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

Abstract

A grainy texture and high syneresis are 2 defects in low-fat stirred yogurt that are often disliked by consumers. In this study, a rheometer controlling the shear rate and temperature was used to simulate the smoothing step of yogurt manufacture. Identical formulations containing whey protein isolate or whey protein concentrate were compared. After the yogurt milk underwent heat 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 protein aggregates) were investigated on d 0, 4, and 7 by laser diffraction and by image analysis using optical microscopy. Optical microscopy was also used to characterize the reorganized protein network embedding microgels. The type of whey protein ingredient had only a slight effect on the induced syneresis of YR and Y20 treated yogurts, and the major effect came from the smoothing temperature. The Y42 treatment presented the highest induced syneresis; YR and Y20 had similar low induced syneresis values. Images showed a heterogeneous microstructure (large microgels, reorganized gel) and serum separation for Y42; the YR and Y20 networks were homogeneous. Both the image analyses and laser diffraction showed that the microgel size depended on the smoothing temperature. However, only the image analyses made it possible to identify a time dependency effect on microgel size during storage. The number of microgels >10 4 µm 2 continued to increase over time, whereas the number of microgels <10 3 µm 2 decreased. Microscopic observations were less destructive than la-ser diffraction and highlighted the presence of microgel aggregation during storage.

Reología del yogur: efectos de las operaciones unitarias en el procesamiento y uso de aditivos

“…La viscosidad podría aumentar durante el enfriamiento a menor temperatura antes del suavizado y esto puede conducir a una mayor ruptura de la red de proteínas. Gilbert et al (2020) obtuvo los mismos resultados al suavizar el yogur con un reómetro 42 °C dando un valor de sinéresis más alto que el yogur suavizado a 20 °C. Diversos autores coinciden en que el batido a 20 °C es ideal para obtener yogur de alta calidad, que se supone que incluye valores de sinéresis más bajos (Robinson, Lucey, & Tamime, 2007), debido a que si se suaviza el yogur a una temperatura cercana a la temperatura de incubación dañaría la estructura de la red de proteínas haciéndola más frágil (Lucey, 2004).…”

The influence of incubation temperature and starter culture type on physical properties of ayran

“…The Rvis was calculated using a protocol described by Küçükçetin et al (2012). The microscopic image of the ayran samples was taken at 10x magnifications by a fluorescent microscope (MCX300, MICROS Produktions und Handels GmbH, Veit/Glan, Austria) (Figure 2 a-h) (Gilbert et al, 2020).…”

Smoothing temperature and ratio of casein to whey protein: Two tools to improve nonfat stirred yogurt properties

“…During the post-fermentation process, stirring and smoothing steps take place in a continuous circuit, where the gel undergoes different levels of structure modification through pipes, pump, cooling devices, and finally smoothing nozzle (Chandan and O'Rell, 2013;Mokoonlall et al, 2016). These processing treatments induce mechanical shearing that creates a new microstructure for stirred yogurt (Mokoonlall et al, 2016;Gilbert et al, 2020b). Microscopic observations (scale between 10 µm and 250 µm, depending on the study) have revealed highly disrupted and reorganized protein network with intact microfragments (microgels) from the initial set gel (van Marle, 1998;Laiho et al, 2017;Gilbert et al, 2020b;Gregersen et al, 2021).…”

“…These processing treatments induce mechanical shearing that creates a new microstructure for stirred yogurt (Mokoonlall et al, 2016;Gilbert et al, 2020b). Microscopic observations (scale between 10 µm and 250 µm, depending on the study) have revealed highly disrupted and reorganized protein network with intact microfragments (microgels) from the initial set gel (van Marle, 1998;Laiho et al, 2017;Gilbert et al, 2020b;Gregersen et al, 2021). Microgels can be defined as dense protein aggregates; their sizes and rigidity have been identified as important parameters for understanding the perceived granularity of stirred yogurt (Cayot et al, 2008;Laiho et al, 2017).…”

“…Microgels can be defined as dense protein aggregates; their sizes and rigidity have been identified as important parameters for understanding the perceived granularity of stirred yogurt (Cayot et al, 2008;Laiho et al, 2017). A correlation between microgel size and syneresis has often been mentioned for stirred yogurts, with some conflicting conclusions (van Marle, 1998;Lee and Lucey, 2006;Zhang et al, 2016;Gilbert et al, 2020b).…”