Food and Industrial Microbiology

Module 4. Microbiology of food preservation

Lesson 17
COMBINATION METHODS OF PRESERVATION

17.1 Introduction

Hurdle technology is a method of ensuring that pathogens in food products can be eliminated or controlled. This means the food products will be safe for consumption, and their shelf life will be extended Leistner in germany referred the combination preservation as hurdle technology. Hurdle technology usually works by combining more than one approach. L. Lestner in germany referred the combination preservation as hurdle technology in 1980’s. These approaches can be thought of as "hurdles" the pathogen has to overcome if it is to remain active in the food. The right combination of hurdles can ensure all pathogens are eliminated or rendered harmless in the final product. Hurdle technology has been defined as an intelligent combination of hurdles which secures the microbial safety and stability as well as the organoleptic and nutritional quality and the economic viability of food products .

17.2 Common Hurdle Technology

The most important hurdles generally used in food preservation are temperature (high or low), water activity (a_w), acidity (pH), redox potential (Eh), preservatives (e.g., nitrite , sorbate, sulfite ), and competitive microorganisms (e.g., lactic acid bacteria ). However, more than 60 potential hurdles for foods, which improve the stability and/or quality of the products, have been already described, and the list of possible hurdles for food preservation is by no means complete. Some hurdles (e.g., Millard reaction products) will influence the safety and the quality of foods, because they have antimicrobial properties and at the same time improve the flavour of the products. The same hurdles could have a positive or a negative effect on foods, depending on its intensity. For instance, chilling to an unsuitable low temperature is detrimental to some foods of plant origin (‘chilling injury’), whereas moderate chilling will be beneficial for their shelf life. Another example is the pH of fermented sausage which should be low enough to inhibit pathogenic bacteria , but not so low as to impair taste. If the intensity of a particular hurdles in a food is too small it should be strengthened, if it is detrimental to the food quality it should be lowered. By this adjustment, hurdles in foods can be kept in the optimal range, considering safety as well as quality, and thus the total quality of a food. For each stable and safe food a certain set of hurdles is inherent, which differs in quality and intensity depending on the particular product, but in any case the hurdles must keep the ‘normal’ population of microorganisms in this food under control. The microorganisms present (‘at the start’) in a food should not be able to overcome (‘leap over’) the hurdles present during the storage of a product; otherwise the food will spoil or even cause food poisoning .

Severe heat treatments can impair the organoleptic properties and nutritional value of foods. Excessive Low temperature treatment may reduce food quality by destroying food surface (chilling injury) or due to release of enzymes by dead microbes. Too low pH of a food may impair taste of a food. MAP in excess may change color, flavor, and texture of food. Irradiation at high doses can be harmful to food like production of free radicals. Hurdle effect was first introduced in 1978. Leistner and co-workers acknowledged that the complex interactions of temperature, water activity, acidity, redox potential, preservatives etc; are significant for the microbial stability and safety of most foods. In modern food preservation techniques using bacteriolytic enzymes, irradiation, high pressure or pulsed technologies, secondary hurdles are employed to achieve the desired preservation. Pulsed electric fields can be combined with other hurdles such as pH, aw, temperature or preservatives. Effect of high pressure can be improved if combined with heat, antimicrobials or ionizing radiations.

Various factors like the microbial load determines the type and amount of hurdles required.

· If only few microorganisms are present at the start, than a few or low hurdles are sufficient for the stability of the product

· If microbes present are sub lethally injured, they lack vitality & are easier to inhibit by few hurdles

· Food rich in nutrients and vitamins will enhance the growth of microorganisms (booster or trampoline effect), thus the number & intensity of hurdles should be increased

· Water content is an essential component of food, if an increased water activity is compensated by other hurdles (pH, Eh etc.), the food becomes more economical

· If energy preservation is the goal, than energy consuming hurdles such as refrigeration are replaced by other hurdles that do not demand energy but still ensures a stable and safe food

17.2.1 Basic aspects of hurdle technology

17.2.1.1 Homeostasis

Strong tendency of organisms to maintain their internal environment stable and balanced so that homeostasis is in balanced condition. If homeostasis is disturbed by preservative factors (hurdles) in foods, they will remain in lag phase or even die before their homeostasis is re-established. Repair of disturbed homeostasis demands much energy, thus restriction of energy supply inhibits repair mechanism and leads to synergistic effect of preservative factors. Energy restrictions are caused by anaerobic conditions, low _{a w} , low pH and low redox potential.

17.2.1.2 Metabolic exhaustion

It leads to auto sterilization of foods. Counts of variety of bacteria, yeasts and molds that survive the mild heat treatment decrease quite fast in the products during unrefrigerated storage because the hurdles applied do not allow growth. Microorganisms in hurdle technology foods try every possible repair mechanism for their homeostasis. By doing this, they completely use up their energy and die, that leads to auto sterilization of foods.

17.2.1.3 Stress reactions

Bacteria become more resistant or even more virulent under stress – heat shock proteins. Protective stress shock proteins are induced by heat, pH, Aw, ethanol, starvation etc. Activation of shock protein genes would be more difficult if different stresses are received at the same time.

17.2.1.4 Multi target preservation

A Synergistic effect could be achieved if hurdles in a food hit, at the same time, different targets (e.g., cell membrane, DNA, enzyme system, RNA) within the microbial cell and thus disturb the homeostasis of the microorganisms. Repair of homeostasis as well as the activation of stress shock proteins would become more difficult. Nisin, damages the cell membrane, in combination with Lysozyme and citrate, which then easily penetrate the cell and disturb the homeostasis with different targets.

Example for application of hurdle technology

The hurdle technology approach is used for non fermented foods like Italian pasta. In this reduced aw, mild heating are principle hurdles with modified atmosphere in packaging and chilling during storage are the hurdles. Other food items preserved by the hurdle \ technology are foods, dairy products, fish, meat and cereals for shelf stable food preparations.

Fig. 17.1 Example for application of hurdle technology

17.3 The Hurdle Concept

In the hurdle concept, multiple factors or techniques are employed to affect the control of microorganisms in foods. Barrier technology, combination preservation, and combined methods are among some of the other descriptions of this concept. Referred to as “hurdle technology” since the mid-1980s by L. Leistner in Germany, the practice has been applied to some foods for over a century. A simple example of the hurdle concept or barrier technology is demonstrated by preventing the germination of spores of proteolytic or group I strains of Clostridium botulinum. Among the intrinsic and extrinsic parameters that are known to prevent their germination and growth are: pH <14.6; aw <0.914; NaCl of 10% or more; NaNO2 ca. 120 ppm; incubation temperature<10◦C; and a large aerobic bacterial biota. Foods that employ the hurdle concept in their formulation would embody a series of the above, thus making for a multitargeted approach to preventing germination and growth of these spores. In order for C. botulinum to grow, it must “hurdle” a series of the barriers noted. Note that the hurdles listed above include “aerobic bacterial biota”, which is microbial interference. The important parameters of pH and a_w may be controlled by the growing food biota, especially the lactic acid bacteria. The concept of growth/no growth (G/NG) has been advanced to better quantify the hurdle concept by employing the synergy that exists between two or more parameters. Implicit in this concept is the interaction between two or more parameters to a point where growth ceases,the G/NG interface. Precise definitions and determinations of those factors/parameters that permit and prevent growth of a given organism should make it possible to devise models for the hurdle concept.