HEAT STERILIZATION POUCHES a Multilayered Laminate Material With
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HEAT STERILIZATION POUCHES A multilayered laminate material with an internal food contact surface of polypropylene was used as a base material for making the pouches. A polypropylene based laminate was preferred to polyethylene on the basis of its stability at sterilization temperatures (melting point of around 170 ºC). A pouch that can be used to ohmically reheat and sterilize food and later reused to stabilize waste will significantly reduce the Equivalent System Mass during a long term space mission. Reheating of food and stabilization of waste have been successfully done in the past using a V-shaped electrode pouch using pulsed ohmic heating. A retort pouch is a type of food packaging created by aseptic processing, made from multiple layers of flexible laminate, allowing for the sterile packaging of a wide variety of food and drink, ranging from water to fully cooked, thermo-stabilized (heat-treated) high-caloric (1,300 on average) meals such as Meals, Ready-to-Eat (MREs) which can be eaten cold, warmed by submersing in hot water, or through the use of a flameless ration heater, a meal component introduced by the military in 1992. Retort pouches are used in field rations, space food, camping food, and brands such as Capri Sun, Tasty Bite, etc. A retort pouch is a plastic and metal foil laminate pouch that is used as an alternative to traditional industrial canning methods. A pouch is a bag with 3 or 4 wide seals, which can store liquid within. Some varieties have a bottom gusset and are known as Stand-Up Pouches. The retort pouch was invented by the United States Army Natick R&D Command, Reynolds Metals Company, and Continental Flexible Packaging, who jointly received the Food Technology Industrial Achievement Award for its invention in 1978. Retortable pouches are extensively used by the U.S. military for field rations (called Meals, Ready-to-Eat, orMREs). A retort pouch is constructed from a flexible metal-plastic laminate which is able to withstand thermal processing used for sterilization. The food is first prepared, even raw or cooked, and then sealed into the retort pouch. The pouch is then heated to 240-250°F (116-121°C) for several minutes under high pressure, inside retort or autoclave machines. The food inside is cooked, similar to pressure cooking. This process reliably kills all commonly occurring microorganisms (particularly Clostridium botulinum), preventing it from spoiling. The packaging process is very similar to canning, except that the package itself is flexible. The lamination structure doesn't allow permeation of gases from outside into the pouch. The retort pouch construction varies from application to application, as a liquid product needs different barrier properties than a dry product, similarly an acidic product needs different chemical resistance than a basic product. Some different plastic layers used in retort pouches: Polyester PET Provides a gloss and rigid layer, may be printed inside. Nylon Bi-Oriented Polyamide, provides puncture resistance. Aluminum Al Provides a very thin but effective gas barrier. Food-grade cast polypropylene or CPP is used as the sealing layer. Polyethylene (PE) can be used instead of PP as a sealing and bonding layer. This multi-layer structure prevents the retort pouch from being recycled into other retort pouches or food packaging. However this material can be recycled into an aluminized resin or up-cycled into textile materials. The weight of a pouch is lighter than regular cans or bottles, and the energy required to produce each pouch is less than competing packaging from metals, paper, and glass. Containers made out of plastic do not have the strength to resist sudden changes in internal pressure during thermal processing. Thus, the heating and cooling steps must be carried out slowly or air overpressure must be applied inside the retort all the time during the process to ensure that the pressure in the retort is always greater than the pressure inside the container. Processing with air over-pressure, however, occurs under a non saturated steam atmosphere. Heat transfer is slower than in a saturated steam or saturated water medium. Another problem with steam/air processing medium is the possibility of large variations in the temperature within the retort and the difference in heat transfer coefficients with different concentrations of air in the medium. To solve these problems, processing may be done by complete immersion of the product in water, or water may be sprayed on the product throughout the process, or water may be cascading over the product during the process. Fig 1.Retort system designed for steam/air mixtures as heating medium. A recent development in thermal processing is the use of a mixture of steam and air instead of water or saturated steam for heating. This system has been touted as ideal in processing of products in retortable pouches and glass. The advantages are elimination of a need for exhausting, and no sudden pressure changes on heating or cooling preventing breakage of the fragile containers. Heating rates on which the scheduled process is dependent are strongly dependent on the heat transfer coefficient when steam-air is used for heating. The heat transfer coefficient is a function of velocity and mass fraction of steam. Thus, a retort designed for steam- air heating must be equipped with a blower system to generate adequate flow within the retort to maintain uniform velocity and uniform temperature. Various types of aseptic packaging fillers are available, which can handle particulate materials. These include the ‘intasept’ and scholle fillers which pack in laminate bags and the combibloc filler, which packs in laminate cartons. The ‘stork steripart’ system allows liquid and particulate fractions to receive different heat treatments. The liquid fractions can flow at a high velocity and are subjected to a heat treatment comparable to that of an ultra-high temperature process. The particulates, which may vary in thermal size, can be held in the main flow during preset times and are subjected to a heat treatment suited to their relevant size. The system incorporates heat exchangers with one or more ‘rota-hold’ Thermal sterilization is the most common technique used for food preservation today. Strict regulations and procedures are established by government agencies for the thermal processing of low-acid canned foods because of widespread public health concern about the anaerobic Clostridium Botulinum, a spore-forming microorganism that produces a toxin deadly to human, even in very small amounts. Two different methods of thermal processing are known, the aseptic processing in which the food product is sterilized prior to packaging, and canning in which the product is packed and then sterilized (Barbosa-Canovas, Ma, & Barletta, 1997). In the design of thermal food process operations, the temperature in the slowest heating zone and the thermal center of the food during the process must be known. Whereas traditionally this temperature course is measured using thermocouples, there is a growing interest towards the use of mathematical models to predict the food temperature during the thermal treatment. Since the pouch has been introduced only recently to the market, little information is available on the temperature distribution within the pouch during the sterilization process. The results of sterilization of food in cans cannot be extended to pouches due to the complicated geometry of the later. Pouch analysis will require further computer modeling in a three-dimensional domain. Bhowmik and Tandon (1987) and Tandon and Bhowmik (1986) developed a model to evaluate thermal processing of a two-dimensional pouch containing a conduction heated food. In this study, transient temperature, velocity profiles, and the migration of the SHZ during natural convection heating of food in a three-dimensional pouch are simulated and presented for the first time. This investigation can be used to optimize the industrial sterilization process with respect to its temperature and time. Using the CFD model developed in this work for pouch, it is possible to predict the necessary sterilization time required for any pouch, containing new food products. Fig.2. Geometry of the pouch In order to construct the geometry of the pouch, body fitted coordinate (BFC) is used. For generating curvilinear BFC grid within the sub-domain command, the option of solving differential equations for the corner coordinates within the currently active domain is used. The experimental measurements were conducted at Heinz Watties Australasia Research and Development Laboratories, located at Hastings in New Zealand. A single pouch filled with carrot±orange soup was placed in an ``Easteel Pilot Plant Retort-Full Immersion Process'', which operates using steam at 121°C. The measurements were performed for a single pouch having a width of 11 cm, height of 4 cm, and length of 22 cm, respectively. The pouch was sitting on a tray in a horizontal position and heated for 50 min for complete sterilization. This option involves the solution of Laplace equations for the cartesian coordinates of the cell corners. The finite-difference equations solved for cartesian coordinates are expressed in linearized form, so that they can be solved by means of linear equation solvers. Heat generation due to viscous dissipation is negligible, this is due to the use of high viscous liquid with very low velocities (Mills, 1995). The assumption of no-slip condition at the inside wall of the pouch is valid. The thermal boundary conditions are applied to liquid boundaries rather than the outer boundaries of the can. This is true since the pouch is composed of very thin layers of polypropylene, aluminium and polyester with a total thickness less than 0.1 mm. Fig.3. (a) Experimental measurements and theoretical predictions of temperature of a pouch heated in a retort by condensing steam at 121°C (at x .