Preserving sensory and nutritional value during thermal processing
A core step in cooked foods productions is heating. The severity of the heat treatment is determined by the pH of the food; the composition of the food; the heating behavior of the food; the nature, size and shape of the container; and the nature and mode of application of the heating medium. The success of thermal processing does not depend on the complete destruction of all microorganisms which would result in low product quality caused by the long heating required. Maximizing the nutrient retention during thermal processing has been a considerable challenge for the food industry. The adoption of the proper time–temperature processing conditions in relation to the specific food product and its target essential nutrient permits to reduce inevitable nutrient losses. Depending on the heating method, commodity, and the nutrient, the loss caused by blanching can be up to 40% for minerals and vitamins (especially vitamin C and thiamin), 35% for sugars and 20% for proteins and amino acids. In addition to nutrients, the toxic constituents (e.g. nitrates and cadmium in spinach) naturally present in the vegetable may also be leached and the level of contaminating microorganisms may be reduced, which are advantages gained by blanching. Although texture degradation is characteristic of most heat treatments, low-temperature blanching has been shown to improve the texture of some products (carrots, beans, potato, tomato and cauliflower) owing to activation of pectin methyl esterase. The individual quick blanching (IQB) technique is an innovation based on a two stage heat-hold principle and has been shown to improve the nutrient retention significantly. Other non-conventional blanching procedures use moisturized hot gas, microwave or ohmic heating techniques generally together with air cooling to minimize leaching of water-soluble components, including vitamins, minerals and sugars.
Novel thermal processing techniques
As alternative methods, novel thermal processing techniques, such as ohmic heating, high frequency heating and microwave heating, and non-thermal processing techniques, such as high hydrostatic pressure processing and pulsed electricity method, have been receiving more and more interest from both food scientists and industries. The majority of ohmic installations are found in Japan for the production of fruit products, while the largest application in the food industry for high frequency heating is in the finish drying or post-baking of biscuits, expanded cereals and potato strips. Industrial applications of microwave heating are found for most defrosting or thawing of frozen foods prior to further processing. Further application of microwave heating is for drying in combination with conventional hot-air drying: pasta, vegetables and various cereal products, where puffing by rapid expansion of the interior of the food matrix can also be accomplished using microwave energy. Other incremental thermal process changes include better temperature control and air flow, less energy use through a combination of different energy sources and better heat conduction, improved cleanability and ergonomics. Finally, advances in statistics and computing enable a variety of modelling applications that can simulate a range of scenarios from heat flow in equipment design to virtual environments of eating situations.
by Milena Lambri – Istituto di Enologia e Ingegneria Agro-alimentare, Università Cattolica del Sacro Cuore, Piacenza
