Alternative heat treatments



For years researchers have been studying heating & cooking systems that contribute to save energy and reduce cooking times: microwave and sous vide cooking are now an alternative used in the food industry.

Most industrial food processes require the cooking of foodstuff or the use of other heat treatments as for instance pasteurization, sterilization, thawing, tempering, blanching, concentration, and drying. Often these processes are both long and energy-expensive, and have a double impact on the final cost of the product. For years, researchers have been studying alternative heating/cooking methods that contribute to save energy and reduce cooking time as compared to traditional methods. Two of these methods – microwave and sous vide cooking – are in use for some years now in the food industry, and recently various scientific journals reviewed their properties, strengths and weaknesses.

Microwaves in the food industry
Microwaves are electromagnetic waves with frequencies ranging between 300 MHz and 300 GHz, generated by an electromagnetic field directly applied to the foodstuff to be processed.  The heat produced by microwaves derives from the capacity of foodstuffs – or better, of their moisture content – of absorbing energy from these microwaves transforming it into heat, especially thanks to so-called dielectric mechanisms (many molecules, such as those of water, are electric dipoles). When the alternating electric field is applied to the water molecules of a foodstuff, the molecules try to align their dipole with the alternating electric field; due to the high frequency of this field, the alignment occurs millions of times a second, generating molecular vibrations that result in the heating throughout the mass. Microwaves can increase the temperature without affecting surrounding materials (as the food package). One of the main advantages of this technology is that a food item can be heated quickly (with considerable reduction of processing times) and uniformly, irrespective of its volume (as long as the food item is uniformly structured and homogeneous), thus minimizing the thermal gradients generated inside the foodstuff, which is typical for traditional cooking methods. There is evidence that compared to traditional cooking methods (especially boiling), microwave cooking results in less damages and modification to nutritional and organoleptic properties of foodstuffs. With regard to these benefits, however, the type of foodstuff to process should be assessed with great accuracy, as its dielectric response may vary considerably according to the water, salts, fibre, and proteins contained in it, and affect the end result. There are mathematical models to predict the temperature reached inside the foodstuff, depending on the distribution of the moisture while it is being processed. Even specific solutions were developed for the microwave cooking of inhomogeneous food materials, which are more prone to develop so-called “hot spots”, i.e. spots within the mass that become quickly very hot, whereas other spots are not sufficiently heated. This problem is particularly feared if microwave cooking is combined with pasteurization, as in the case of non-homogeneous foodstuffs (e.g. “ready to eat” meals consisting of different layers and/or parts), since this treatment would leave spots that have not been adequately processed, and still containing potentially pathogenic micro-organisms.

bread bakery

This problem may be reduced by applying microwaves to the “moving” foodstuff (e.g. on rotary tables), and selecting with great attention the geometry of the food product and package (especially for liquid foods), as well as the shape of the microwave oven chamber. As for the cooking, a variety of microwave applications are known and used for bread, some types of meat, pasta, rice, potatoes, and their derivatives. For bread, for instance, the correct level of moisture and texture may be obtained; whereas it is quite difficult to achieve a brown and crisp crust (easily obtained with conventional hot-air ovens), because microwave ovens do not attain the necessary temperatures to produce Maillard reactions. However, the browning can be obtained using special materials inside the microwave oven, capable of conveying the heat over the entire product surface.  As an alternative, to obtain the desired brown and crispy crust, microwave cooking of ovenable products may be combined with other heat transfer methods, such as infrared radiation, or jet impingement. As for meat, an important parameter to establish the uniformity of cooking is the quantity and distribution of fat:  microwave cooking is particularly efficient in case of evenly distributed fat (as in the case of hamburgers or minced meat), and, unexpectedly, a higher fat content speeds up the cooking process.