for set type yogurt production but these starters contain strains at the different ratios (Chr. Hansen’s Tech- nical Bulletin, 2004). Different studies on fermented goats’ milk (GM) and ewes’ milk (EM) products can be found in the literature, especially from areas where GM and EM have been originally used for the preparation of fermented milk, such as the Middle East, Bal- kans, Greece, Egypt, and Turkey (Kaminerides, Stamou, & Massou- ras, 2007; Karademir, Atamer, Tamucay, & Yaman, 2002; Stelios & Emmanuel, 2004). However, these studies provide information on some chemical properties. To the best of our knowledge, there has been a paucity of reports available for the quality aspects of yo- gurts made from goats’ and ewes’ milk, and their mixture (50% goats’ milk + 50% ewes’ milk) using different yogurt cultures. Therefore, the aim of this study was to determine the effects of the types of milk (goat milk, ewe milk and their mixture milk) and commercial cultures (CH-1 and YF-3331) on volatile com- pounds and free fatty acids in set type yogurts during storage. 2. Materials and methods 2.1. Preparation of milk and yogurt cultures Milk was collected from bulk tank of Damascus (Shami) goat and Awassi ewe, at the Research and Training Farm, Agriculture Faculty of 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.01.090 ⇑ Corresponding author. Address: Faculty of Agriculture, Department of Food Engineering, Mustafa Kemal University, Tayfur Sökmen Campus, 31034-Antakya- Hatay, Turkey. Tel.: +90 3262455844/1060; fax: +90 3262455832. E-mail address: zguler@mku.edu.tr (Z. Güler). Food Chemistry 127 (2011) 1065–1071 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem