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

Abstract

Exopolysaccharides (EPS) synthesized by lactic acid bacteria during fermentation significantly affect the rheology of set‐style acid milk gels and that of acid gel suspensions, produced from the gels by stirring. In this study, shear and uniaxial extensional flow of acid gel suspensions made with seven single strains of Streptococcus thermophilus or Lactococcus lactis was investigated. Six strains produced ropy EPS, and maximum filament length determined by using a continuous stretching method was up to four times higher than filament length of an EPS‐negative control strain. The systems containing ropy EPS showed a different response to shear and extensional deformation. In shear rheology, higher apparent viscosities and an enhanced structural breakdown was observed for acid gel suspensions with more pronounced ropiness. Breakup time and extensional viscosity, determined by using a Capillary Breakup Extensional Rheometer (CaBER™), significantly increased with increasing ropiness. The increase of extensional viscosity with increasing ropiness was, however, much higher than the effects of ropiness on shear viscosity. As relaxation times also depended on ropiness, it is concluded that ropiness is caused by EPS‐EPS interactions that can be better discriminated in extensional rheology. Practical applications To improve the texture of fermented milk, lactic acid bacteria that are able to produce ropy exopolysaccharides (EPS) are increasingly used in the dairy industry. The EPS exhibit a significant influence on processing properties and sensory characteristics of the resulting products, which can be estimated by means of shear and extensional rheology. The current work provides information on these respective properties of acid gel suspensions, which facilitate product design by supporting the selection of appropriate starter cultures.

Exploring Technological, Safety and Probiotic Properties of Enterococcus Strains: Impact on Rheological Parameters in Fermented Milk

“…In similar research, Surber et al (2020) observed that milk fermented with the EPSproducing strain NCC1971 exhibited higher shear stress compared to milk fermented with the non-EPS-producing strain NCC2201 [41]. The shear stress of milk fermented with NCC1971 increased to a plateau around 70 Pa after reaching a maximum shear rate of 186 s −1 , attributed to the resistance of EPS-bacterial cell bonds breaking under maximum treatment.…”

Highlighting the need for high-speed imaging in capillary breakup extensional rheometry

“…With two simple example experiments, we have highlighted the need for high-speed imaging in capillary breakup extensional rheometry rather than using a laser micrometer with a finite thickness as many previous studies have done [6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…In experiments, the filament-thinning evolution is typically tracked using a laser micrometer [6][7][8][9][10][11][12][13][14][15][16][17][18][19] (such as that originally equipped on the commercial HAAKE™ CaBER™ 1 device) or high-speed imaging techniques. Typically, the laser micrometer has a finite thickness h micro = O(1 mm) and, in the case of the commercial HAAKE™ CaBER™ 1 device, its axial location is fixed, and it is the position of the end-plates that is adjusted to ensure that the laser micrometer is located at the axial location of filament breakup.…”

Caracterization and optimization of nonfat yogurt based on texture properties: instrumental texture profile and rheological properties

“…The growing demand for fat-free foods due to new eating habits on the consumers creates problems related to the consistency of yoghurts, presenting an unstable and more fragile structure, which can lead to sensory rejection (Steffe, 1996;Surber, Jaros, & Rohm, 2019). However, optimization studies applied to yoghurts are rarely found (Granato et al, 2010;Popović et al, 2013;Pakseresht et al, 2017) and there are no studies related to optimization of the texture profile and rheological characteristics of yogurt supplemented with transglutaminase.…”