Bifidus Yoghurt - Yoghurt with bifidobacteria

Bifidobacteria are more commonly incorporated into fermented dairy foods including yoghurt because of their potential health benefits. The problem with Bifidobacteria is that they grow slowly in milk and hence they are frequently incorporated with the yoghurt cultures, viz.  Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, along with bifidobacteria for making yoghurt.

The suggested level for incorporating bifidbacteria is at one million viable cells per gram of a product in order to realize health benefits. More importantly, the strains used for therapeutic benefits should originate from human sources, be capable of surviving in the acidic fermented products, and be available in sufficient number in the product prior to consumption. Proper bifidobacaterial strain selection with regard to acid and bile resistance coupled with microencapsulation, incorporation of micronutrients such as cysteine, peptides and amino acids along with improvement in technology of production will improve their survival.

A number of health benefits have been claimed viz. antimicrobial, antimutagenic, anticarcinogenic properties, reduction in serum cholesterol, improvement in lactose tolerance in lactose-intolerant individuals and adherence to intestinal cells.

Bifidus Yoghurt Products

A number of fermented foods employing bifidobacteria are well established and widely available in the market. Of this, yoghurt serves as the best medium for incorporation of bifidobacteria. Bifidobacteria incorporated yoghurt is known by different names in various parts of the world. Some of the commonly available and most popular products containing bifidobacteria are Bio-garde, Bioghurt, Bifidus milk, Bifighurt, Milmil and Progurt.

The trend is changing in yoghurt production from incorporating Lactobacillus acidophilus, to Lactobacillus acidophilus and bifidobacteria. Yoghurts containing Lactobacillus acidophilus and bifidobacteria are referred as `AB' yoghurt. Now, the trend has been to incorporate Lactobacillus casei in addition to Lactobacillus acidophilus and bifidobacteria and such products are known as `ABC' yoghurt. The age old practice is to manufacture yoghurt using Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus as starter cultures. These yoghurt organisms are known to impart many health promoting properties. But the disadvantages of these two bacteria are they are not natural inhabitants of the intestine and cannot survive under the high acidic conditions and bile concentrations usually encountered in the gastrointestinal tract. Therefore, for yoghurt to be considered as a probiotic product, Lactobacillus acidophilus and/or bifidobacteria have to be incorporated as dietary adjuncts. Recent advances in technology have enabled manufacture of fermented milk – yoghurt, with only Lactobacillus acidophilus and bifidobacteria. When bifidobacteria alone is used in the production of fermented milk, it results in longer incubation period and compromising with quality of the end product.

Thus, the normal practice is to make yoghurt with a combination of

    both yoghurt bacteria and Lactobacillus acidophilus or
            Lactobacillus acidophilus and bifidobacteria or
            Lactobacillus acidophilus, bifidobacteria and Lactobacillus casei as adjunct starter culture.

In order to impart desired therapeutic effects, the bifidobacteria must be available and be viable in sufficient numbers. The organisms should be present in a food to minimum levels of one million colony forming units per gram or if possible, daily intake should be about 100 million cells per gram. Such high numbers are essential to compensate for the possible reduction in the numbers of the probiotic organisms during their passage through the gastro-intestinal tract. Most of the developed nations, as on this date, including the United States and Australia do not specify any requirements regarding the numbers of bifidobacteria in yoghurt. But countries like Japan, through the Fermented Milks and Lactic Acid Bacteria Beverages Association has established a standard, which requires a minimum of ten million viable bifidobacteria cells per ml to be present in fermented dairy foods.

Several research findings show that the viability of bifidobacteria is often low in yoghurt and other fermented food products. Most of the popular brands contained very low numbers of bifidobacteria and possible reasons for decrease in viable counts could be attributed to inhibitory substances such as lactic acid produced during fermentation and cold storage, strains used, interactions between species present, final acidity, availability of nutrients, growth promoters and inhibitors, dissolved oxygen and oxygen permeation through the package.

Incorporation of Peptides as Micronutrients

During the yoghurt making process, Streptococcus thermophilus dominates the early stage of yoghurt fermentation. As oxidation-reduction (redox) potential of the milk medium is reduced and the pH lowered from 6.6 to 5.5, growth of Lactobacillus delbrueckii subsp. bulgaricus is stimulated during yoghurt fermentation and the organism produces acetaldehyde and lactic acid, which are responsible for the characteristic yoghurt green apple flavour. Continued acid production lowers yoghurt pH to near 4.6 (which is the iso-electric point of casein), which induces clotting. Fermentation is terminated at pH 4.5.

Bifidobacteria grow slowly in milk because of a lack of proteolytic activity, and the usual practice is to add yoghurt bacteria to reduce the fermentation time. Lactobacillus delbrueckii subsp. bulgaricus produces essential amino acids owing to its proteolytic nature, and the symbiotic relationship of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus is well established; the former organism produces amino nitrogen for the latter. However, Lactobacillus delbrueckii subsp. bulgaricus also produces lactic acid during refrigerated storage. This process is known in the industry as `post acidification'. Acid produced during refrigerated storage (i.e. post acidification) is found to cause loss of viability of bifidobacteria.

To overcome the problem of post acidification, the latest trend is to use starter cultures that are devoid of Lactobacillus delbrueckii subsp. bulgaricus, such as ABT (Lactobacillus acidophilus, bifidobacteria and Streptococcus thermophilus). Such starter cultures may necessitate the incorporation of peptides and amino acids as micro nutrients typically in the form of casein hydrolysate for reducing the fermentation time and improving the  viability of bifidobacteria. Streptococcus thermophilus, which is less proteolytic than Lactobacillus delbrueckii subsp. bulgaricus, is the main organism responsible for fermentation in ABT cultures.

ABT starter cultures increase fermentation time significantly (up to 8 h vs. 4 h with both yoghurt bacteria) as there is no symbiosis without Lactobacillus delbrueckii subsp. bulgaricus and the fermentation is carried out primarily by Streptococcus thermophilus. Longer incubation times are undesirable, given the rigid schedule in modern yoghurt manufacture. Addition of whey protein concentrate and acid casein hydrolysate improves the viability of bifidobacteria particularly in yoghurt made from ABT starter cultures. A nitrogen source in the form of peptides and amino acids from whey protein concentrate and acid casein hydrolysate is responsible for improved viability of bifidobacteria. Thus, it is important to provide adequate peptides and amino acids for improved viability of these bacteria.