Measurement of firmness of stirred yogurt in routine quality control

The objectives of the present work were to evaluate the potential use of the vane method and penetration tests as a tool to measure the variability in firmness of stirred yogurt in routine quality control, and to evaluate the effect of distribution on yogurt firmness. Twelve batches of plain stirred yogurt of a commercial brand were studied, acquired both from a manufacturing plant and three retail stores. Both vane method and penetration tests allowed the detection of changes in the firmness of yogurts, confirming that they could be useful in routine quality control to characterize the firmness of yogurt. The yield stress of samples from the manufacturing plant varied within a range of 250 Pa. This important variation was partially explained by variations in the final acidity during fermentation, showing the importance of a standardized production in order to obtain a uniform texture. The yield stress of yogurts from the manufacturing plant was significantly higher than that of samples from the different retail stores. Averaged values being considered, the yogurts lost 53 Pa of their original yield stress during distribution and handling inside retail stores, probably due to mechanical damage. This mechanical damage also caused the occurrence of syneresis.