CONICET | Buscador de Institutos y Recursos Humanos

Lactose fermentation is the key process in the yogurt manufacture. During this process small amounts of volatile components with high impact on the flavour are formed by the symbiotic action of Str. thermophilus and Lb. bulgaricus, being the carbonyl compounds and volatile acids the main metabolic products. Despite the importance of lactose in aroma development, its presence in dairy products represents a serious problem to persons with lactose intolerance. Production of yogurt with reduced lactose content constitutes a very interesting alternative. However, the lactose reduction could affect the biochemical events related to flavour generation. Thus, the objective of the present study was to compare the volatile fraction of the traditional and delactozed yogurts during fermentation and refrigerated storage. Stirred type yogurts were made by duplicate at laboratory scale. Delactozed yogurts were prepared by adding b-galactosidase enzyme together with the starter culture, which were compared with control yogurts without enzyme addition. Samples were obtained during fermentation (0, 45, 150 and 240 min) and storage at 4ºC (1, 7, 14 and 21 days), and analyzed for pH and lactose content by potentiometric and HPLC-IR methods, respectively. Volatile compounds were isolated by solid-phase microextraction (SPME) and analysed by GC-FID/MS. The postacidification was slightly lower in delactozed than control yogurts. After 45 min of incubation there was a drastic decrease of lactose content in delactozed yogurt compared to controls; in both yogurts the lactose slightly decreased towards the end of storage. Thirty volatile compounds were approximately identified in the samples. Among them, those key compounds to yogurt flavour: acetaldehyde, propanone, butanone, diacetyl, ethanol, 2,3-pentanodione, acetoin, benzaldehyde, acetic, butanoic and hexanoic acids, were considered in the discussion. Only acetoin presented similar content in delactozed and control yogurts during fermentation. The levels of the majority of volatile compounds showed differences between both types of yogurts at 45 and 150 min of incubation. At the end of the process (240 min), the quantities of 2,3-pentanodione, benzaldehyde, butanoic and hexanoic acids were significantly higher in controls than in delactozed yogurts, whereas the remaining compounds had similar values. During storage, minor changes were mainly detected at 14 days. The levels of acetaldehyde, 2-propanone, and 2,3-pentanodione were higher in delactozed yogurts than in controls, whereas acetic acid and benzaldehyde had higher levels in the controls. At the end of storage (21days) only the acetic acid level was significantly different between yogurts, being upper in the controls. Propanone, diacetyl, 2,3-pentanodione, benzaldehyde and volatile acids showed a trend to increase during storage in both types of yogurts. Overall, the results seems indicate that the lactose reduction did not affect markedly the volatile profile at the end of fermentation and throughout the storage time.