Heat treatment, also known as thermal treatment in food preservation, involves subjecting food to a high temperature for a specific period in order to kill or reduce the population of microorganisms responsible for food spoilage.
The use of heat plays a vital role in food preservation because it has strong preservative effects, including the destruction and inactivation of enzymes, toxins, and microorganisms.
Heat treatment is also highly significant in food processing, as it contributes to making food more tender and palatable.
In addition to preservation, heat application brings desirable changes to food, such as improved textures, enhanced colors, appealing aromas, and better flavors, all of which increase consumer acceptance and preference.
Different methods of heat treatment in food preservation
Role of heat treatment in food preservation:
Heat treatment plays a key role in food preservation by inactivating enzymes that could otherwise cause deterioration in food quality.
It lowers the microbial load, thereby reducing the chances of spoilage and extending the shelf life of food products.
The process helps in destroying or inactivating spores and toxins that may pose health risks to consumers.
Application of heat contributes to the development and enhancement of desirable sensory attributes, including aroma, flavor, and texture of foods.
Heat treatment makes food more tender and palatable, improving its overall acceptability.
It assists in peeling processes, such as loosening the skin of fruits and vegetables, making them easier to process.
Heat also facilitates packaging by preparing foods in a way that makes them more suitable for safe and effective storage and distribution.
Things to consider while choosing safe and proper heat preservation treatment
When selecting a safe and proper heat preservation treatment, it is essential to ensure the desired storage life and production of food that is free from pathogens and toxins.
The correct time and temperature combination must be determined to effectively inactivate the most heat-resistant pathogen present in the food.
Knowledge of the food composition and the heat penetration characteristics of the food matrix is crucial, as these factors influence how efficiently heat is transferred during processing.
The type of can or container used for packaging must also be considered, since it affects heat distribution and the overall effectiveness of the preservation process.
Advantages of heat treatment
Heat treatment is a simple and controllable method, making it easy to apply in food preservation and processing.
It helps produce shelf-stable foods that can be stored for longer periods without requiring refrigeration.
The process leads to the destruction of anti-nutritional factors (ANF) such as tannins, phytates, oxalates, and enzyme inhibitors, which otherwise reduce nutrient absorption.
Heat application improves the availability of nutrients, making them more accessible to the body.
It enhances digestibility by breaking down proteins and complex carbohydrates into simpler forms that are easier to digest.
Heat treatment also assists in the release of bound nutritional components, further improving the overall nutritional quality of food.
Disadvantages of heat treatment
Heat treatment can cause quality loss, as excessive heat may destroy the organoleptic characteristics of food such as taste, texture, color, and aroma.
It leads to nutrient loss, particularly the destruction of heat-sensitive vitamins and other nutritional factors.
Improper or uncontrolled heat treatment may result in the formation of harmful compounds, including some with carcinogenic potential.
The method is not effective against all microorganisms, since certain heat-resistant bacteria and spores can survive and later multiply in the food.
Foods preserved by heat treatment often have a limited shelf-life compared to other preservation methods like freezing or dehydration.
Heat treatment is not suitable for heat-sensitive foods, as they can easily get damaged or lose their desired properties.
The process often requires specialized equipment, which can be expensive and may consume large amounts of energy during operation.
The most common heat treatment methods in food preservation are:
Blanching
Pasteurization
Heat Sterilization
1. Blanching
A heat treatment process in food preservation where vegetables and fruits are submerged in boiling water or steam.
Its purpose is to inactivate enzymes and microorganisms that may cause spoilage and nutrient loss.
Types of blanching methods:
Steam blanching.
Water blanching.
Microwave blanching.
Ohmic blanching.
Blanching is mostly used as a pretreatment before other preservation techniques such as canning, freezing, drying, and various non-thermal techniques.
Heat transfer during blanching:
Heat is transferred convectively from steam or hot water to the food surface.
Heat then moves by conduction from the surface to the interior of the food.
Blanching eliminates undesirable enzymatic, nutritional, and sensory changes and prevents the revival of spoilage microorganisms during storage.
Two heat-resistant enzymes, catalase and peroxidase, are present in vegetables and used as indicator enzymes to determine blanching success.
Time and temperature of blanching depend on the type of food.
Typical time–temperature combinations: 1 to 15 minutes at 80–100°C.
Inactivation of peroxidase/catalase and retention of a specified proportion of vitamin C are the main concerns in the blanching process.
Factors affecting blanching conditions:
Shape and size of the food.
Thermal conductivity of food.
Blanching temperature and time.
Advantages of blanching:
Removes microorganisms from vegetable surfaces and fruit skins.
Inactivates enzymes that can deteriorate food quality.
Facilitates peeling and dicing of raw food.
Improves color, texture, and flavor when optimum time–temperature is applied.
Defects of blanching on foods:
Causes physical and metabolic changes within the food.
Leaching losses:
Heat increases cell membrane permeability, allowing water and solutes to move in and out of cells.
This results in nutrient losses, as vitamins and minerals leach into the hot water.
Overblanching can lead to unfavorable softening, loss of sensory attributes, and discoloration of food.
Some minerals, water-soluble vitamins, and other nutrients are lost:
Riboflavin: 15–20% loss.
Niacin: 10% loss.
Ascorbic acid (Vitamin C): 10–30% loss
Folic acid: >50% loss.
Vitamin B1 (thiamine): destroyed (heat-sensitive)
Vitamin C losses also occur due to exposure of tissues to the atmosphere.
Recent developments in blanching:
Microwave blanching – applied for mushrooms, turnip greens, and peanuts.
Ohmic blanching – applied for mushrooms and peas, highly effective in destroying heat-resistant enzyme peroxidase in a short period.
Ultrasonic-assisted blanching.
High-humidity hot-air impingement blanching – applied for Fuji apples and grapes.
High-pressure blanching.
2. Pasteurization
A mild heat treatment primarily used to destroy pathogenic organisms and inactivate enzymes present in low-acid or acidic food, thereby extending shelf-life.
Can be applied to food either in packaged containers or to unpackaged food products.
Extends the shelf-life of foods:
Several days (e.g., milk).
Several months (e.g., bottled fruit juice).
When combined with other preservation methods such as refrigeration or drying, it further prolongs shelf-life.
The completion of pasteurization in milk can be confirmed by the Alkaline Phosphatase (ALP) test.
Degree of heat treatment required for pasteurization:
Depends on the acidity of food and the targeted heat-resistant enzymes or pathogens.
For low-acid foods (pH > 4.5, e.g., milk), pasteurization is based on a 12D reduction in pathogens such as Brucella abortus, Mycobacterium tuberculosis, and Coxiella burnetii.
General purpose of pasteurization:
Destroy pathogens.
Destroy spoilage microorganisms.
Inactivate enzymes.
Pasteurization conditions of different foods:
Acidic foods (pH < 4.5):
Fruit juice: Inactivation of pectinesterase & polygalacturonase → 65°C for 30 min, 77°C for 1 min, or 80°C for 10–60 s
Beer: Destruction of wild yeasts, Lactobacillus spp., and residual Saccharomyces spp. → 65–68°C for 20 min, 72–75°C for 1 min (in bottle)
Low-acid foods (pH > 4.5):
Milk: Destruction of pathogens and ALP enzyme.
LTLT = 63°C for 30 min.
HTST = 71.7°C for 15 s.
90°C for 0.5 s.
Ice cream: Destruction of pathogens → 69°C for 30 min, 71°C for 10 min, or 80°C for 25 s.
Cream r chocolate: Destruction of pathogens → 66°C for 30 min or 75°C for 15 s.
Liquid egg: Destruction of Salmonella → 64.4°C for 2.5 min, or 60°C for 3.5 min.
Types of pasteurization:
Before packaging: Product is pasteurized first and then filled in a sterile container.
In-package pasteurization: Product is filled in a sterile container first, then pasteurized.
Batch pasteurization (LTLT): Product heated to 68–71°C and held for 30 min.
Continuous pasteurization (HTST / Flash pasteurization): Product heated to a few degrees above pasteurization temperature for a short time, e.g., fruit juice rapidly heated for 1 min at 5°C above pasteurization temperature.
Ultra-High Temperature (UHT) pasteurization: Operates at very high temperature with a holding time of about 3 seconds.
Effect of pasteurization on foods:
Causes only minor changes in nutritional and organoleptic properties.
May cause loss of volatile aroma compounds in fruit juices.
Causes losses of heat-labile vitamins such as thiamine and vitamin C.
Things to consider before or after pasteurization:
Deaeration of fruit juice before pasteurization helps prevent loss of vitamin C, carotene, discoloration, or enzymatic browning.
Milk should be homogenized before pasteurization for better quality.
Fruit juices are often pasteurized twice:
First at 95–98°C for 10–30 s → inactivates enzymes and pathogens.
Second at 95°C for 15 s before filling → prevents recontamination.
Carbonated juices and other high-acid juices can be pasteurized safely at 65°C, which destroys most yeast cells.
Non-carbonated juices must be pasteurized at 80°C.
Pasteurized milk must be refrigerated after treatment since pasteurization does not make it sterile.
Pasteurization does not kill spores.
3. Heat Sterilization
Heating (thermal process).
Non-heating (non-thermal process).
Heat sterilization:
Refers to the destruction of vegetative microbial cells, spores, and inactivation of enzymes by heating.
An important unit operation in food preservation and food processing technology.
Destroys yeasts, molds, vegetative bacteria, and spore formers.
Commercial sterilization vs Complete sterilization:
In practice, commercial sterilization is preferred because complete sterilization deteriorates product quality and nutritional value.
Commercial sterilization ensures that all pathogenic and toxin-forming microorganisms are destroyed.
Some heat-resistant spores may remain but will not multiply under normal storage conditions.
Most canned and bottled foods are commercially sterile, with a shelf-life of more than 2 years.
A major concern in food preservation is the destruction of Clostridium botulinum and C. sporogenes.
Principle of sterilization:
Treated products must be free from spoilage microorganisms or toxins they produce.
Heat treatment should be at least 121.1°C for 3 minutes to achieve a 12D reduction in microorganisms.
Up to 15 minutes of treatment is required to destroy bacterial spores completely.
Processing must consider the slowest heating point (cold point) with proper time–temperature combinations to ensure sterilization.
Factors affecting degree of heat sterilization:
Processing equipment design.
Type of heating media.
Shape and size of packaging container (can or bottle).
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