Moderate Electric Field (MEF) heating of heterogeneous food systems

Abstract

In recent years, consumers’ focus to healthy lifestyle is increasing significantly. Therefore, on food market the demand for safe, minimally processed and healthy food products is growing exponentially. Food industries, consequently, started redesigning their market strategy and production in favour of healthy and nutritious food products. In the context of processes related to the food industry, thermal treatments, such as heating, drying, sterilization, pasteurization, are a set of processes able to guarantee shelf-life enhancement of the treated product and improvement of the food safety. Conventional heating methods are characterized by high temperatures gradients and long-time lasting processes due to slow heating regions within the processed food. To prevent the possibility that in the slowest heating regions of the food products there is an insufficient thermal inactivation of pathogen microorganisms, in conventional heating methods foods are often over processed. This brings to the degradation of important food components, such as vitamins, proteins and other desirable food compounds. In addition, they can influence the organoleptic properties and quality of the treated food products, bringing to a loss of nutrients, texture and colour. To address these problems, novel thermal technologies, based on electro-heating, such as Moderate Electric Field (MEF), have been developed. MEF heating process is based on the passage of an electric current through a conductive material. This method allows to reduce energy consumption and processing time, compared to conventional heating methods, due to its volumetric heating nature. MEF has the potential to heat homogenous food materials rapidly and uniformly. However, the heating uniformity of heterogeneous food products, with components, and thus properties, of different nature, represents a challenge for MEF heating applications. There is, indeed, a lack of information regarding MEF heating of heterogeneous systems. For this reason, this PhD work focused on the investigation and analysis on the MEF heating feasibility to heterogeneous food systems. Being based on the passage of electric current in a food item, MEF requires food product having a certain electrical conductivity (and thus a certain ionic content) to be effective. So, a first part of the work was devoted to analyze how the different ionic content influenced the heating of an heterogeneous food system (meatballs in reconstituted potato puree), regardless of the taste of the food. The first part of the work consisted in the study of the relevant aspects of this process, also in order to be able to design and build a MEF unit at University of Salerno, Italy. Then, a first experimental campaign was developed at University College Dublin (Dublin, Ireland) including a training on a MEF system already available among the facilities of the School of Life Sciences. First of all, MEF heating of a homogeneous food system, composed by reconstituted potato flakes puree at 5.7% and 10.9% ionic content, was investigated. Obtained results showed a good uniform heating in the three key points analyzed. Subsequently, a heterogeneous food system composed by reconstituted potato flakes puree (RPF), at 5.7% and 10.9% ionic content, and meatballs was investigated. Meatballs were heated in reconstituted potato flakes considering different system configurations (2PSiRP, 2PAiRP, 3PiRP). Results showed that, even if the components are characterized by different electrical conductivities (5.60 S/m for RPF at 5.7%, 9.75 S/m for RPF at 10.9% and 1.46 S/m for meatballs), a heterogeneous system can be evenly heated by MEF, with the right choice of ingredients. Particularly, meatballs in reconstituted potato flakes puree with 5.7% ionic content showed a heating dynamic similar to the reconstituted potato flakes in which they were dispersed, while a further increase of the ionic content (10.9%) shortened the time required to reach a target temperature, but it also increased

the difference in heating between meatballs and potatoes, producing a less even heating. Meatballs’ heating did not depend on their relative position, but it was affected by the number of meatballs in the system. Moreover, results showed that spherical geometry gives to the meatballs a different and higher local potential variation than reconstituted potato flakes in which they were heated. Furthermore, for a symmetric heterogeneous system (2PSiRP) the effect of different applied voltages (20 V, 30 V, 40 V) was investigated: it was proven that, both for meatball and potatoes, the heating time to target increased as the applied voltage decreased. In the second year at University of Salerno, to carry out further experimental tests and studies a MEF heating system was designed and developed. A first set of MEF heating experimental tests was carried out, at different applied voltages (30 V, 40 V, 50 V, in order to test the behaviour of the system at different electric field strenghts), on two homogeneous food system, composed, respectively, by only reconstituted potato flakes, one with butter among the ingredients (RPFB) and one not (RPFNB), at different salt compositions (RPFB at 0%, 2.9% and 5.7% salt content; RPFNB at 0.37%, 0.74%, 1.39% and 2.75% salt content). For both analyzed homogeneous system, a good uniform heating was obtained in the three investigated points. Obviously, higher the salt content, higher the electrical conductivity and higher the temperature reached from the different analyzed homogeneous system at the target time. Obtained results of the homogeneous system RPFB were compared with the ones obtained from the system RPF: the two homogeneous systems were affected from the different applied electric field strength. RPFB system was subjected to an electric field strength higher (0.43 V/cm) than RPF (0.30 V/cm). RPFB reached at the same target time a higher temperature than RPF. Therefore, it was proved that higher the electric field strength applied and greater the heating. Another set of experimental tests was carried out on a heterogeneous food system composed by reconstituted potato flakes (RPFB) at different salt contents (0%, 2.9% and 5.7%) and meatballs. Chicken meatballs were heated in RPFB using different configurations (2PSiRP, 2PAiRP, 3PiRP). For configuration 2PSiRP, results showed that meatballs reached, at target time, a higher temperature than reconstituted potato flakes. This was linked to the proximity to the electrodes and to the fact that, due to the spherical geometry, meatballs were more affected from electrical field strength than reconstituted potato flakes. Finally, a mathematical modeling of MEF assisted heating of foods, has been embedded in a simulation tool able to explore further MEF heating scenarios and to propose the design of more efficient MEF treatment cells. [edited by Author]