Effect of ‘ropy’ strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt

Abstract

The TA-TX2 Texture Analyser and the Brookfield RVT Viscometer have been used to investigate the contribution of ropiness to the texture of stirred yogurts made using ropy strains of bacteria. Back extrusion and texture profile analysis, not commonly used to quantify rheological properties of semi-solid foods, have been found useful in distinguishing the contribution of exopolysaccharides to different texture attributes (Toba et al., 1990). Thus ropiness, a characteristic which is imparted to the product as a result of fermentation with particular polysaccharide-producing strains, contributes to 'adhesiveness', while 'firmness' and 'elasticity' are likely to be influenced more by the protein matrix of the yogurt than by secretion of the polysaccharide by the ropy strains. Effects on viscosity and ability to recover viscosity after disruption were apparent, although the contribution of ropiness was not always positive. Ropy strains increased viscosity of stirred yogurts when compared to yogurt made with non-ropy cultures. But, whilst a ropy Lactobacillus delbrueckii ssp. bulgaricus (Lb r ϩ ) combined with a non-ropy Streptococcus thermophilus (St r Ϫ ) produced a viscous product which recovered its viscosity well, a yogurt made by combining both ropy strains did not recover its viscosity as well as yogurt made by combining two non-ropy cultures and lost its structure more rapidly during the destructive testing. These results show therefore that inclusion of a ropy strain will not always lead to improved texture attributes, that while ropy strains may increase viscosity they may not influence 'firmness' and lend support to the view that this latter attribute is more influenced by protein-protein interactions.

Exopolysaccharide production by Streptococcus thermophilus SY: production and preliminary characterization of the polymer

“…Lactic acid bacteria usually produce only small amounts of EPS (100-200 mg l -1 ; Cerning 1995) but yields up to 4 g l -1 have been obtained with Lactobacillus sakei O-1 (van den Berg et al 1995). Small amounts of EPS produced in fermented milk (Rawson and Marshall 1997) and during cheese production (Low et al 1998;Perry et al 1998) have a strong impact on the texture and properties of these products. Although the fermentative yield of EPS produced by LAB is still very low compared with that of commercial EPS, such as xanthan, EPS produced by LAB may provide an interesting alternative to EPS produced from from nonfood-grade organisms (xanthan, gellan, etc.)…”

Culture Conditions Determine the Balance between Two Different Exopolysaccharides Produced by Lactobacillus pentosus LPS26

“…Among microbial EPS, those produced by lactic acid bacteria (LAB) are receiving increasing attention because these microorganisms have a 'food-grade' status. The rheological properties of EPS produced by LAB have found a major application in the manufacture of fermented dairy products such as yogurt, cheese, or fermented milks, although they also play a role in the elaboration of fermented meats and vegetables (2,33,42,45). In addition, they have also been proven to have beneficial effects on human health such as cholesterol-lowering, antitumoral, and inmunomodulating activities and prebiotic effects (8,29,37).…”

Redox Potential: Monitoring and Role in Development of Aroma Compounds, Rheological Properties and Survival of Oxygen Sensitive Strains During the Manufacture of Fermented Dairy Products

“…Indeed, the gel of yoghurts produced under N2 -H2 conditions is weaker despite greater EPS production [23]. It is a common assumption that EPS produced by bacteria contribute to the rheological properties of yoghurt [31][32][33] but, as reported by Hassan et al [34], van Marle [35] and Martin et al [23], no correlation between the viscosity of yoghurt and EPS concentrations was found.…”