Characterization of syneresis phenomena in stirred acid milk gel using low frequency nuclear magnetic resonance on hydrogen and image analyses

Water retention is an important quality attribute for yogurt. Classically, stirred yogurt water retention is investigated using induced syneresis measurement (centrifugation), which does not characterize spontaneous syneresis. Low-frequency nuclear magnetic resonance (H-1-LF-NMR) is a non-destructive technique to detect spontaneous syneresis. Experimental yogurt from pasteurized skim milk, and commercial stirred yogurts were analyzed with H-1-LF-NMR. After Laplace's transformation of the signal, hydrogen atoms pools were differentiated according to their mobility. Each hydrogen pool stood for a type of water mobility in the matrices characterized by a relaxation time (T-2(i)), and a signal intensity (I-2(i)). Yogurt water retention was assessed by induced syneresis and their structure was characterized using microscopy. Low frequency H-1-NMR detected four different water mobility groups in the matrices. Among these, there was a signal from bulk water, and another attributed to the separated serum (spontaneous syneresis). In experimental yogurts, spontaneous syneresis was visible, resulting in induced syneresis higher than 50%. Moreover, induced syneresis and spontaneous syneresis detected by H-1-LF-NMR were similar. In commercial yogurts, bulk water mobility reduced with increasing protein content and protein network density. Induced syneresis and bulk-water mobility correlated only in yogurts without gelatin. In the presence of gelatin, the network was more open, probably favoring bulk water mobility. This study shows that H-1-LF-NMR associated with microscopy image analysis efficiently assesses and describes yogurts water retention and spontaneous syneresis.