Short communication: Effect of stirring operations on changes in physical and rheological properties of nonfat...

The rheological and physical properties of stirred yogurt depend on several parameters, including the mechanical stress caused by stirring, smoothing, and cooling conditions (duration, intensity, or temperature). However, the literature reports little information about the effects of mechanical stress from all of the stirring operations on changes in yogurt properties during storage. The aim of this study was to determine, by means of a technical scale unit, the combined effects of stirring in the yogurt vat, smoothing, and cooling on changes in the rheological properties of nonfat yogurt during storage at 4°C. The yogurt was standardized to 14% total solids, 0% fat, and 4% protein, and was stirred with a technical scale unit using 2 stirring durations (5 min or 10 min), 2 types of cooling systems (plate or tubular heat exchanger), and 2 smoothing temperatures (38°C for yogurts smoothed before cooling or 20°C for yogurts smoothed after cooling). All yogurts were stored for 22 d at 4°C, and we determined the combined effect of the stirring operations on changes in syneresis, apparent viscosity, firmness, consistency, and flow time. During storage, post-acidification was the same for all stirred yogurts and involved restructuring of the protein network, which resulted in an increase in all properties except syneresis, which decreased. The combined stirring operations did not modify changes in syneresis during yogurt storage but did affect flow time, viscosity, consistency, and firmness. Changes in flow time depended on smoothing temperature, and viscosity and consistency depended on the cooling system used. Firmness was the property most affected by all combined stirring operations during storage. Therefore, the technical scale unit was effective for quantifying the combined effects of stirring, smoothing, and cooling on changes in yogurt properties during storage. This study also confirmed that the restructuring of stirred yogurt depended on the mechanical stress that occurred during the stirring process.