The role of lactic acid bacteria in yogurt production
Methods to improve the quality of yogurt include increasing the dry matter of milk (by adding fat, protein, sugars such as sucrose and fructose) or adding stabilizers (pectin, starch, alginate, and gelatin). However, these methods do not meet consumer demand for products with minimal additives as much as possible. Exopolysaccharide (EPS) produced by lactic acid bacteria, which are GRAS (generally recognized as safe), is a significant source of natural alternatives. Lately, exopolysaccharides produced by lactic acid bacteria have received considerable attention in the fermented dairy industry due to their potential use as viscosifiers, texturizers, and emulsifying agents.
According to the results, sensory properties such as mouthful, shininess, clean cut, ropiness, and creaminess in yogurt can be improved with exopolysaccharide produced by yogurt starter cultures. In addition, the starter of yogurt cultures producing exopolysaccharides may decrease the extent of syneresis. Syneresis is associated with extensive gel network rearrangements and is considered a major defect in yogurt. At present, the countries of Northern Europe, Eastern Europe, and Asia use exopolysaccharides extensively in the production of fermented dairy products. There is a lot of diversity in lactic acid bacteria exopolysaccharides that the type of lactic acid bacteria strains, culture conditions, and medium composition are effective factors in this amount of diversity.
It is significant to select the strains with higher EPS production. Yogurt starter cultures are usually a 1: 1 cocci–rods ratio of a mixture of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. These organisms grow in a protocooperative relationship, and as a result of this symbiotic relationship, acidification occurs rapidly. since Streptococcus thermophilus has poor proteolytic activity, its growth is stimulated by free amino acids and small peptides released by rods from casein. The production of formic acid and carbon dioxide by cocci increases the growth of Lactobacillus delbrueckii ssp. bulgaricus.
Depending on the type and activity of the starter culture, certain characteristics of taste, texture, and aroma are created in the products, which besides lactic acid, other metabolites such as carbon dioxide, acetic acid, diacetyl, acetaldehyde, large molecular weight exopolysaccharides are involved. High molecular weight extracellular polysaccharides (EPS) produced by lactic acid bacteria are now used in the yogurt industry and, by enhancing the viscosity of yogurt independent of the fat content, replace stabilizers and gelling agents to improve the product’s texture.
The physical properties of stirred yoghurt depend to a large extent on the choice of culture starter. Therefore, in order to improve the physical stability of the fermented product, it is very important to choose strains with high exopolysaccharide production. The production of polysaccharides is affected by the incubation temperature selection. Low incubation temperatures increase the production of ropey polysaccharides. Polysaccharides produced by lactic acid bacteria are mainly composed of glucose, galactose, and rhamnose. However, individual polysaccharide substances can vary.
Although many studies have been performed to isolate and characterizing the composition of exopolysaccharides produced by different strains of lactic acid bacteria, the function of exopolysaccharides in fermented milk is still unclear. It seems that an important factor in the rheological properties is the type, charge, and molecular mass of exopolysaccharide. The amount of exopolysaccharide is only correlated to the viscosity properties. Micrographs obtained by conventional electron microscopy (SEM) have shown that the effects of exopolysaccharide on yogurt tissue are due to a possible attachment to the casein matrix, and the connections between polysaccharides and casein filaments are responsible for increasing yogurt viscosity.
Nowadays, research on fermented milk focuses mainly on lactic acid bacteria and focuses on aspects of molecular biology, genetics, and genetically engineered food products related to product quality. In addition, the mechanism of controlled production of exopolysaccharides from lactic acid bacteria as natural thickeners and improving yogurt tissue properties are still under investigation.