Heat processing is at the very center of food preservation and is one of the most well-known and practiced areas of food manufacturing. The heat process has a critical role in ensuring that foods are safe from microbiological contamination and remain high in nutritional and sensory attributes.
Heat process validation tests the effectiveness of the heat process to ensure the safety of the product.
The well-known heat processes which are used in food industry are;
Blanching
Pasteurization
Cooking
Retorting
Hot- Holding
All these process can control or destroy the microbial growth.
Heat process validation is complex, so it may be best to hire a professional who familiar with the process and the product to validating.
The basic steps of the heat process validation are;
1.Determining the target pathogen
2.Pathogen reduction
3.Verifying pathogen reduction
4. Identifying HACCP controls & critical limits
1.Determining the target pathogen
Target pathogen is most heat resistant pathogen; a public health significant, that is reasonably likely to occur in the product.
The goal of the heat processing is to eliminate the target pathogen or reduce to growing up level that is unable to cause illness under normal storage and handling.
Pathogen may occur naturally on raw food or more introduce with handling & processing.
Most common target pathogens associated with ready to eat food are;
Clostridium botulinum
Listeria monocytogenes
To identify the target pathogen need complete description on finished product including;
-Final packaging
-Storage
-Distribution
If the process changes to pH, water activity and water phase salt or any combination of these in the final product, these changes may provide additional barriers that will affect the target pathogen chosen.
The product packaging and distribution also have a significant impact on the target pathogen.
Main two types of packaging used for ready to eat foods are-
* Oxygen permeable packaging-
Oxygen transmission rate with 10,000 cm3 /m2 per 24 hours at 24(C, can be regarded as oxygen permeable.
* Reduced oxygen packaging –
Any packaging with the transmission rate less than above consider as reduced oxygen packaging.
Reduced oxygen packaging include; vacuum packing, modified atmosphere packaging, controlled atmosphere packaging.
Reduced oxygen environment can form within oxygen permeable packaging under certain circumstances such as;
-Within deep containers
– Products packaged in oil
-Products compacted preventing the flow of oxygen throughout the entire container
Product distribution and stores will usually in the one of those categories;
*Refrigerated storage
*Frozen storage
*Shelf – stable
As mentioned in previously the most common pathogen in cooked ready to eat products are Clostridium botulinum and Listeria monocytogenes.
Clostridium botulinum falls in to two categories as non-proteolytic and proteolytic, both of which form protective spores that allow pathogens to alive extreme like cooking. Clostridium botulinum is an anaerobes, meaning it growth best and produce toxins in the absence of the oxygen.
Listeria does not produce spores, so it most susceptible to destruction by cooking. However Listeria is facultative anaerobe, meaning that is grow with or without oxygen.
These pathogens grow under different conditions created by the product final packaging.
2. Determining the target pathogen
Second step is determine the process time & temperature to achieve pathogen reduction. The heat process should typically design to achieve the 6D Reduction.
6D Reduction –
D – Time needed to kill 90% of the target organisms at a specific temperature.
90% Reduction – Reducing 1 million organisms to one hundred thousand: a one log reduction.
6D Reduction – 6 times the D value, a six logarithmic reduction in the target pathogen.
6-log reduction – 99.9999% reduction in the number of the target pathogen in the product.
Processors can conduct the heat resistance study to determine the D value needed for the specific product or if the scientific literature will support using different D values.
The normal active growing stage of the microorganisms is the vegetative cells. Under stress conditions some microorganisms form spores. The spore is with thick wall that highly resistance to extreme conditions. Generally spores are more heat tolerance than vegetative cells. Spores may survive pasteurization and cooking. Different spores has different levels of heat resistance. Hence some spores are more difficult to kill than others.
Some of the food characteristics make it easier or harder to destroy pathogens in it. Sugars and oils in food tend to shell the pathogen form effect of heat. Pathogens will kill more easily food with acidic pH or with high moisture content.
When developing new products, processors need to collect their own data to establish a heat process for the target pathogen. And processors responsible to validate that heat processed consistently render a safe product.
3. Verifying the pathogen reduction
The effectiveness of the heat process depend on the three factors;
-How heat is distribute to the product within the heating vessel
-The rate that heat penetrate in to the product
-How the target pathogen response to the heat
How heat is distribute to the product within the heating vessel??
This is a complex temperature distribution study. Temperature distribution studies evaluate how evenly heat is distributed throughout equipment. Temperature distribution is influence by the type of the system, heating medium (hard water/steam), design of the equipment and product (amount, distribution & position)
The temperature distribution study will help ensure that every unit of product in the heating equipment exposed to heating media at the desired temperature.
The rate that heat penetrates in to the product;
After establishing time & temperature combination needed to achieve the 6D reduction, the next step is to determine how long takes the coldest point of the product to reach the target temperature. This is determine by performing heat penetration study.
Many factors affect heat penetration such as;
-Size and initial temperature of raw food
-Type of heating medium
-Proportion of liquid to solid
Apply 6D time and temperature combination to the cold spot under worst possible conditions that could occur doing regular processing.
Heat penetration in to the food can be measured using thermos couples or other measuring devices, place within the product prior to starting the heat process. These devices measure how long it takes the time to reach the appropriate temperature until cold spot.
There are two principle ways of heat transfer with the contained food;
* Conduction – heat transferring from one particle of the food to the next, heat transfer is slow. With conduction slowest part of the food heat is typically the center. Hence the thermocouple have to place in center of the container.
* Convection – The faster method of heat transfer. Here as the food heat, it rises usually along the sides of the container. At the same time the cool of food at the center force crating the occurring that moves heat throughout the cooking vessel. Convection only occur when food can move within the cooking vessel. Examples include liquid food like soups, sauces. In this case the coldest point is usually along the container vertical center line, above the 1/3 of height from the bottom of the container.
*Combination (Convection & conduction) – when the heat is penetrate through the combination of these two process, the coldest point would be the geometric center of the vessel.
The final cooking time should be long enough to inactivate or destroy the pathogens in cold spot.
Individual pieces of equipment have their own rates of heating and temperature variations leading cold spot. So every unit of product is subject to minimum time and temperature requirements.
How the target pathogen response to the heat??
This evaluate how the target pathogen responds to the heat. The combination of heat penetration and temperature distribution should allow heat process to achieve 6D reduction.
Following characters have to include in the successful heat process;
-Correct amount of heat
-Appropriate heating method
-Result in 6D reduction
With considering;
-Characteristics of food
-Packaging
-Processing equipment
-Target pathogen
4. Identifying HACCP controls & critical limits
Validating of the heat process, making sure that appropriate controls are place to the target pathogen and the critical limits necessary to ensure that consistence safe product is incorporate with HACCP plan.
Critical limits may be based on other factors besides time and temperature. Critical limits are based on the factors defined at the heat distribution study and the process equipment.
Establishing monitoring system is also very important to continual monitoring and improvement process.
Records of validation procedures and studies should include;
-Method used to establish D- value
-Heat resistance and penetration data
-Temperature distribution data
-Verification of the final process
It is important to seeking professional assistance for heat process validation, while the initial cost may be high, knowing the product has been process properly and safe for the consumer will be worth the cost while minimize food borne associate illness.
References –
Thermal process compliance – Campden BRI, Food and drink innovation
Validation of heat processes- G.S. Tucker, Campden & Chorleywood Food Research Association, Chipping Campden
Food Desk 