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Preservation of foods using Hurdle Technology
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Tuesday, 01 September, 2015, 08 : 00 AM [IST]
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Akanksha Wadehra, Prasad Patil
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fiogf49gjkf0d Introduction
Hurdle technology is a method of ensuring that food products will be safe for consumption and extending their shelf life. It also ensures that pathogens in food products will be eliminated.
The technology is used in food industry for the gentle but effective preservation of foods. The shelf life, sensory and nutritional qualities of foods are based on applications of combined preservative factors (called hurdles). This is true for traditional foods with inherent hurdles as well as for novel products for which hurdles are intelligently selected and intentionally applied. The principal hurdles used for food preservation are given in Table 1.
Table 1: Principal hurdles used for food preservation (Leistner, 1995)
Parameter | Symbol | Application | High temperature | F | Heating | Low temperature | T | Chilling, freezing | Reduced water activity | aw | Drying, curing | Increased acidity | pH | Acid addition or formation | Reduced redox potential | Eh | Removal of oxygen or addition of ascorbates | Bio preservatives | | Competitive flora such as microbial fermentation | Other preservatives | | Sorbates, sulfites, nitrites |
Need for hurdle technology
With the fast growing economy and more and more working women, income amongst middle-class has increased and the ongoing trend has been to eat out. Therefore, the demand for processed food has increased in recent times. Moreover, consumers now a days are demanding fresh, natural and minimally processed ready-to-eat food products. The most important hurdles used are food preservatives (e.g. nitrite, sorbate, sulfite) and competitive microorganisms (e.g. lactic acid bacteria).
In hurdle technology, hurdles are deliberately combined to improve the microbial stability and the sensory quality of foods as well as nutritional quality of foods and economic properties. Thus, hurdle technology aims to improve the total quality of foods by application of an intelligent mix of hurdles.
In general, there are three main principles on which the physiology and behaviour of microorganisms in foods could allow more intelligent applications of hurdles. These are Homeostasis; Metabolic exhaust; and Stress reactions.
Homeostasis is the tendency to achieve uniformity and stability in the internal status of organisms. For instance, the maintenance of a defined pH is a prerequisite and feature of living cells, and this applies to higher organisms as well as microorganisms. When homeostasis of bacteria is altered, the bacterial cells react by spending their energy in maintaining their physiological status rather than in multiplying, thus repair needs more energy. In this case, microorganisms remain in the lag phase or even die before their homeostasis is re-established by repair. Energy restrictions for microorganisms are caused by anaerobic conditions, such as vacuum or modified atmosphere and low aw, pH and redox potential, and these act synergistically in combination when applied in foods.
Metabolic exhaustion is the “auto-sterilisation.” It was first observed that in a mildly heated sausage inoculated with Clostridium spores and adjusted to different water activity by addition of salt and fat, and stored 37°C, Clostridium spores surviving during the heat treatment vanished in the product during storage, especially in unrefrigerated conditions.
Further experimental evidence showed that auto-sterilisation occurred in the unrefrigerated conditions, and more hurdles accelerated metabolic exhaustion. The traditional “air-dried” fermented sausages from Germany showed a good record of safety due to the fermentation at low temperature (<15°C) and due to extensive ageing, which compensated the requirement of higher levels of acids and antimicrobial nitrite.
Some bacteria become more resistant or even more virulent under stress (i.e. stress reaction), since they generate stress shock proteins. The synthesis of protective stress shock proteins is induced by heat, pH, aw, ethanol, and oxidative compounds as well as by starvation. Therefore, multi-target preservation of foods could be the key to avoiding synthesis of stress shock proteins, which otherwise could threaten the microbial stability and safety of hurdle technology. Nevertheless, further research in stress shock proteins and different mechanisms that switches them on or could inactivate them warranted, in relation to hurdle technology. Various types of hurdles used for food preservation are presented in Table 2.
One example of such a product is traditionally fermented seafood from Japan “Sushi.” Fermentation of Sushi employs hurdles that favour growth of desirable bacteria but inhibit the growth of pathogens. The important hurdles in the early stages of fermentation are salt and vinegar. Raw fish is cured in salt (20-30%, w/w) for one month before being desalted and pickled in vinegar. The main target of these hurdles is Clostridium botulinum. Growth of lactic acid bacteria during fermentation results in acid production from metabolism of added sugars and rice. The result is a pH hurdle important in controlling growth of C. botulinum.
Another example of such a product is traditionally fermented pickles, which is made in every Indian home. Vegetables are first cut into pieces and dry salted and kept for some time to remove the excess moisture present in them. After this only they are mixed with other ingredients and immersed in oil/ vinegar for storing them for long time. In this, salt as well as vinegar act as major hurdles for increasing the shelf life of the product and inhibiting the growth of undesirable microorganisms.
Table 2: Types of hurdles used for food preservation
Type of hurdle |
Examples |
Physical |
Aseptic packaging, electromagnetic energy (microwave, radio frequency, pulsed magnetic fields, high electric fields), high temperatures (blanching, pasteurisation, sterilisation, evaporation, extrusion, baking, frying), ionising radiation, low temperatures (chilling, freezing), modified atmospheres, packaging films ( active packaging, edible coatings), high hydrostatic pressures, ultra sonication, UV radiation |
Physico- chemical |
Carbon dioxide, ethanol, lactic acid, lactoperoxidase, low pH, low redox potential, low water activity, maillard reaction products, organic acids, oxygen, ozone, phenols, phosphates, salt, smoking, sodium nitrate/ nitrite, sodium/potassium sulphites, herbs and spices |
Microbial |
Antibiotics, bacteriocins, competitive flora |
Conclusion
The physiological responses of microorganisms during food preservation (i.e., their homeostasis, metabolic exhaustion, and stress reactions) are the basis for the application of advanced hurdle technology. The disturbance of the homeostasis of microorganisms is the key phenomenon of food preservation. Microbial stress reactions may complicate food preservation, whereas the metabolic exhaustion of microorganisms present in stable hurdle technology foods could foster food preservation. The novel and ambitious goal for optimal food preservation is the multi-target preservation of foods, in which intelligently applied gentle hurdles will have a synergistic effect. After the targets of different preservative factors within the microbial cells have been elucidated, this will definitely become a major research topic in the future and preservation of foods can progress far beyond the state-of-the-art of the hurdle technology approach, as we know it today.
(The authors are Ph.D. scholars at dairy technology division and dairy microbiology division, National Dairy Research Institute, Karnal, respectively. They can be contacted at smartakanksha@gmail.com and patilprasad11@gmail.com)
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