Abstract
Traditional hard Xinotyri cheese was manufactured using raw or pasteurized goat milk, without starter cultures, and the changes in microbiological and biochemical characteristics were studied during ripening and storage. Mesophilic lactic acid bacteria (LAB) predominated (>8.5 log CFU/g) in freshly fermented Xinotyri cheeses (pH 4.5–4.6), regardless of milk pasteurization. Enterobacteria, pseudomonads and staphylococci were suppressed below 6 and 4–5 log CFU/g in fresh cheeses from raw and pasteurized milk, respectively. Salmonella and Listeria spp. were absent in 25 g cheese samples. Coagulase-positive staphylococci exceeded the 5-log safety threshold in fresh raw milk cheeses, which also had 10-fold higher levels of enterococci than pasteurized milk cheeses. Non-LAB groups declined <100 CFU/g, whereas yeasts increased to 5–6 log CFU/g in both cheeses during ripening. Milk pasteurization affected the protein, fat, ash, moisture, nitrogen fractions, total free fatty acids and total free amino acids content of cheeses. Primary proteolysis, detectable by urea-PAGE, was more intense in raw milk cheeses than in pasteurized milk cheeses. However, the hydrophilic and hydrophobic peptides and their ratio in the water-soluble fraction were similar in both cheeses. Cheeses discriminated clearly according to the milk kind (raw, pasteurized) and the stage of ripening, based on the examined biochemical characteristics.
Highlights
During ripening, a cheese undergoes microbiological and biochemical changes, resulting in the development of its characteristic texture and flavor
The levels of enterococci, which constitute an important part of the native microbiota in traditional raw or thermized milk cheeses made in Greece or other Mediterranean countries [16,17,18], were, on average, 6.4 log CFU/g in fresh raw milk cheeses, and 0.7-log units lower in the pasteurized milk cheeses (Table 1)
The microbiota of all Xinotyri cheese curds before salting was dominated by mesophilic lactic acid bacteria (LAB) types, which had grown abundantly after 24 h of milk fermentation at 20–25 ◦ C, followed by 2–3 h of draining at 16 ◦ C. This prolific LAB growth was normal in the raw milk cheese curds [19], but this speed was surprising in their pasteurized counterparts in the absence of commercial or natural starters [20,21]
Summary
A cheese undergoes microbiological and biochemical changes, resulting in the development of its characteristic texture and flavor. The microbiological changes include the death and lysis of starter cells, the growth of adventitious microbiota (non-starter lactic acid bacteria) and, in many varieties, the development of a secondary microbiota. The biochemical changes that occur during cheese ripening may be grouped into primary events, which are proteolysis, lipolysis and glycolysis, and secondary events, which include the metabolism of fatty acids and amino acids and affect the development of volatile flavor compounds. The biochemical transformations are catalyzed by enzymes from residual coagulant, milk ( plasmin) and proteinases and peptidases from bacteria, starters, non-starters and adjunct cultures [1]. Cheeses made from raw milk generally develop a stronger aroma and more intense taste than the same cheeses made from pasteurized milk. Sometimes, raw milk cheeses can have a non-controlled microbiota, which may cause spoilage or food poisoning
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