Food deteriorate in quality due to a wide range of chemical and enzymatic reactions, added to this the consumer demands faster production rate for high quality food with “fresh-like” characteristics and long shelf life, leading to the application of thermal processes for food preservation. However this thermal treatments cause undesirable effects on food flavor, color and nutritional attributes such as protein and vitamin destruction. These market conditions together with the disadvantages of the traditional food preservation technologies forced the food products manufacturers to seek for improvements in existing methods and the development of new preservation technologies.
Called non-thermal food preservation methods, this new and emerging preservation techniques work by killing the microorganisms and inhibiting its enzymatic activity exerting a minimal impact on the nutritional and sensory properties of foods for an extended shelf life. Additionally non-thermal methods allow the processing of foods below temperatures traditionally used during thermal control processes, so flavors, vitamins and essential nutrients undergo minimal or no change.
“They are also considered to be more energy efficient and to preserve better quality attributes than conventional thermally based processes. Non-thermal processes also meet industry needs by offering value-added products, new market opportunities and added safety margins (Morris et al., 2007)”. Foods can be non-thermally processed by high hydrostatic pressure, ultrasound, filtration, irradiation, hurdle technologies and electric methods such as pulsed electric fields, oscillating magnetic fields and high-voltage discharge. “Due to technological developments, high pressure processing and high electric field pulse processing have received increased attention during the last decade (Butz & Tauscher, 2002) ¨.
The main purpose of this assignment is to give a detailed review and approach to the non-thermal preservation technologies of pulsed electric field processing and high hydrostatic pressure processing by individually analyzing the objectives, equipment and process employed, effects in the food components and properties, and comparing and contrasting the advantages and disadvantages of each method to conclude which process can preserve in longer and in better shape the characteristics that define a “commercially” high quality product.
PULSED ELECTRIC FIELD (PEF)
Constituting a non-thermal treatment for food preservation which has a variety of uses in the field of food conservation, ranging from microorganism or enzyme inactivation and deceleration in liquid food to mass transfer process in plant materials (Wang et al., 2013). The application of PEF includes the implementation of short repeated high voltage pulses to form cell membranes and develops an alternative to the traditional pasteurization process in the food industry based on thermal processing (Janositz et al., 2011). Liquid, semi-liquid and solid food can undergo pulsed electric field processing. Objective
Pulsed electric field processing use a strong defined system that rarely changes its foundations allowing it to keep the general objective of the non-thermal preservation technologies, inhibiting the enzymatic activation of that causes food decomposition using continuous intensive electrical pulses between electrodes to treat foods that consequently, inactivate microorganisms Equipment
The food products that will be treated is a critical factor in design and equip with the adequate machinery the PEF system chamber, its hygienic design is also determining for the quality and safety of foods that will be treated and produced. The commonly used equipment for the pulsed electric field system consists of a high-voltage power source, and energy storage capacitor bank, a charging current limiting resistor, a switch to discharge energy from the capacitor across the food, and a treatment chamber (Ortega-Rivas, 2012).
Treatment systems used in the pulsed electric field treatment can be classified as batch or continuous (Zhang et all., 2010). Batch or static system chamber consist in two parallel plates that provide the most uniform electric field area. Continuous have the same equipment for the high voltage pulse generator but also incorporate a pumping continuous system that allows the nonstop processing, necessary for the industrial application. Process
The bank of capacitors is charged by a direct current power source from the amplified current main source. An electrical switch is used to discharge energy stored in the capacitor bank across the food held in the preservation/treatment chamber Mechanism of Action
Pulsed electric field manages different types of voltages that cause different implications in the processed food, generating a short burst of high voltage to a food placed between two electrodes. When subjected to a higher electric voltage, a large flux of electric current flows through food surface acting a conductors because of the present of charge carriers like ions (Ortega-Rivas, 2012). Although the detailed mechanism that pulsed electric field uses to inactivate the enzymes and microorganism has not been fully clarified, two theories have been proposed: the dielectric breakdown theory and the electroporation theory (Zhang et all., 2010).
In the dielectric breakdown mechanism a cell membrane with induced potential starts to compress from reversible pores to irreversible on the membrane, on the other hand the electroporation theory mechanism on a cell membrane makes a osmotic unbalance that swells and leads to a the membrane rupture. HIGH HYDROSTATIC PRESSURE (HHP)
High hydrostatic pressure alters the equilibrium between protein-protein and protein-solvent interactions at different levels of pressure and time combinations causing minimal loss of vitamins and flavor compounds maintaining the sensory and nutritional quality attributes of food products. Known for its versatility in its applications, HHP provides new alternatives in food products such as cakes, jams, sauces and fruit juices making the further investigation of high hydrostatic pressure is necessary for seeking more advantages in other food processing products (Yang et all., 2012). Like pulsed electric field process, high hydrostatic pressure is an alternative to conventional thermal pasteurization for food preservation (Yang et all., 2012). Objective
Using up to 600 MPa of high pressure, high hydrostatic pressure objective is to achieve microbial inactivation or to alter the food attributes in order to obtain consumer-desired qualities maintaining its natural freshness and extending its shelf life, causing minimal changes in the original
characteristics of food by eliminating thermal degradation. Equipment
Although the equipment used for high hydrostatic pressure is expensive and specialized, due to its popularity is available in a variety of types and quality. Every high hydrostatic pressure system consist of multiple components, its main constituents are the pressure vessel and its enclosure, the pressure generator system, the temperature control device and materials handling adaptation (Ortega-Rivas, 2012). The main factors food producers must consider when building the main components of an HHC system like pressure vessels are stress endurance, resistance and corrosion. High-pressure vessel is usually made of low-alloy steel (Bhat, 2011). Process
Place the food package in a sterilized container and load it in the pressure chamber. Fill the pressure chamber with water and hold under pressure for the time required to pressurize the vessel. Depressurize the chamber and remove processed food. . After pressurization, the food is kept under high pressure for the required process time. Depressurization can be done faster. Mechanism of Action
HHP removes air from the food packaging the products. During the pressurizing process the time required to pressurize the vessel is influenced by the compressibility and the nature of the food material but independent to the quantity of food placed in the pressure vessel (Ortega-Rivas, 2012). There are two types of pressurization systems, direct and indirect (Ortega-Rivas, 2012). The indirect pressurization system is pumped through a high-pressure intensifier into the pressure vessel, the intensifier is used to increase the pressure to desired levels.
This system requires high-pressure tubing and appropriate fittings to convey the medium into the pressure vessel. In direct system, the pressure intensifier and the pressure vessel worked as a large single unit, so a piston is used to deliver the high pressure to the product. The limitations of this system are that it requires heavy duty seals that can withstand the repeated opening or closure and the need of a large number of seals between the pressure vessel and the piston. EFFECTS
The wide field of application that both PEF and HHP have led to find in the literature many of experiments where the positives effects of this two process are observed in any kind of food products. Their most common effects are the inactivation of microorganisms and the enhance of biological compounds. Pulsed Electric Field
Extension of shelf life produced by PEF has been reported. Antioxidant components such as anthocyanins, carotenoids, phenolic compounds and vitamin C are very sensitive to heat, effects of PEF is very low making it an excellent alternative to the thermal processes that lead to the loss of antioxidant and bioactive compounds (Bhat,2011). PEF effects in important parameters that determine the “commercial” quality of a final product have been demonstrated. Food parameters like drying, extraction and pression of solid texture can improve if using the suitable electrical load (Zhang et all., 2010). Enzyme inactivation after a PEF treatment is the secondary most important effect. High Hydrostatic Pressure
By enabling the enzyme activation HHC prevents the rapidly and uniform propagation of microorganism throughout the food, making this the main effect in benefit of the product quality. APPLICATIONS
As established at different points throughout the text, the applications of this two methods are used in all the food industry fields. Vegetable and meat product industries use these technologies to maintain fresh-like sensory qualities on its products. Sea food and juices and beverages focus more in increasing the water uptake and water holding properties (Klonowski et all.,2006). ADVANTAGES
The main advantages of PEF and HHP in comparison with the thermal preservation process is the fact that their quality has been proved in products that globe all the types of food industries. Competition between the various types of processes makes companies optimize everyday technologies to deliver better quality products. DISADVANTAGES
Due to the difficulty that represents obtaining the materials to build the machinery and the time that it takes, makes using PEF and HHP a privilege for those companies that can afford the high cost of manufacturing and maintenance. CONCLUSION
HHP has a clear advantage over PEF due to the effects that water pressure have against inactivation of microorganisms, more effective than the electric pulses. REFERENCES
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