Hurdle Technology

Hurdle Technolgy in Food Preservation

In traditionally preserved foods, such as smoked fish or meat, jams, and other preserves, there is a combination of factors that ensure microbiological safety and stability of the food, and thus enable it to be preserved. In smoked products, for example, this combination includes heat, reduced moisture content, and anti-microbial chemicals deposited from the smoke onto the surface of the food. Some smoked foods may also be dipped or soaked in brine or rubbed with salt before smoking, to impregnate the flesh with salt and thus add a further preservative mechanism. Smoked products may also be chilled or packed in modified atmospheres to further extend the shelf life. In jams and other fruit preserves, the combined factors are heat, a high solids content (reduced water activity), and high acidity. These preservative factors also strongly influence the sensory characteristics of the product and contribute to important differences in flavor, texture, or color between different products.

In vegetable fermentation, the desired product quality, and microbial stability are achieved by a sequence of factors that arise at different stages in the fermentation process: the addition of salt selects the initial microbial population which uses up the available oxygen in the brine. This reduces the redox potential and inhibits the growth of aerobic spoilage micro-organisms and favors the selection of lactic acid bacteria. These then acidify the product and stabilize it. Further treatments may include pasteurization and packaging to extend the shelf life and facilitate distribution. The demand by consumers for high-quality foods having ‘fresh’ or ‘natural’ characteristics, that require a minimum amount of preparation has led to the development of ready-to-eat and convenience foods that are preserved using mild technologies. 

The main preservation technique is refrigeration, but because of the difficulty in maintaining sufficiently low temperatures throughout the production, distribution, and storage chain, additional barriers (or ‘hurdles’) are required to control the growth of spoilage or pathogenic micro-organisms. The concept of combining several factors to preserve foods has been developed by Leistner (1995) and others into the Hurdle effect (each factor is a hurdle that micro-organisms must overcome). This in turn has led to the application of Hurdle Technology, where an understanding of the complex interactions of temperature, water activity, pH, chemical preservatives, etc. are used to design a series of hurdles that ensure the microbiological safety of processed foods. 

The hurdles are also used to improve the quality of foods and the economic properties (for example, the weight of water that can be added to a food, consistent with its microbial stability). To be successful, the hurdles must take into account the initial numbers and types of micro-organisms that are likely to be present in the food. 

The hurdles that are selected should be ‘high enough’ so that the anticipated numbers of these microorganisms cannot overcome them. However, the same hurdles that satisfactorily preserve food when it is properly prepared (Fig.a), is overcome by a larger initial population of micro-organisms (Fig. b) when for example raw materials are not adequately cleaned. In this example, the main hurdles are low water activity and chemical preservatives in the product, with storage temperature, pH, and redox potential having a smaller effect. Blanching vegetables or fruits has a similar effect in reducing initial numbers of micro-organisms before freezing or drying. If in given fig., the same hurdles are used with a different product that is richer in nutrients that can support microbial growth (Fig.c), again the hurdles may be inadequate to preserve it and a different combination may be needed or the height of the hurdles increased. It should be noted that although the hurdles in Fig. are represented as a sequence, in practice the different factors may operate simultaneously, synergistically, or sequentially. The combination of hurdle technology and HACCP in process design is described by Leistner (1994). By combining hurdles, the intensity of individual preservation techniques can be kept comparatively low to minimize loss of product quality, while overall there is a high impact on controlling microbial growth. 

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