Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Final Report Summary - FOOD PRO (Ohmic heating for Food Processing)

The production of safe food products in most cases requires a heat treatment. In the usual heating methods heat is generated outside the food and transmitted to it by conduction and/or convection.
%The thermal processing of fruit purees and jams is traditionally difficult, essentially due to their rheological properties. The problem is aggravated when fruit particles are present in the slurry, as in the case of fruit purees to be incorporated in yoghurts. In fact, in order to process them fully to meet food safety requirements, an over-processing of the liquid phase is necessary. This is mainly due to the heat exchange mechanism used (conduction) and leads to important losses both in nutritional and organoleptic terms. The use of conventional heat exchangers is not possible and scraped surface devices are normally used, instead. The contact of the slurry with a hot surface is promoted and mixing is achieved by means of rotating blades. These are responsible for mechanical damage to the fruit particles affecting the final quality of the product and diminishing its acceptability to the consumer. The maintenance of such scraped surface heat exchangers is also more expensive than that of the most usual plate-and-frame or shell-and-tube options.

The same thermal overprocessing problems leading to nutritional and flavour losses are present in the production process of canned soups, which normally integrate food solid particles (e.g. peas, carrot cubes).

In another example, it is widely accepted that raw milk cheeses are among the most appreciated for their unique flavour and taste, having an excellent market value, but it is also widely known that they present a safety problem to the consumer due to the use of raw milk. However, traditional pasteurisation implies the loss of nutritional and organoleptic qualities of the end product, be it milk itself or the cheese made from it, due to the high shear stresses imposed on milk during heat treatment in plate-and-frame heat exchangers (the ones normally employed to heat treat the milk to be used in cheese production). These high shear stress values destroy the fat globules and free their fatty acid content which rapidly oxidises, producing off-odours and off-flavours. This effect is particularly true in the milk of small ruminants (sheep and goat). A safer product is obtained, then, but its properties are no longer compatible with those demanded for such high market value products.
%Also in the case of pet food production, the same kind of problems is posed: nutritional and flavour losses due to thermal overprocessing of the product.
%In all these cases, the problems can be tackled by the use of ohmic heating as an alternative to the thermal processing of fruit purees, jams, soups, milk and milk products and pet foods, as it has been reported to reduce the losses of vitamins and flavours/aromas, while keeping or improving the microbiological safety of the treated products.
%The main innovation brought by ohmic heating is the way through which it allows the heating of foods (internal heat generation). This makes all the difference from other commercially available heat processing technologies where heat is either transmitted by conduction or convection (as exemplified above for the case of the scrapped surface heat exchangers but being also the case for every other type of heat exchanging equipment). Also, when compared to microwave heating, ohmic heating presents advantages once in the former heating is achieved only in a certain depth of the product (ca. two centimetres below the surface) while the latter heats all the volume, no matter its size.
%The aim of the FOOD PRO project was to develop an alternative heating process through the use of ohmic heating, where the heat was generated through passing an electric current through the food heating it as a result of electric resistance. Thus overheating could be avoided and food with improved taste and nutritional content, and with maintained and in some cases improved microbial safety, might be produced.
%The objectives of the project were, by order of importance:
- to build pilot scale ohmic heater units and test them in five different food producing small and medium-sized enterprises (SMEs);
- to determine physical, chemical, microbiological, rheological and biochemical characteristics in food from the test runs in order to model the process and to provide a basis for optimisation of the equipment and of the different process conditions;
- to perform sensory evaluation and to study consumer acceptance of the foods produced with ohmic heating through the use of consumer panels;
- to develop optimal packaging materials to maintain the quality of the products.

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United Kingdom
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