Faculty of Food Technology and Biotechnology, University of Zagreb
Abstract
The effect of temperature on the growth of yeasts during the production of Sethemi, South African naturally fermented milk (NFM), was studied by incubating raw milk and milk inoculated with selected yeast strains at 7, 15, 25 and 37 °C. The different temperatures were selected to represent the average ambient temperatures around Bloemfontein, South Africa, during winter, spring, summer, and in the human body, respectively. The yeast strains used had previously been isolated from Sethemi and identified as Kluyveromyces marxianus, Saccharomyces cerevisiae, Candida albicans and Debaryomyces hansenii. The yeast strains were inoculated into raw milk separately and also as a mixture of the four strains. The yeast counts, lactic acid bacteria counts and pH were monitored over a period of 20 days. It was observed that although all the yeast strains grew in the milk at all temperatures, the fastest growth was at 37 °C but there was a prolonged lag phase at 7 and 15 °C. The highest yeast counts of 8.30 log (CFU/mL) were obtained at 25 °C in the milk inoculated with K. marxianus. At all temperatures, the initial yeast count in the control was significantly (p<0.05) lower than the counts in the inoculated milk. Lactic acid bacteria also grew to high numbers both with added yeast and in the control. The highest LAB counts of about 11.59 log (CFU/mL) were obtained in the presence of S. cerevisiae after about 4 days of incubation at 25 °C. The addition of different yeast strains did not affect significantly the growth of LAB at all temperatures. After 3 days, the LAB counts decreased rapidly at 37 °C, while from day 2 to day 5 the LAB numbers remained stable at 25 °C. There was a rapid decrease in pH at higher temperatures than at 7 or 15 °C, corresponding to the LAB growth. A temperature of 25 °C was found to be ideal for producing fermented milk with high LAB counts, low pH and a visually acceptable coagulum.Inkubacijom svježeg mlijeka i mlijeka inokuliranog odabranim sojevima kvasca na 7, 15, 25 i 37 °C istražen je utjecaj temperature na rast kvasca pri dobivanju južnoafričkog prirodno fermentiranog mlijeka (naturally fermented milk – NFM) Sethemi. Odabrane su različite temperature fermentacije koje predstavljaju prosječnu zimsku, ljetnu i proljetnu temperaturu zraka u okolici grada Bloemfontein (Južna Afrika), te prosječnu temperaturu ljudskoga tijela. Upotrijebljeni su sojevi kvasca Kluyveromyces marxianus, Saccharomyces cerevisiae, Candida albicans i Debaryomyces hansenii izolirani iz mlijeka Sethemi. Sojevi su inokulirani zasebno i kao mješovita kultura u svježe mlijeko. Tijekom 20 dana praćen je broj kvasaca i mliječno-kiselih bakterija te pH-vrijednost. Iako je primijećen rast sojeva kvasca pri svim vrijednostima temperature korištenim u istraživanju, najbrže su rasli pri temperaturi od 37 °C, a na 7 i 15 °C došlo je do produljenja lag faze. Najveći broj kvasaca, i to od 8,30 log (CFU/mL), postignut je u mlijeku inokuliranom s K. marxianus pri temperaturi od 25 °C. Početni broj kvasaca u kontrolnom uzorku bio je značajno (p<0,05) niži nego u inokuliranom mlijeku pri svim temperaturnim vrijednostima. Porastao je i broj mliječno-kiselih bakterija (LAB) u kontrolnom i inokuliranom uzorku. Najveći broj LAB (11,59 log (CFU/mL)) postignut je u prisutnosti S. cerevisiae nakon 4 dana inkubacije na 25 °C. Dodatak različitih sojeva kvasca nije značajno utjecao na rast LAB bez obzira na temperaturnu vrijednost. Nakon 3 dana na 37 °C drastično je smanjen broj LAB, dok je nakon 2-5 dana pri 25 °C ostao nepromijenjen. Pri temperaturama višim od 7 i 15 °C uvelike se snizio pH zbog povećanja broja LAB. Temperatura od 25 °C ocijenjena je idealnom za proizvodnju fermentiranog mlijeka zbog velikog broja LAB, niske pH-vrijednosti i prihvatljivog izgleda koaguluma
