Protein in an aqueous whey solution is degraded by adding a protease to the aqueous whey solution. After that, a whey degradation medium is prepared by adding yeast extract or the like to the aqueous whey solution. The whey degradation medium is inoculated with bacteriocin-producing lactic acid bacterium and the lactic acid bacterium is cultured while the whey degradation medium is maintained at pH of not lower than 5 and not higher than 6. After the completion of the culture, the whey degradation medium (culture solution) is centrifuged to thereby separate a concentrated cell suspension containing the lactic acid bacterium in a concentrated form. The concentrated cell suspension has extremely high antibacterial activity (several tens of thousands AU) and is usable as a food preservative. Further, yogurt is produced by fermenting a yogurt mix to which 0.01% to 0.1% by weight of the concentrated cell suspension is added. With this process, it is possible to suppress an increase in the acidity of the yogurt.
