Changing lifestyle and nuclear families are leading to exponential increase in the demand of ready to eat processed food in developing countries like India. A consumer would prefer traditional food on a daily basis, provided a safe, tasty and processed option is available. Therefore, this modern urban dynamic lifestyle leads to the genesis of discovery of Ready to Eat, Ready to serve and convenient food.
Retorting is one of the major technique used for the thermal processing of food products which are packed either in semi rigid flexible laminates or in metal or alloy cans. Retort Technology provides the advantage of processing the food and packaging together which makes the filled product more commercial stable. This technique is used for commercialization of traditional or ethnic dishes which are important due to their delicacy and are high in demand. It can be defined as the heating of low acid food prone to microbiological spoilage in hermetically sealed container to extend shelf life.
Operating Principle
Retort is the main processing chamber for where the packed food is sterilized, pasteurized or cooked properly to achieve safety and quality. The major objective of this thermal processing is to destroy microorganisms and to deactivate the enzymatic activity which are responsible for shortage of shelf life and changing of physical and chemical properties of food. Retort consists of basket in which packaged food is kept. Then these food products are exposed to the sterilizing temperature (121 Deg. C) mainly by the application of steam and high pressure (15 psi). Compressed air and water are used for sterilizing and cooling for the food products respectively. Principally, it involves seven steps
a) Preparing the Retorts: Cleaning by washing and steaming for a short time, checking the thermometer, pressure gauge, recorder, inlet, outlet valves etc. before loading the cans
b) Loading: Cans are Jumble stacked for better steam circulation However, this reduces the retort capacity by about 25% and care should be taken to avoid denting.
c) Venting: Removal of air from the retort before process timing begins.
d) Come-up-time: This is the time required to bring up the retort to processing temperature after steam is turned on. Most important thing is to see that all air is driven out through vents before processing starts.. Vent should be closed when process time starts.
e) Processing: When there is no automatic control, it should be seen that the retort temperature does not fluctuate more than 0.50C, throughout the process.
f) Blowing Down: Stopping steam supply and opening the vents. The retort pressure should be gradually brought to ‘zero’. Too quick blowing down may result in buckling of cans. Open the retort only after pressure reaches zero.
g) Unloading: Unload the cans and cool outside promptly.
The processing techniques also differ from product to product and from package to package. Hence, the retort designs and operating procedures also vary. Traditionally, metal cans were used for used for retort processing because of its rigidity but it could not efficiently transfer the heat due to which food in the middle takes larger time to get sterilized. Therefore, to combat this problem of uneven heating, retort pouches were developed which provides efficient heat transfer.
Components of Retort
1. Steam inlet: It is a valve-controlled pipe of suitable size and the opening through which steam is admitted to the retort. The steam inlet to each retort shall be large enough to provide steam for proper operation of the retort, and shall enter at a point to facilitate air removal during venting.
2. Steam Spreader: It is a pipe with perforations for uniform distribution of steam inside the retort. Perforated steam spreaders shall be maintained to ensure that they are not blocked during operation.
3. Safety Valve: It is set to open, when the working pressure has exceeded the safety limit pressure of the retort.
4. Vent: These are large valve, openings controlled by gate, plug cock, or other adequate valves used for the elimination of air during the venting period
5. Bleeder: It is a small opening used to remove air entering the retort with the steam and to provide circulation of steam in the retort and shall be open during the entire process, including the come-up time. Bleeder should be positioned at sufficient distances and also near the thermometer and pressure gauges.
6. Drain Valve: It is an opening used to remove the condensed or cooled steam (water). A non-clogging, water tight drain valve shall be used; screens shall be installed over all drain openings.
7. Pressure gauge: is used to maintain correct processing pressure corresponding to the temperature to show the pressure inside the retort at all times.
8. Thermometers: To maintain correct processing temperature and to show the temperature inside the retort during heat processing.
9. Rails: In case of horizontal retorts only, used for pushing the trolley.
10. Water inlet and spreader: It is used for spraying cold water on the can for cooling after heat process is over.
11. Crates, baskets and devices: To hold the cans.
Types of Retort
1. Batch Retort: It can be either horizontal type or vertical type. The retort has a lid or door with good fastening. It has necessary controls for temperature, pressure and safety devices. They can also be of circular or rectangular cross section. In batch systems, the retort is filled with product, closed and then put through a processing cycle.
2. Continuous sterilizer in the continuous sterilizers, the cans enter and exit the sterilizer in a continuous manner. There is continuous container handling and intermittent product agitation. The retort system is constructed of a series of cylindrical vessels called shells (typically 1.5 m diameter). Processing and cooling occurs in separate shells connected by transfer valves. To maintain pressure in the shells, containers enter and exit through self-sealing inlet and discharge valves. Agitation occurs when the containers roll on the bottom of a shell. Product agitation increases rate of heat penetration, allows use of higher processing temperature (up to 138°C), and improves product uniformity and quality. Large initial investment and additional critical factors to monitor and control.
Retort processing has evolved one of the major sterilization technique significantly since its incorporation into Department of Defence and NASA food system. While the technology still relies on aggressive application and penetration of heat throughout foods, recent advancement in process engineering coupled with evolution of packaging technology have allowed for overall improvement of the technology. Retort processing presents the advantage of increased shelf life with less loss of nutrients due to which it can be applied to process ready to meal, marinades, vegetable curry, soups and rice.
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