We have demonstrated that ohmic heating is a very unique thermal process. Ohmic heating is considered a 'minimal process' besides the 'HTST' process. Potential uses of ohmic heating include:15,47-50
2 Sterilisation and pasteurisation.
However, as mentioned earlier, there has been only limited research quantifying the potential benefits of ohmic heating processes in terms of nutrition preservation. More research is needed to realize the advantages of ohmic heating and to promote the commercialisation of the process.
There are other major challenges hindering the commercialisation of the ohmic heating process. They are: (1) lack of temperature monitoring techniques for locating cold/hot spots in continuous throughput systems, (2) differences in electrical conductivity between the liquid and solid phases, and their dynamic responses to temperature changes, which cause irregular heating patterns and complexity and difficulty in predicting or modeling heating characteristics of par-ticulates in carrier medium, and (3) a lack of data concerning the critical factors affecting heating (residence time distribution, particulate orientation, ratio of electrical conductivity, loading rates, etc.).
These problems must be addressed before the process can be fully commercialised and gain approval from FDA. Below are listed some areas identified as research priorities for ohmic heating processing.
19.7.1 Quantification of effect of ohmic heating on major nutrients
As mentioned earlier, there is serious lack of data demonstrating the changes in major nutrients in food products and quantifying the advantages of ohmic heating over conventional heating in terms of nutrition retention. Kinetic studies are desirable to provide information that will be useful for process and product design. Occasionally, improvements in product throughput 'accidentally' result in better nutrient retention and sensory quality attributes, and directed studies on optimising critical process factors to achieve food safety and improve nutrition retention with ohmic heating are highly recommended.
19.7.2 Reliable modeling and prediction of ohmic heating patterns
Predicting the heating patterns of ohmic heating is a very difficult task because of its unique heating characteristics. The heating rate is critically dependent on parameters such as the electrical conductivity, temperature dependence of electrical conductivity, and volumetric specific heat. Furthermore, possible heat channeling, causing hot spots and cold spots, complex coupling between temperature and electrical field distributions, and sensitivity to process parameters, e.g. residence time distribution, particle shape and orientation, etc., all contribute to the complexity of the process. To ensure sterilisation, the heating behavior of the food must be known. Without the information, process validation - an actual demonstration of the accuracy reliability, and safety of the process - is impossible.
Mathematical modeling allows insight into the heating behavior of the process. Spatial and temporal temperature distribution obtained from a reliable mathematical model which incorporates the critical factors can provide information for the calculation of lethality and cook value. It will also save time and money for validation experiments, process and product design. Modeling of a continuous ohmic heating process is extremely difficult due to a number of different physical phenomena occuring during the heating process. De Alwis, Fryer, Sastry, Palaniappan, and their co-workers are pioneers in modeling the ohmic heating process. Their published models have been used to predict the temperature within particles for very specific heating conditions. The models are of limited usefulness in establishing the heating characteristics of a commercial product because of their inability to model a multicomponent system undergoing a continuous process. The verification of the models is also limited in selected regions within the system. Another limitation is the lack of understanding about some interactions within the system. For example, limited information is available for the temperature dependence of the electrical conductivity, and a reliable method does not exist to measure the convective heat transfer coefficient at the liquid-particle interface. These types of limitations require that actual physical measurements of the temperature of the product and its constituents be conducted when establishing a process. Some of these limitations can be compensated for by using appropriate conservative assumptions at the expense of the product quality. A more accurate and reliable model is needed.
19.7.3 Well-defined product specifications and process parameters
Product specification includes information that defines the product and its physical/chemical aspects that play important roles in determining how much lethal treatment is delivered during the process. Critical factors may include particle size and shape, liquid viscosity, pH, specific heat, thermal conductivity, solid liquid ratio, and electrical conductivity. It is also important to know how these factors interact and how they are influenced by the process, for which only limited information is available.
Particulates are the centerpiece around which an ohmic heating formulation is built. Contrary to conventional heating where we would expect no difference due to the change in particle orientation, the heating pattern of an ohmically heated food system would be greatly affected by particle orientation. De Alwis and Fryer51 showed the heating of identically-shaped potato particles parallel and perpendicular to the electrical field. The particle heating rate changed considerably as a result of the change in orientation. De Alwis and Fryer51 explained that this uniqueness is due to the fact that the orientation of the particles changed the electrical field and thus the heating rate.
Though there seems no limit to the particle size which can be processed in an electrically uniform mixture, cooling of particulates will always be controlled by thermal conduction and the cooling rates possible may impose an upper limit on the particle size. This is important in HTST processes, since rapid cooling is desired. The center of large particles may cool too slowly and thus become overprocessed during prolonged cooling. Unlike conventional heating where the outside may be overcooked, here the inside might be. Particulate size is typically limited to 2.54 cm3. Fundamental particulate considerations include size, shape, concentration, density, conductivity, and specific heat capacity. The fluid phase cannot be neglected. Liquid viscosity should be determined at various temperatures to assure adequate suspension of particulates. Moreover, the liquid viscosity may affect the liquid/particle interface heat transfer and thus the heating and cooling rates and process control. More research is needed to address and understand the many aspects of the product and process design and their effects on the product quality.
19.7.4 Reliable real-time temperature monitoring techniques for locating cold/hot spots
Pioneers of ohmic heating researches have documented that a particle does not heat uniformly during an ohmic heating process because of the non-uniform nature of the electric field and the food materials within the ohmic system.52 As in other thermal processes, it is important to have information on the temperature-time history of the coldest point within the liquid-particulate system undergoing ohmic heating.
It is assumed that the agitation of a continuous system minimises these variations in temperature profiles. However, there is insufficient published evidence to indicate what the temperature is within a particle, let alone how the temperature profile changes during a continuous process. It does appear that for a particle with a homogeneous electrical conductivity, if the particle heats faster than the liquid phase, the particle's coldest spot is at its surface.53 There is little published information for particles with heterogeneous electrical conductivity (i.e., fatty meat). The location of the coldest spot is especially important because that is the place where the thermal lethality must be ensured and this is the key factor in determining the processing time. Conventional tools such as thermocouple and optic fiber are apparently invasive when used to measure the ohmic heated food system. A non-destructive and non-invasive technique which can be used to monitor the spatial distribution of temperature is important for understanding and control of ohmic heating technology. In addition, a non-destructive and noninvasive temperature mapping technique is essential for the model development and the validation of this novel process. MRI seems to be a valid approach to this problem. There is a need to improve the technique further, to collect more data under various product specifications and processing conditions with the technique, and to use this technique to validate mathematical models.
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