Studying stirred yogurt microstructure using optical microscopy: How smoothing temperature and storage time affect microgel size related to syneresis

摘要

A grainy texture and high syneresis are 2 defects inlow-fat stirred yogurt that are often disliked by consumers.In this study, a rheometer controlling the shearrate and temperature was used to simulate the smoothingstep of yogurt manufacture. Identical formulationscontaining whey protein isolate or whey protein concentratewere compared. After the yogurt milk underwentheat treatment, inoculation, and fermentation at 42°C,the yogurt was smoothed at 42°C (Y42) or 20°C (Y20)or during a cooling ramp from 42°C to 20°C (YR).Induced syneresis (serum expelled by centrifugation)was measured on d 3. Sizes of microgels (dense proteinaggregates) were investigated on d 0, 4, and 7 by laserdiffraction and by image analysis using optical microscopy.Optical microscopy was also used to characterizethe reorganized protein network embedding microgels.The type of whey protein ingredient had only a slighteffect on the induced syneresis of YR and Y20 treatedyogurts, and the major effect came from the smoothingtemperature. The Y42 treatment presented the highestinduced syneresis; YR and Y20 had similar low inducedsyneresis values. Images showed a heterogeneousmicrostructure (large microgels, reorganized gel) andserum separation for Y42; the YR and Y20 networkswere homogeneous. Both the image analyses and laserdiffraction showed that the microgel size depended onthe smoothing temperature. However, only the imageanalyses made it possible to identify a time dependencyeffect on microgel size during storage. The number ofmicrogels >104 μm2 continued to increase over time,whereas the number of microgels <103 μm2 decreased.Microscopic observations were less destructive than laserdiffraction and highlighted the presence of microgelaggregation during storage.