Plasma is defined as the fourth state of matter that is energetically distinguishable from solids, liquids and gases and can either be thermal or non-thermal, depending on the conditions in which they are created. PEF lethality is also affected by the lifecycle stage of the target micro-organisms with rapidly growing and dividing microbial populations being the most sensitive while those in stationary or lag phases showing some resistance. High conductivity foods are not thought to be suitable for treatment with PEF, neither are those with a high salt content or liquid products containing particulates. PEF efficacy towards microbial inactivation mainly depends on the intensity of the electric field, the pulse width, treatment time (number of pulses times pulse duration), pulse waveshape, temperature (operational temperatures of 50–60oC result in increased microbial inactivation) and product conductivity. Studies have shown that combining the PEF treatment at moderate temperatures with the presence of an antimicrobial, effective against both Gram-positive and -negative pathogenic bacteria, would significantly enhance process efficiency, reduce the number of pulses and outlet temperature and lead to significant energy cost savings. Others believe that PEF could be a suitable technique for use with additional processes/hurdles such as mild thermal treatment or control via pH/water activity. PEF has the potential to replace thermal techniques for the pasteurization and sterilization of liquid foods due to the retention of the products’ nutritional value, sensory characteristics, safety and overall quality profile, yet at lower operational costs. It can be industrially implemented to produce juices that better resemble the freshly squeezed, mainly with regards to their aroma profile. Studies have shown that PEF processing results in better retention and storage stability of carotenoids, phenolic compounds and flavonols, in comparison to thermal processing. Recognized for its ability to inactivate bacteria present in liquid food products at low temperatures, PEF is a food preservation technique whereby the food to be processed is exposed to a pulsed high voltage field for less than 1 second. High capital expenditure and limited throughput are its main current limitations. HPP has been employed in the processing of a number of liquid and semi-solid foods such as fruit juices, purees, smoothies, jellies and guacamole. HPP wasn’t used in the commercial preservation of foods until 1990 but has since been used successfully while the number of commercial applications is growing worldwide. Factors such as the type of microorganism, food composition, pH and water activity are shown to affect its microbial inactivation efficiency. HPP is shown to inactivate pathogenic and spoilage bacteria, yeasts, molds and viruses however, its efficacy is limited against bacterial spores and enzymes. However, microbial spore survival during processing requires this technique to be used in conjunction with another hurdle such as low pH or refrigeration. The advantages of HPP over the conventional thermal processing techniques involve reduced process times and thus reduced heat damage, as well as retention of product flavor, texture, color and vitamins. HPP is a non-thermal pasteurization technique whereby products are held at pressures typically between 300 and 600 MPa for up to 10 minutes. Novel processing technologies such as high-pressure processing (HPP), pulsed electric field (PEF) and cold plasma are thought to be among the most promising of novel technologies.
Improvement of product quality has always been the main goal of food and beverage manufacturers. The trend towards the use of “natural” ingredients, (colors, flavors or preservatives) although technically challenging, has created the need for research into milder and more energy efficient but equally effective processing technologies that are able to preserve the structure and thus, function and benefits of novel ingredients whilst at the same time maintaining the nutritional and other food product qualities.
The need for novel processing technologies in the food industry is a direct result of consumer demand for fresh, high quality and healthy products that are free from chemical preservatives and yet are safe.