A high proportion of the probiotic microbial population is microaerophilic (requires very little oxygen for their growth) or even anaerobic (grows in absence of oxygen). Oxygen at higher concentration can be detrimental for the growth and proliferation of microorganisms. Excess of oxygen in the system will result in the formation of reactive oxygen species or high energy containing free radicals. These reactive oxygen agents can damage the membrane system and other organelles, creating ionic imbalance thereby killing the cells. In order to protect them from the damaging effects of oxygen, many bacteria possess oxygen scavenging enzymes that can neutralize the toxic effects of oxygen. However, most of the probiotic bacteria due to their origin in low oxygen content environment lack the enzymes or the basic mechanism to retaliate to such conditions. As such, the viable count of the probiotics seems to be highly compromised under packaged condition or in food products like yoghurt when exposed to oxygen. Since, the viability of the probiotics is an important issue in order for it to act effectively, methods should be devised to make them fight the menace of oxygen toxicity. Work on both natural and artificial ways of inhibiting it is in progress.
Natural Oxygen Scavengers:
The probiotic bacteria particularly the Bifidobacterium strains can be protected by the use of other non probiotic, high oxygen consuming friendly bacteria. Viljoen and his group have shown that such strains of S. thermophilus which are highly aerobic (require higher concentrations of oxygen for their metabolic needs) in nature can be used successfully in yoghurt to mask the bifidobacterial strains from oxygen thereby acting as a potent savior of the probiotics. Other groups have proven the effectiveness of Streptococcus thermophilus in utilizing oxygen dissolved in the yoghurt and related food products. However, the major loophole in using such friendly microflora as potential protectors is that the strains make the medium highly acidic through rapid accumulation of acids.
The strains of both L. acidophilus and Bifidobacterium although can withstand some levels of the acid but such an elevated presence of it can negatively impact the growth and overall viability of the probiotics. Another important aspect is that the method can only be useful during the initial stages of the yoghurt production and does not ensure protection during the long storage of the food products. Also, the food products like yoghurt are stored at relatively low temperatures (4-8) °C which can send the protective strains into a state of dormancy.
In addition, oxygen may infiltrate the dairy foods through the material of the packaging system and there has been little answer to it by the use of this technique. Alternatively, search should be on for the identification of probiotic strains that show high resistance to oxygen. There are many pathogenic strains of microbial species with similar properties. With the use of modern day genetic manipulation technology, it is possible to knock-in the oxygen scavenging enzyme encoding genes like the oxidase, peroxidase and their different variants into the beneficial probiotic bacteria.
Use of Amino Acids and Vitamins:
Many amino acids and vitamins are potent anti-oxidant agents besides their other well known features and roles within the human system. The use of vitamin C (ascorbic acid) and L-Cysteine as scavengers of oxygen was previously known. Some researchers have also tried to show the efficacy of using ascorbic acid as an anti-aging drug. Accordingly Collins et al. have tried to figure out the beneficial aspects of Vitamin C and Cysteine for their ability to maintain the conditions congenial for the prolonged survival of the probiotic strains in yoghurt. Whether the two compounds can act as scavengers of oxygen with the maintenance of a reduced redox potential under the fermentation conditions as prevalent during probiotic synthesis was yet another domain to contemplate.
From their study, it was established that the presence of both ascorbate and Cysteine can reduce the concentration of available oxygen with a concomitant reduction of redox potential within the growth medium. As a result there was significant improvement in the viability of both Bifidobacterium and L. acidophilus strains. But both the anti-oxidant compounds can cause a drastic reduction in the level of oxygen required for the growth and activity of the yoghurt starter culture S. thermophilus. Also, L-Cysteine was found to inhibit the growth potential of L. delbrueckii strains used for yoghurt manufacture.
This process can therefore interfere with the quality of the yoghurt manufactured with a compromise in its nutrient content and texture and can have undesirous impact on the yoghurt industry. Other available compounds with anti-oxygen activity can be evaluated for their role in the protection of probiotics as well as their non interference with the growth of the starter cultures of dairy products. At the same time, microaerophilic bacteria that can be used in the manufacture of yoghurt like food products needs to be screened.
Role of Microencapsulation and Packaging Material:
Microencapsulation techniques have been efficiently employed to protect the different probiotic strains. It ensures the physical segregation of the beneficial bacteria through certain artificial matrices that prevents the direct exposure of the cells to oxygen with an overall improvement in their rate of survival and viability count. Talwalkar et al. has demonstrated increased survival of the probiotic strains of Bifidobacterium and L. acidophilus when they were grown in culture medium as also in manufactured yoghurt post encapsulation in calcium alginate.
The nature of the material used for packing yoghurt and related dairy products acts as a rich source of oxygen and has been one of the crucial factors for such high oxygen content in yoghurt. The materials like polystyrene or polythene are highly permeable to oxygen thereby allowing the diffusion of the gas during the storage period of the food products. On the other hand, usage of oxygen exclusion principles for the production of dairy products can be quite costly. People have used glass bottles as compared to plastic cups for the storage purpose with better success rates. Scientists like Miller have developed the technique of using a cheap, non-permeable layer of gas over the polystyrene packaging with good effect to inhibit oxygen inflow into the food products.
The inhibition to oxygen toxicity is a critical factor for the viability of the probiotics and current research is well directed to address the issue.