Effects of treatment with nisin and natamycin on the quality and shelf-life of a Greek soft acid-curd cheese “Galotyri”

Galotyri is a traditional Greek Protected Destination of Origin (PDO) cheese. It is a soft, white, acid – curd, spread cheese. In the present study, the effect of two natural antimicrobial substances, nisin and natamycin, as well as their combinations, were assessed on the quality, freshness and shelf-life extension of fresh Galotyri cheese, during its refrigeration storage (4 C). Galotyri samples stored under aerobic packaging conditions, without any antimicrobial substances added to the product, were evaluated (control samples). Microbiological analyses (primarily total yeasts counts) and sensory evaluation parameters were used in order to estimate the shelf-life of the cheese. The microbiological tests included the enumeration of total mesophilic bacteria, yeasts and moulds, coliforms, lactic flora (Lactobacilli and Lactococci). Furthermore, physico-chemical quality parameters included determination of pH, fat content, color (L, a, and b) and lactic acid. Sensory parameters being evaluated, included odor, taste, flavor, texture and appearance of the cheese products, both in the control and treated samples. Results of microbiological analyses showed that nisin added, at concentrations of 100 and 200 IU/g, had a small inhibitory effect on the population of Lactobacilli and Lactococci, as well as, on the total mesophilic bacteria. Nisin, as expected, seemed to be not effective against yeasts. In contrast, the addition of natamycin, added at concentrations of 10 and 20 ppm (with or without nisin) inhibited the growth of yeasts during the storage period (28 days) maintaining sensory characteristics (unaffected). Moreover, our results, surprisingly, demonstrated that natamycin concentrations had a small, but significant inhibitory effect against Lactobacilli and Lactococci. Coliforms were not detected.