Effect of Initial Acidity of Plain Yogurt on Acidity Changes during Refrigerated Storage

The relationship between initial acidity of plain yogurt and the changes in this acidity during refrigerated storage has been investigated. Yogurt samples with initial pH 4.5 ± 0.1, pH 4.2 ± 0.1 and pH 3.9 ± 0.1 were prepared under laboratory conditions and stored at 4°C and 7°C for 3 wk. pH and titratable acidity were monitored weekly. Results indicated that samples with the lowest initial acidity (high pH) showed the highest increase in acidity during refrigerated storage (pH 4.59–4.15). Samples with high initial acidity (low pH) remained fairly stable (pH 3.82–3.77). Acidity changes during refrigerated storage were more pronounced at 7°C compared to 4°C (pH 4.59–4.27 at 4°C; pH 4.59–4.15 at 7°C). Changes in acidity were maximal for the first week and minimal thereafter.