Individual and sequential effects of stirring, smoothing, and cooling on the rheological properties of nonfat yogurts stirred with a technical scale unit

Abstract

Few studies have considered the impact of unit operations during stirred yogurt manufacture because their operational sequence is difficult to replicate at the laboratory scale. The aim of this study was to investigate the individual and sequential effects of stirring in the yogurt vat, smoothing, and cooling on the rheological properties of yogurts, using a technical scale unit simulating some industrial conditions. The yogurts were prepared from a milk mixture that was standardized to contain 14% total solids, 0% fat, and 4% protein, and then homogenized, heated (94.5°C, 5 min), and inoculated at 41°C with the same thermophilic lactic starter. The operating parameters under investigation were 2 stirring durations in the yogurt vat (5 or 10 min), 2 cooling systems (plate or tubular heat exchanger), and 2 smoothing temperatures (38°C for smoothing before cooling; 20°C for smoothing after cooling). Sampling valves were installed at critical points on the technical scale unit so that the effect of each operation on the properties of stirred yogurt could be quantified individually. Syneresis, apparent viscosity, firmness, and consistency were analyzed after 1 d of storage at 4°C. In general, as the yogurts moved through the technical scale unit, the properties of the yogurts (evaluated after 1 d) changed: viscosity increased but syneresis, firmness, and consistency decreased. The individual effects of the operations showed that smoothing and cooling, compared with stirring duration, made the greatest contribution in terms of modifying yogurt properties. The stirring parameters (5 or 10 min) had similar effects on the yogurts. The use of a plate heat exchanger promoted a decrease in syneresis, whereas a tubular heat exchanger had a greater effect in terms of increasing firmness and consistency. The type of cooling system had no effect on stirred yogurt viscosity.Smoothing at 38°C had a greater effect on the increase in firmness, whereas smoothing at 20°C contributed more to a decrease in syneresis and increases in viscosity and consistency. This study confirms that each unit operation has a defined effect on the rheological properties of a nonfat stirred yogurt, which also depends on the operation sequence.

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

“…When the yogurt was stirred at 42°C, induced syneresis was highest (Figure 3); significantly lower values were found for YR and Y20. In a study by Guénard-Lampron et al (2019), no differences in induced syneresis were observed, whether the yogurt was smoothed at 38°C or 20°C. In that study, the contribution of each unit operation (stirring, pumping, smoothing, and cooling) to the total change in induced syneresis (as a percentage) was determined.…”

“…The steps in the yogurt stirring operation are also known to affect gel functionality. Afonso and Maia (1999) and Guénard-Lampron et al (2019) found that the smoothing and cooling step performed with a plate heat exchanger was responsible for major changes in viscosity, elasticity, and firmness throughout the process. Moreover, when the gel was smoothed at 38°C versus 20°C, the yogurts were firmer (Guénard-Lampron et al, 2019).…”

“…Afonso and Maia (1999) and Guénard-Lampron et al (2019) found that the smoothing and cooling step performed with a plate heat exchanger was responsible for major changes in viscosity, elasticity, and firmness throughout the process. Moreover, when the gel was smoothed at 38°C versus 20°C, the yogurts were firmer (Guénard-Lampron et al, 2019). Viscosity and firmness are related to the stirred yogurt microstructure, which depends on interactions between molecules or particles ( van Marle, 1998;Zoon, 2003;Mokoonlall et al, 2016).…”

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

“…El uso de estabilizantes puede ayudar a proporcionar una consistencia más uniforme. Sin embargo, puede haber defectos de textura relacionados con el uso de estabilizantes, incluida la estabilización excesiva (sobre estabilización) que da como resultado un yogur elástico "gelatinoso", lo que puede generar una separación débil del cuerpo o del suero "líquida" (Vedamuthu, 1991), lo que ocasionan un efecto negativo en las propiedades físicas y sensoriales del producto (Guénard-Lampron, St-Gelais, Villeneuve, & Turgéon, 2019).…”

Shear and extensional rheology of acid milk gel suspensions with varying ropiness

“…Product texture is of outstanding importance for consumer acceptance and affected by, for example, fat content reduction as a response to consumer demands for low‐fat products (Lucey & Singh, ). To improve texture different strategies can be applied, including changes in processing (Guénard‐Lampron, St‐Gelais, Villeneuve, & Turgeon, ), or the incorporation of additional protein or commercial thickeners (Loveday, Sarkar, & Singh, ). As an alternative, mesophilic or thermophilic lactic acid bacteria with the ability to produce extracellular polysaccharides (EPS) are increasingly used in the dairy industry.…”