Thermal Food Engineering Operations. NITIN KUMAR

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СКАЧАТЬ style="font-size:15px;">      Therefore, this section brings, in particular, all the possible novel heat processing technologies for decontamination of spoilage and pathogenic microorganism present in the food. The table mentioned below provides a summary of the studies done on the microbial techniques and outcome which were achieved for the different food items and their change in properties after the treatment.

2.3 Inactivation Mechanism of Targeted Microorganism

The effective application of innovative technology for food preservation depends on the development in the field of inactivation mechanisms. A requisite acquaintance with the physiological behavior of microorganisms in the direction of decontamination factors is indispensable for the advancement of secure food products [20]. It is vital to consider and understand the crucial environmental factor for identifying the resistance in the food product. This will support easy construction of the mathematical model and interpret the kinetics which is done based on the parameters with an effective prediction of the decontamination in a wider range [21]. Therefore, designing a process becomes easier by gathering information on different preservation agents. The efficiency of any method utilized depends upon product type, the process used, and targeted microorganisms [22]. Inactivation mechanisms depend on the technique utilized for the above action and are also influenced by the structure and number of the microbial cell. Environmental stress kills or injures cells of the microorganism but in some cases, it might cause sublethal injury or also might cause total lethal cell death [23].

      Destruction of the cell wall is the main factor which leads to the inactivation of microorganism and it also depends upon the cell morphology and shape of the cell. Destruction of the cell wall can be achieved by either chemical process by modifying the wall leading to leakage of the cellular content or by altering the constituents chemically. As already known, heat is the major factor that causes destruction of the membrane as well as protein denaturation but what type of heat, it is tough to state [24].

      Application of significant high pressure with heat can lead to disorientation of the cellular structure of the microorganism and also lead to protein denaturation, whereas this is hardly seen in the bacterial spore because of their morphology. As visually seen nevertheless, that hydrostatic pressure treatment can induce for growth of bacterial spores and a combination of swift decompression pressure and elevated temperature leads to the destruction of the spores which re-germinated [25].

      Tolerance of microbial stress is affected both by intrinsic and extrinsic factors as resistance during stationary growth and during exponential growth varies because of varied stress sigma factors [26, 27]. With the intention of endurance, micro-organisms incline to produce biofilms, the utmost prevailing microbial defensive structure. Bacteria produce carbohydrate matrices and might consequently condense the efficiency of approaches for inactivation [28].

      2.3.1 Action Approach and Inactivation Targets

Damage to cell structure or any type of physiological change can indicate the death of the cell. So, during the process of cell structure disruption, cell envelopes break, change the anatomy of DNA, alters the ribosome, and disintegrates the protein [29]. Whereas during the physiological change fluctuation of permeability in addition to the loss of enzyme functionality leads to death of the cell. So, during the inactivation process the above changes can occur solely or combinedly, and therefore, identifying the particular event becomes problematic for the researchers while coming across events in this area. Therefore, keeping in mind, the following situation, this can be possible to have multiple inactivation actions that combine to cause the death of the cell. It is likewise probable that the important aim solitary affected when a secondary structure is beforehand injured. For example, due to heat, there is nutrient loss, ions loss, the disintegration of DNA, denaturation of crucial proteins and enzymes [30]. Therefore, exactly capturing the real event of death of the cells becomes difficult as the structure of the cell is also affected by higher temperature. The only possible way to find out the exact reason is by examining the relation between inactivation degree and the modification dress of the targeted microorganism concerning various environmental conditions.

2.4 Environmental Stress Adaption

      The cell contains some extracellular protein, which warns or senses the probability of danger when there is any environmental stress for instance heat [31]. These processes comprise fluctuations of events of protein and gene expression with the persistence of averting and/or lessening injury to cells. As already discussed, resistance is higher in the stationary phase of growth rather than the exponential one because of the sigma factor [32]. If a bacterium survives, it can exist in a VBNC state. This denotes a state in which the cells cannot be detected by standard culture on enriched agar media, although remaining viable and capable of resuscitation under favorable conditions. VBNC was mainly observed among Gram-negative bacteria and has been proposed as a strategy for survival in natural environments [33]. More than 60 bacterial species are described as being able to enter into a VBNC state, among them Gram-positive (e.g., L. monocytogenes, Enterococcus, Micrococcus luteus) and Gram-negative (e.g., E. coli, Vibrio cholerae, Vibrio vulnificus, Legionella pneumophila, Campylobacter jejuni, Salmonella enterica, Pseudomonas aeruginosa, Helicobacter pylorii) bacteria can be found [34].

      2.4.1 Sublethal Injury

Sometimes when microorganisms survive any environmental stress, they might again revive and regrow themselves with available appropriate conditions [35]. This shortcoming can cause the wrong estimation of the exact lethality after treatment as it might not detect properly only the cells which are injured. There is a chance therefore to get repair during the treatment and reviving phase. So, this can be avoided by adding some further agents for presentation so the reviving process of the cell can be containing and better inactivation can be achieved [36]. Hence, uncovering and classifying the sublethal injury by innovative preservation techniques is indispensable for optimizing a varied combination of methods for elevated effects of microbes for inactivation.

2.5 Resistance of Stress

      As already discussed, microbes tend to grow resistant for some time when coming in contact with varied environmental stress which causes serious issues when it comes to food safety.

      Researchers have been studying for many years now how to deal with varied adaptation techniques to do the inactivation process better. Modifying the sigma factor with different RNA polymerase is probably the most significant controlling method in bacterial cells [37]. This factor governs the transcription of the genes in Gram-negative bacteria which are resistant to oxidative, heat, and osmotic stress. Therefore, inducing these sigma factors would help to activate during the cell undergoing different growth phases, may it be in stationary or exponential state [38]. But for Gram-positive bacteria a substitute sigma factor with alike physiological roles are studied in [39, 40]. This infers that a similar process for multiple stress resistance is seen for cells of Gram-negative and Gram-positive. The utmost problem that should be taken care of is to prevent microorganisms from adapting to the stress because that helps them to create a barrier and create greater protection for different other succeeding stress. This is the reason for the emergence of different novel techniques for preservation; a direct heat and traditional thermal method is causing sublethal injury as well as augmenting the sensitivity of the cells to stress when applied mechanically. This is because of some temperature-induced variations in the cell envelopes of microorganisms [41].

      2.5.1 Oxidative Stress

      The main cause of the oxidative stress in bacteria is due to the following reasons, i.e., imbalance of macromolecules changes, cellular and intracellular antioxidant and oxidant concentration which is related to lipids, proteins, and DNA repair enzymes [42]. Enzymes are considered to be the shield of microbes and catalyses Hydrogen peroxide which is the prevailing СКАЧАТЬ