Food Preservation Techniques PDF

OTHER PRESERVATION TECHNIQUES Introduction  Non-thermal processing is a promising and useful approach for fruit juice and beverage preservation.  The products based on these techniques show many advantages such as the retention of sensorial qualities and nutritional values ov...

OTHER PRESERVATION TECHNIQUES Introduction  Non-thermal processing is a promising and useful approach for fruit juice and beverage preservation.  The products based on these techniques show many advantages such as the retention of sensorial qualities and nutritional values over traditional thermal processing.  Examples of non thermal processing are: high hydrostatic pressure (HHP), pulsed electric field (PEF), and ultrasound (US) etc.  High Pressure Processing (HPP) Principle  Also described as high hydrostatic pressure (HHP)  HPP uses up to 900MPa to kill many of the micro organisms found in foods, even at room temperature without degrading vitamins, flavor and colour molecules in the process.  When high pressures up to 1000MPa are applied to packages of food that are submerged in a liquid, the pressure is distributed instantly and uniformly throughout the food (isostatic).  Typically a pressure of 350MPa applied for 30min or 400MPa for 5min will cause a tenfold reduction in vegetative cells of bacteria, yeasts or moulds.  High pressure inactivate vegetative microorganisms and since the process does not involve heating, the sensory and nutritional qualities of HPP similar to the unprocessed products.  High pressure processing has no “heating or cooling” periods and there is a rapid “pressurization/depressurization” cycle, thus reducing processing times compared to thermal processing.  Bacterial spores are resistant at high pressure, therefore products that are currently available in the market are chilled or refrigerated storage.  Shelf life of HPP products varies according to product type, e.g fruit based chilled HPP products have shelf life of 21-28 days whilst meat products have shelf life of 30-60 days (involve additional hurdles to prevent growth of C. botulinum using curing agent).  PULSE ELECTRIC FIELD (PEF)  Non thermal pasteurization technique  Used on liquid food – liquid egg, milk, fruit juices  Main purpose: to inactivate pathogenic microorganisms  Energy loss due to heating of foods is minimized  Superior to traditional heat treatment of foods because it avoids or greatly reduces the detrimental changes of the sensory and physical properties of foods.  Involved the application of pulses of high voltage (typically 20 – 80 kV/cm) to foods placed between two electrodes  Frequency (eg – 3.5 Hz), pulse duration (nano or microseconds), field strength or electric field intensity (eg 20 to 80 kV/cm) and number of pulses  Limited application : restricted to food products that can withstand high electric fields  PEF treatment chamber consists of at least two electrodes and insulation that forms a volume, i.e. PEF treatment zone, where the foods receive pulses. The electrodes are made of inert materials, such as titanium  Mechanism of action (PEF)  Pulsed electric field treatment of food material involves the application of short pulses (microseconds) of high voltage (kilovolt).  When exposed to such electric field pulses cell membranes develop pores which may be permanent or temporary, depending on the intensity and treatment conditions.  The possible application of this non-thermal technology in a wide range of the food industry is based on this effect on biological cells which is called electroporation (see Fig. 1).  The pore formation increases membrane permeability which results in the loss of cell content or intrusion of surrounding media.  Processing of apple juice  Apple juice concentrate treated with PEF at 50 kV/cm, 10 pulses, pulse width of 2 ms and maximum processing temperature of 45oC had a shelf life of 28 days compared to a shelf life of 21 days of fresh squeezed apple juice  No physical or chemical changes in ascorbic acid or sugars  A sensory panel found no significant differences between untreated and electric field treated juices  Processing of milk i. Shelf life of raw skim milk (0.2% milk fat), treated with PEF at 40 kV/cm, 30 pulses and treatment time of 2 ms using exponential decaying pulses. ○ The shelf life of the milk was 14 days stored at 4oC ii. Treatment of raw skim milk with 80oC for 6 s followed by PEF treatment at 30 kV/cm, 30 pulses and pulse width of 2 ms increased the shelf life up to 22 days (heating + PEF) Mechanisms of microbial inactivation  Damage cell membrane of the microorganism by altering membrane permeability (electroporation)  High voltage electric discharge (electroporation)  The application of electrical fields to biological cells causes buildup of electrical charges at the cell membrane  Electrical breakdown and electroporation or distruption of cell membrane.  PULSE LIGHT  A non thermal method  Three main components : power supply, pulse configuration device and the xenon lamp  Convert high speed electronic pulses into high energy light pulses  Use of high intensity flashes of pulses of broad spectrum light (UV to infrared) at short duration High voltage feed  High energy delivered to the lamp produced an intense pulse of light focused on the treatment area  Exposed the food to intense, very brief flashes of light (UV to near infrared), which disrupt the cell membranes of bacterial cells  Effectiveness depends on the intensity and number of pulses delivered (frequency of flashing)  Mechanism of action  Photochemical mechanism: chemical modication and DNA cleavage  Photo thermal mechanism: bacterial disruption due to temporary overheating. The water content of the microorganism vaporize generating a small steam flow induces membrane distruption.  Limitation  To packaging material efficient if it is UV transparent  Low degree of penetration, decontaminate only at surface of food and transparent media  Depends on food composition, not suitable for high protein or oily foods  Depends on the types of microorganism  ULTRASOUND WAVES  Defined as energy generated by sound waves of over 20 000 vibration per second  Sonic waves with frequencies over the threshold of human hearing (16-20 kHz)  Generated by mechanical vibrations of frequencies higher then 18 kHz  During implosion, very high temperatures and pressures are reached inside these bubbles.  Potentially most useful for sterilization of liquids – milk and juices  Disadvantages: has limited lethal effect : effective if applied ultrasound under pressure (Mano Sonication – MS) or ultrasound in combination with under pressure and heat (Mano Thermo Sonication –MST)  Mechanism of action  High temperatures and pressures of collapsing bubbles lead to generation of free radicals such as hydrogen atoms and hydroxyl radicals  Responsible for the inactivation of bacterial cells by oxidative damage  Kills microorganisms by disrupting cell membranes apparently as a result of the formation and subsequent implosion of small bubbles (cavitation) OSCILLATING MAGNETIC FIELD (OMF)  The flow of electrons is caused by the attraction between electric fields in the conductor.  The oscillation of electron in a conductor caused by the alternation in the direction of attraction on electrons.  The oscillating magnetic lines created by the electrons is radiated to space perpendicular to their oscillations.  When electron oscillate at higher voltages, a radio transmitter creates high amplitude oscillating magnetic lines. OSCILLATING MAGNETIC FIELD (OMF)  Static magnetic field (SMF) : intensity of magnetic field is constant with time  Food is treated with magnetic pulses with pulse duration of millisecond range  1 to 100 pulse may be delivered with a total exposure time of 25 to over 100 ms  Food preservation : A frequency of 5 to 50 kHz and intensity or field strength of 5 to 50 T (tesla)  Effective depends on the number of pulses and frequency applied  Mechanism of action  OMF could loosen the bonds between ions and proteins  Effect on the microorganism: might be due to the rotating electric field formed by the variable magnetic field.