Newer methods of thermal processing
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Thermal processing of foods involves applying heat for a specified time to reduce or eliminate microorganisms. There are three main categories: blanching, pasteurization, and sterilization. Blanching uses mild heat (up to 100°C) to inactivate enzymes as a pretreatment for other processes. Pasteurization uses heat below 100°C to destroy heat-sensitive microbes and extend shelf life. Sterilization employs high heat to eliminate all microbes, including spores, through methods like canning. The time and temperature used depends on the thermal resistance of the target organisms.
Newer methods of thermal processing
- 1. OBT554 Unit 3 Dr K.Geetha Associate Professor, Dept of Biotechnology, Kamaraj College of Engg & Tech, Madurai
- 2. Definition: the combination of temperature and time required to eliminate a desired number of microorganisms from a food product.
- 3. The basic purpose for the thermal processing of foods is, to reduce or destroy microbial activity, reduce or destroy enzyme activity, to produce physical or chemical changes, to make the food meet a certain quality standard.
- 4. There are three main temperature categories employed in thermal processing: Blanching - Mild processes Pasteurization - Mild processes Sterilization - More severe processes like Canning, Baking, Roasting, Frying
- 5. The primary purpose of blanching is to destroy enzyme activity in fruit and vegetables. It is not intended as a sole method of preservation, But as a pre-treatment prior to freezing, drying and canning.
- 6. Blanching is carried out at up to 100°C using hot water Some use of fluidised bed blanchers, utilising a mixture of air and steam. There is also some use of microwaves for blanching.
- 7. Steam Blanchers – preferred method for foods with large cut surface areas as lower leaching losses. food material carried on a mesh belt or rotatory cylinder through a steam atmosphere, residence time controlled by speed of the conveyor or rotation. Hot Water Blanchers – Includes various designs which hold the food in hot water (70 to 100°C) for a specified time, then moves it to a dewatering/cooling section. In blanchers of this type the food enters a slowly rotating
- 8. faster, more uniform heating, good mixing of the product, reduction in effluent, shorter processing time and hence reduced loss of soluble and heat sensitive components
- 9. Pasteurization is a relatively mild heat treatment in which food is heated to <100°c. It is widely used throughout the food industry. It can be used to destroy enzymes. It can be used to destroy relatively heat sensitive Micro-organisms It is used to extend shelf life.
- 11. Thermoduric: organisms that can survive exposure to relatively high temperatures but do not necessarily grow at these temperatures e.g. Streptococcus and Lactobacillus. Thermophilic: organisms that not only survive relatively high temperatures but require high temperatures for their growth.
- 12. Pasteurizers may be batch or continuous. A simple batch type may be a water bath in which crates of the food are heated to a pre-set temperature, and then cooled by draining and adding cold water. A continuous version may convey containers through a hot water batch followed by a cold water bath.
- 13. 1. Batch (holding) Method In this method every particle (e.g. milk) must be heated to at least 63°C and held for at least 30 minutes.
- 14. In this method the heating of every particle of milk to at least 72°C and holding for at least 15 seconds. Carried out as a Continuous process. Ultra Heat Treatment (UHT) a sterilisation treatment, can also be performed using higher temperatures and shorter times e.g. 1 s at 135°C Eg: Tunnel Pasteurizer
- 15. Sterilization is a controlled heating process used to completely eliminate all living micro-organisms, including thermo resistant spores in milk or other food. It can be achieved by, Moist heat, Dry heat, Filtration, Irradiation, Chemical methods
- 16. Phase 1 = (heating phase) heating medium (water or steam) the product temperature is increased from ambient to the required sterilization temperature Phase 2 = (holding phase) temperature is maintained for a defined time. Phase 3 = (cooling phase) the temperature in the can is decreased by introduction of cold water into the autoclave.
- 17. Autoclaves or retorts Simple small autoclaves Larger autoclaves Rotary Autoclaves
- 18. If a population of microbes is held at a constant high temperature, the number of surviving spores or cells plotted against time (on a logarithmic scale) will look like the following graph – which is referred to as the ‘death rate curve’.
- 19. If D-value versus time is plotted – again on a logarithmic scale, the graph looks very similar to the one previously. This one is called the Thermal death time (TDT) curve.
- 20. Factors affecting heat resistance of microorganism Age of cell: younger cell are less heat resistance Initial concentration of spore or cells: more the number greater the heat treatment The medium in which growth has occurred: a more nutritious medium increases heat resistance Moisture content: dry food tend to require more serve heat treatment during sterilization pH of medium: cell or spore have great heat resistance at or natural pH values
- 21. Two strategies: A) High Temparature-Short Time (HTST) processing, with associated aseptic packaging B) By delivering heat in new ways e.g. by Microwaves or by Electric resistance (“OHMIC”) heating of food Examples: Retorts Using plastic packaging in retort operations Ohmic heating Microwaves sterilization
- 22. The Campden and Chorleywood Food Research Association is a centre of technical excellence for the food industry.