Heat Sterilization Technology Summary of heat process calculations
- Slides: 48
Heat Sterilization (Technology)
Summary of heat process calculations Microbiological input Heat penetration input D, z-values for the target microorg. T vs. time data F, the equivalent time necessary f, j-values processing conditions: initial temperature heating medium temp. cooling medium temp. established process (processing time to meet microbiological, heat penetration and processing requirements)
Sterilization implies the destruction of all viable microorganisms. Determining the time and temperature for a thermal sterilization process takes into account thermal inactivation kinetics (heat resistance) of the target microorganism, as well as its sensitivity to oxygen, p. H, and temperature. Pasteurization is a relatively mild heat treatment given to foods with the purpose of destroying selected vegetative species of microorganisms, such as the many pathogens that cause food-borne illness, as well as to accomplish inactivation of enzymes. Pasteurization does not eliminate all vegetative microorganisms, nor does it eliminate more heat-resistant spore-forming bacteria. Therefore, pasteurized foods are not shelf-stable and must be stored under refrigeration and/or with modified-atmosphere packaging, which slow the growth of microorganisms Blanching is a relatively mild form of heat treatment consisting of a few minutes of exposure to boiling water or atmospheric steam, and is not a food preservation process. It is normally applied to fresh foods (fruits and vegetables) prior to further processing
Commercial sterilization The use of a thermal process to achieve a shelf-stable food product is referred to as commercial sterilization. The commercial sterilization criterion was arbitrarily established (12 D concept). This criterion states that the minimum thermal process should reduce the initial microorganism’s concentration by 1012 times. This is the well-known 12 D concept (also referred to as a “botulinum cook”). In commercial practice, actual processes normally go far beyond 12 D. This is because of additional considerations: * Safety margins * Cooking requirements * Prevention of the growth of thermophilic microorganisms that cause spoilage • Currently, a common treatment for Clostridium botulinum (is a highly heat-resistant, rod-shaped, spore-forming, anaerobic pathogen that produces the botulism toxin) is 6– 8 min at 250 °F.
Acid foods to mean foods that have a natural p. H of 4. 6 or below.
Sterilization of Foods Can be a) Sterilization in containers (in cans) b) Sterilization out of containers before filling/canning/packaging… (in heat exchangers etc. ) a) Heating and cooling characteristics of the product in the container. Ø heating and cooling characteristics: conduction, convection, mixed or intermediate. Ø the slowest heating location in the container must receive a sufficient thermal process. Ø Typical medium for heating is high-pressure steam which can afford high temperature. Ø Vessel used for the process must withstand high pressures, are referred to as retorts.
Cold point in cans with solid (a) and liquid (b) content Horizontal retort
Description of commercial sterilization systems for containers Three types of heating media utilized in commercial sterilization processes: 1) High-pressure, saturated steam (sufficiently high pressures to achieve 135 to 140ºC) 2) Hot water, less than 100ºC, for acid foods. 3) Flames
1) Heating by high Pressure-Saturated Steam (it provides a very effective heat transfer environment) Careful attention: a) at the beginning of thermal process to bring steam into retort, careful procedures for venting the retort ensure that the cavity is entirely filled with steam and without air pockets. b) during cooling, care is required to ensure that pressure gradients within the containers are maintained at low levels to avoid damage to the containers or seals.
Venting: Flushing the retort with incoming steam to remove air from the retort Reason for removing air from retort: Air forms a thin boundary layer around containers, which reduces heat transfer from steam to containers
It may be difficult to heat solid and highly viscous foods. Cooking for longer time to attain desirable temperature at thermal center may cause overcooking. Therefore, to increase heat transfer • Containers are agitated • Thinner containers are used More important for solid and highly viscous foods such as canned potatoes, tomato paste etc. Slow temp rise at thermal center Overcooking
2) Heating by Hot Water • Used for mainly glass containers, flexible pouches and polymer trays • Glass containers are thicker and have lower thermal conductivity • Hot water provides lower thermal shock to containers than steam • Flexible pouches and polymer trays are thinner. Therefore, they can be heated more rapidly.
3) Heating by Flames • Used mainly for small cans • Very rapid heat transfer Flame temperature up to 1770ºC • Short processing times • High internal pressures (up to 2. 75 x 105 Pa at 130ºC) • Used mainly for mushrooms, sweet corn, green beans, pears, beef cubes
Retorting Types of Containers used in the retorts: • Metal cans • Glass jars or bottles • Flexible pouches • Rigid trays
In retort operations it is important to: a) have adequate venting of air from the retort and container surfaces to avoid air pockets (Remember: air reduces heat transfer from steam to containers), b) minimize thermal shock to the food, c) limit thermal and pressure strain on the containers by: 1. control of heat-up, cool-down rates, 2. use of pressurized air during cooling to balance increased internal pressure in the container, 3. processing jars immersed in water
Internal pressure increase of containers: Thermal expansion of food Thermal expansion of headspace gas Increased vapor pressure of water
Exhausting • It is the removal of air from the headspace of containers Air Head space
Reasons for Exhausting of containers • To prevent air expansion with heat. Helps to remove distortion of containers • Removal of oxygen prevents internal corrosion in containers • Removal of oxygen prevents oxidative changes in foods • Generating partial vacuum in the can
Principle of Exhausting • Gases occupy greater volume at a higher temperature - Apply heat to expand the air, then, Seal the container and Cool the container. - Air will shrink to a smaller volume, thus causing partial vacuum in headspace
Methods Used for Exhausting Containers • Hot filling the food • Cold filling the food and then heating the container and contents with partially sealed lid • Applying vacuum to remove air • Flushing the cans with steam
1) Sterilization of foods in container -Physical and thermal properties of different foods determine the heat requirements for the foods. - Heating and cooling rates is different at different locations within the container Conduction Heating: - solid foods, packed tightly in a container - the slowest heating location at or near the geometric center of the container - rate of temperature rise at the slowest heating point will be limited primarily by thermal properties of the food product.
Convection Heating: For liquid foods with low viscosity, the slowest heating location is at the radial center. - Rate of temperature rise at the slowest heating point is dependent more on the viscosity of the liquid than on thermal properties of the liquid product.
Three steps in retorts • Pre heating • Sterilization • Cooling For both batch type and continuous retoting
Retort Equipment Batch type • Horizontal or vertical Empty cans being cleaned by steam injection
A vertical batch retort
For code to symbols see previous slide for vertical ret) A horizontal retort
Crateless retort - A variation of the batch or still retort completion of thermal process and the initial phase of cooling, - the bottom of the container can be opened - does not use baskets to hold containers
End-over-end agitation Horizontal Orbitorts Axial agitation
The end-over-end rotation in retorts like Steristar rotary retorts (Malo Inc. ) can create turbulence in products being sterilized, resulting in the greatest rate of heat penetration. In some cases, this can significantly reduce total process time. Actual process time is subject to viscosity of the product, container headspace, size and configuration of containers, etc
End over End Agitation of containers
Hydrostatic sterilizer It is a very popular, high-capacity system. Steam pressure: maintained by introducing columns of water of sufficient height. Time: established and maintained by the rate of movement of the containers from entrance to exit. -imparts mild agitation to a canned product. It is called a hydrostatic sterilizer because the steam pressure inside the cooker is supported by water pressure. It consists of a feed leg, come-up leg, sterilization chamber, come-down leg, and the cooling and discharge leg. The steam pressure in the sterilization chamber pushes the water down, resulting in a rise in the water level in the two legs. At equilibrium, the hydrostatic pressure in the water legs will be equal to the steam pressure in the sterilization chamber. The level of water column in the water leg therefore determines the steam pressure in the sterilization chamber. Every 34 ft provides a pressure equivalent to one atmosphere. In effect, this would be equivalent to a steam temperature inside of about 250ºF.
Hydrostatic sterilizer
Hydrostatic Sterilizer provides continual processing of almost all container sizes and types, including metal, glass, and plastic. The hydrostatic sterilizer is ideal for processing products that require long cook and cool times, high throughputs, and for those deriving little or no benefit from agitation
Variables Monitored in Retorts • • • Temperature of the raw material Temperature of steam Temperature of cooling water Processing time Heating and cooling rates
b) Sterilization of food outside container Ø High temperature processing (T 1500 C) by means of high speed heat exchangers reduces processing time substantially (to few seconds) and improves product quality. Ø Such processes are called high Temperature Short Time processes (HTST -applied to sterilization of milk). Ø Improved product quality is due to the fact that destruction of nutrients and flavor components in foods (vitamins, colors, antioxidants, enzymes, amino acids) are similar to destruction of bacteria with considerably higher z-values.
Ultra High Temperature/Aseptic Processing
Aseptic processing Sterilized food packed in sterile containers under aseptic conditions. Advantages: 1. Product with higher organoleptic and nutritional quality, 2. Possibility to use large containers to pack the food, 3. Extended possibilities for using packaging materials of many package sizes, shapes and materials, 4. Handling of containers during subsequent sterilization is avoided, recontamination risk during cooling is minimized.
Limitations: 1. Large capital investment. Pumping at high pressures, product must be relatively homogeneous. 2. Need for specific design of systems for a specific product. 3. Complex operation requiring careful control and sophisticated instrumentation, need for highly trained personnel 4. Relatively limited filling rate (200 packages per min. versus 600 tin cans per min
UHT/Aseptic Processing • Processing at a very high temperature for a very short time UHT Canning Temperature time
UHT is used for liquid foods Step 1 Step 2 Sterilize the liquid food before filling in containers Sterilize the containers Fill the food in the containers in a sterilized environment
Examples of liquid foods processed using UHT • • • Milk Fruit Juices and concentrates Wine Cream Yogurt Salad dressing Ice Cream Mix Cottage cheese Baby foods Contain Tomato products suspended Soups and rice desserts particles
Principle of UHT • For a given increase in temperature, the rate of destruction of micro-organisms and many enzymes increases faster than the rate of destruction of nutrients and sensory components. Many nutrients and sensory components in foods processed using UHT have greater z values than the microbes
Time temperature combinations for UHT and canning T h erm a l d ea t h t i m e ( s) 40% Thiamin 10% Thiamin Microbial 1 % lysine 3% Thiamin Therefore, by heating food at a high temperature and short time, desirable enzymes and sensory components can be retained, but microorganisms can be destroyed
A disadvantage of UHT • In milk some enzymes such as proteases and lipases are more heat resistant • They can cause flavor changes during prolonged storage
Enzymes and probiotics with low heat sensitivity can be added to the Food After Sterilization Raw Food UHT Sterilized Food Example: Enzymes and probiotics What is probiotic? Opposite of Antibiotic: - Friendly bacteria -Used in animal feed, yogurt and fermented dairy cultures In the dairy industry, lactic acid based bacterial cultures are added to the milk products
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