Since food is seldom exposed directly to the primary source of heat (except in gas-fired ovens or in wood burning bread and pizza ovens) some form of heat exchanger is required. In the food industry, the main sources of heat are steam (produced in boilers) and hot air (heated by electricity, or indirectly by heat exchangers from combustion gases). Heat exchangers were mainly developed for the chemical industry rather than the food industry but were adapted for the food industry, mainly by replacing the mild steel construction by stainless steel a material that is easier to clean and is more acceptable as a food contact material. The simplest type of heat exchanger is in the form of two concentric tubes with the material being heated flowing through the central tube and the heating material flowing through the annular space formed between the two tubes. Of course, heat exchangers are therefore limited to liquid foods since they must flow but have been developed into forms that can accommodate even very viscous liquids or liquids containing suspended solids. Capacity limitations ensured that the concentric tube system never move beyond a laboratory system and industrial versions were developed as the shell and tube system (where a large number of tubes through which the fluid could flow, were bundled into a single cylindrical shell through which the heating mediium could flow. That such a system never became popular in food factories is due more to hygiene and cleaning difficulties than to efficiency.
The cleaning problem was overcome y the development of the plate heat exchanger. In this system a large number of stainless steel rectangular plates are arranged vertically with a rubber gasket around the edge of each plate effectively forming a narrow space between each pair of plates. An ingenious system of gasket encased entrance and exit holds allows the food liquid to be pumper between every second pair of plates with the heating medium pumped between the intervening plates. This system is used almost universally for both pasteurisation and sterilization in the liquid food industry, especially for milk and fruit juices.
The initial form of sterilization as developed by Appert (see History), took place in a sealed metal can or glass jar. The container of food is heated to approximately 120C and held at that temperature for the time required to inactivate the most heat resistant bacterial spores. A holding time of several seconds at this temperature may be sufficient depending on the nature of the bacteria present. However, the holding time required is dependent on the temperature and increases dramatically as the temperature falls. At 100C, the holding time could be several hours. It is easy to see that in the early days of canning development when the only heating possibility was immersion of the cans in boiling water, the very long heating and holding times at the 100C boiling water temperature would result in severe thermal damage to the product and its internal structure. Boiling vegetables at 100C for a few hours is more likely to result in a disintegrated vegetable puree! It took the development of superheated steam boilers (a side-effect of railway developments) in the mid 1800’s to provide higer heating temperatures and more realistic heating times.
More recently, it has been possible to sterilize liquid foods using plate heat exchangers using two different mechanisms. In one, the indirect heating method, the liquid (very often milk) is heated to approximately 120C and the passes through a holding tube where the length of time it takes to traverse the tube (termed the holding time) is equal to or in excess of the time required to inactivate all the bacteria present at that temperature. The sterile liquid is then cooled by first using it as the heat source for the cold incoming raw material to the heat exchanger, hereby improving the energy efficiency of the process. Further heat exchangers using cold water and chilled water lower the temperature still further. Finally, the cold sterile liquid needs to be packaged aseptically to ensure it is not recontaminated during storage, distribution and sale.
The second method, the direct method, uses many of the features of the indirect method except that the final heating to approximately 20C is achieved not in a plate heat exchanger but by the direct injection of superheated culinary grade steam into the product. This instantaneously raises the food liquid to sterilization temperatures. Regulations in many countries demand that any steam that condenses in the food liquid and therefore dilutes it, must be subsequently removed. This is achieved by flash evaporation, a process where the hot sterile liquid food is sprayed into a vacuum chamber where it instantly boils and the previously condensed is evaporated.
Most food heating processes would not normally rank highly in any listing of green processes since the amount of energy needed to raise a food through a given temperature range is the same no matter what process is employed. However, the ‘greenness’ of ohmic heating comes from two sources. Firstly, it is a very rapid process and therefore the time available for heat losses from the product being heated is small and environmental losses are minimised. Secondly, and more importantly, it is a direct application of electrical energy to the product and consequently the significant energy loss implicit at each energy transformation or exchange step in the process is significantly reduced if not entirely eliminated. Consequently, it can justifiably be regarded as a green process and within heating processes will rank as one of the greenest.
While ohmic heating as applied to foods has developed significantly over the past two decades, it has been known for significantly longer than that. In the 1950s and 60s significant research was undertaken into the process, largely at the behest of electrical utility companies rather than the food industry, but those efforts did not result in significant industrial uptake, primarily because of problems of electrode design, electrode polarisation and fouling, difficulties in applying the electric current directly to the food and difficulties with food particles of differing conductivity to the main food matrix.
While the basic physics and mechanisms of ohmic heating are dealt with in more detail in section 2, it is worth noting in this introduction that the process involves the direct passage of an electric current through the food and the consequential heating of the food due to its electrical resistance. Simple in concept, it is anything but simple in application as is apparent from the later sections of this chapter. However, the absence of an intermediate step between the energy source (electricity) and the food such as is common in other processes (e.g. using the electricity to first heat a resistive element which in turn heats air or water with subsequent heat transfer to the food) brings with it the energy savings and efficiencies noted above. Finally, the advances in materials forming the electrical electrodes used to conduct the electrical energy to the food together with the deeper understanding of the heating process of complex foods due to improved modelling and computational techniques, give this ‘green’ process a very bright future.
Principles: Voltage, current and resistance are the primary characteristics of any electrical circuit. Voltage is the electrical driving force and can be supplied from a variety of sources such as the ac mains supply, battery, or a generator. This driving force causes a flow of electric current measured in amperes and the physical makeup of the circuit (wires, etc) contribute a resistance that opposes the flow and is measured in ohms. In ohmic heating, this resistance is provided by the food material through which the current is passed. For a material to be ohmically heated, it must be physically capable of conducting electricity. For a material to be classified as a conductor, electrical charges must be able to move from one point to another within it to complete an electrical circuit. While we are well used to the concept of metals being the best conductors of electricity (wires, etc.) and display metallic conduction due to the relatively free movement of electrons through metallic lattices, even solid foods are vastly different from metals. However, most foods contain high levels of water and dissolved salts and these solutions can conduct electricity through electrolytic conduction.