How Thermal Preservation Kills Microorganisms in Food

Every time you drink a glass of pasteurised milk, open a can of vegetables, or eat commercially packaged fruit juice, you are benefiting from thermal preservation. This technique-one of the oldest and most reliable in food science-uses heat to destroy harmful microorganisms and inactivate enzymes that cause spoilage. But how exactly does heat kill bacteria, and why do different foods need different temperatures and treatment times? Let’s break it down.

What is thermal preservation? 🔗

Thermal preservation is the process of applying controlled heat to food in order to eliminate or reduce the population of spoilage and pathogenic microorganisms. Heat works by denaturing microbial proteins, disrupting cell membranes, and inactivating enzymes that would otherwise degrade food quality. The result is a product that is safer to eat and has a significantly longer shelf life.

The fundamental principle is straightforward: when microorganisms are exposed to temperatures above their maximum growth range, their cellular machinery breaks down. Proteins unfold, membranes lose integrity, and nucleic acids become damaged. The key challenge for food processors is to apply enough heat to destroy dangerous organisms without ruining the food’s taste, texture, or nutritional value.

Major methods of thermal preservation 🔗

Pasteurisation 🔗

Pasteurisation is a relatively mild heat treatment designed to kill pathogenic bacteria and reduce spoilage organisms while keeping the food’s sensory qualities intact. Developed by Louis Pasteur in the 1860s to prevent spoilage of beer and wine, the technique is now most widely associated with dairy products, fruit juices, and eggs.

Two common approaches are used in the dairy industry:

Low-temperature, long-time (LTLT): The product is heated to approximately 63°C (145°F) and held at that temperature for 30 minutes. This traditional batch method is still used in smaller operations.

High-temperature, short-time (HTST): The product is heated to about 72°C (161°F) for just 15 seconds. This continuous-flow method is the standard for commercial milk pasteurisation because it is fast, efficient, and preserves milk quality well.

A third option, ultra-high temperature (UHT) processing, heats milk to 138°C for two or more seconds. UHT-treated milk can be stored at room temperature for months in sealed, aseptic packaging-a major advantage in regions with limited refrigeration access.

It is important to note that pasteurised food is not sterile. Spoilage organisms may survive in small numbers, so refrigeration is still necessary for LTLT and HTST products.

Sterilisation 🔗

Sterilisation involves far more intense heat treatment than pasteurisation. The goal is to destroy all viable microorganisms, including heat-resistant bacterial spores. In practice, most food processors aim for commercial sterilisation-a level at which all pathogens and toxin-producing organisms are eliminated and any surviving spores cannot germinate under normal storage conditions.

Canning is the most familiar application of sterilisation. Food is sealed inside containers and then subjected to high temperatures (typically 121°C or higher) under pressure in equipment called a retort. This is especially critical for low-acid foods like meat, fish, poultry, and vegetables, where the anaerobic bacterium Clostridium botulinum poses a serious safety risk. Commercially sterile canned products can remain safe for two years or more without refrigeration.

Aseptic processing is another form of sterilisation. Here, the food and packaging are sterilised separately, and the product is filled into the container under sterile conditions. This approach is common for liquid products like juices, soups, and dairy beverages, and it reduces the thermal damage that occurs when food is heated inside its container.

Blanching 🔗

Blanching is a brief heat treatment-usually at 80-100°C for one to fifteen minutes-applied to fruits and vegetables before further processing such as freezing, drying, or canning. Its primary purpose is to inactivate enzymes like peroxidase and catalase that cause browning, off-flavours, and texture loss during storage. Blanching also reduces surface microbial load, removes trapped air from plant tissues, and helps retain colour.

Blanching is not a standalone preservation method. It is always followed by another process-typically freezing or canning-that provides long-term microbial control.

How heat kills microorganisms 🔗

Heat destroys microorganisms primarily through protein denaturation. Proteins are complex molecules folded into precise three-dimensional shapes that allow them to function as enzymes, structural components, and transport channels. When temperatures rise beyond a microorganism’s tolerance, these proteins unfold and lose functionality. Critical cellular enzymes stop working, cell membranes are disrupted, and nucleic acids are damaged-all of which contribute to cell death.

Moist heat (steam or hot water) is more effective than dry heat because water molecules accelerate the denaturation of proteins. This is why autoclaving at 121°C for 15-20 minutes can achieve sterilisation, while dry-heat ovens need 170°C for two hours to accomplish the same result.

Understanding D-value, z-value, and F-value 🔗

Food microbiologists rely on three critical parameters to design safe thermal processes.

D-value (decimal reduction time) 🔗

The D-value is the time required at a specific temperature to reduce a microbial population by 90%, or one logarithmic cycle. For example, if a bacterium has a D-value of 1 minute at 72°C, then after 1 minute at that temperature, only 10% of the original population survives. After 2 minutes, just 1% remains, and so on. The D-value is a direct measure of an organism’s heat resistance at a given temperature-higher D-values mean greater resistance.

z-value 🔗

The z-value represents the temperature increase needed to reduce the D-value by a factor of ten. For Clostridium botulinum spores, the z-value is approximately 10°C. This means that raising the processing temperature by 10°C achieves the same level of microbial destruction in one-tenth of the time. The z-value allows processors to calculate equivalent treatments at different temperatures.

F-value 🔗

The F-value expresses the total lethality of an entire thermal process as the equivalent number of minutes at a reference temperature (usually 121.1°C). For low-acid canned foods, the standard safety target is a 12-D reduction of C. botulinum spores. Since the D-value of these spores at 121°C is about 0.21 minutes, a 12-D process requires an F₀ of approximately 2.52 minutes. This “botulinum cook” has ensured the safety of canned foods for over a century.

Factors affecting the effectiveness of thermal preservation 🔗

Applying heat alone does not guarantee food safety. Several factors influence how effectively thermal treatment destroys microorganisms.

Type of microorganism 🔗

Different organisms have vastly different levels of heat resistance. Vegetative bacterial cells are generally killed at relatively low temperatures (60-70°C), while bacterial endospores-particularly those of thermophilic species-are far more heat resistant and may require temperatures above 100°C for extended periods. Yeasts and moulds typically have heat resistance similar to vegetative bacteria, though ascospores of certain moulds like Byssochlamys and Neosartorya can survive pasteurisation temperatures and cause spoilage in fruit products.

Microbial load 🔗

The initial number of microorganisms in a food product directly affects the treatment needed. A product with a higher microbial load requires longer processing times or higher temperatures to achieve the same level of reduction. This is why good manufacturing practices-including proper raw material handling, hygiene, and cold chain management-are essential even before thermal treatment begins.

pH of the food 🔗

Acidity plays a major role in thermal processing requirements. Foods with a pH below 4.5 can be safely preserved with milder pasteurisation treatments (at or below 100°C), because dangerous spore-forming bacteria like C. botulinum cannot grow or produce toxin at such acidic levels. Low-acid foods (pH above 4.5)-including most vegetables, meats, and seafood-require sterilisation at temperatures above 100°C under pressure. This pH threshold of 4.5 is one of the most important dividing lines in food safety.

Water activity 🔗

Water activity (a_w) measures the amount of free water available for microbial growth. A decrease in water activity significantly increases the thermal resistance of microorganisms. This is because dry conditions protect cellular proteins from the denaturing effects of moist heat. Foods with high sugar or fat content, or those that have been partially dehydrated, often harbour microorganisms that are harder to kill with heat. Processors must account for this when designing thermal treatments for products like nut butters, chocolate, spice powders, and dried fruits.

Food composition 🔗

The physical and chemical makeup of food affects both heat transfer and microbial heat resistance. Fats, proteins, and sugars can have a protective effect on microorganisms, essentially shielding them from heat. Dense, viscous, or solid foods also present challenges for heat penetration-the centre of a large can of stew, for instance, takes much longer to reach the target temperature than a thin liquid like juice. This is why processors must carefully study the heat penetration characteristics of each product and container combination.

Time-temperature combination 🔗

The relationship between temperature and time is the foundation of every thermal process. Higher temperatures require shorter holding times, and lower temperatures require longer times to achieve the same microbial kill. Both parameters must be carefully controlled-under-processing leaves dangerous organisms alive, while over-processing degrades flavour, texture, and nutritional content. Achieving the right balance is both a science and a practical engineering challenge.

Why thermal preservation remains essential 🔗

Despite the emergence of newer technologies like high-pressure processing, pulsed electric fields, and cold plasma treatment, thermal preservation remains the backbone of the global food supply chain. It is well understood, backed by over a century of scientific research, and applicable to an enormous range of products-from canned tuna to pasteurised juice to shelf-stable milk.

Thermal processes also serve as a critical control point in Hazard Analysis and Critical Control Points (HACCP) systems used by food manufacturers worldwide. When properly designed and monitored, they provide a reliable safety margin against foodborne pathogens.

That said, no single method is perfect. Thermal treatment can reduce heat-sensitive vitamins (especially vitamin C and thiamine), alter flavour compounds, and change texture. Food scientists continue to refine time-temperature combinations and explore combination approaches-sometimes called hurdle technology-that pair mild heat with other preservation barriers like low pH, reduced water activity, or modified atmosphere packaging to achieve safety with less thermal damage.

Common applications at a glance 🔗

Dairy industry: Pasteurisation of milk, cream, and cheese milk using HTST or UHT methods to eliminate pathogens such as Salmonella, Listeria monocytogenes, and Campylobacter jejuni.

Canning industry: Sterilisation of low-acid vegetables, meats, soups, and seafood in retorts to destroy C. botulinum spores and other heat-resistant organisms.

Beverage industry: Pasteurisation and aseptic processing of fruit juices, teas, and flavoured beverages.

Frozen food processing: Blanching vegetables like peas, carrots, and broccoli before freezing to inactivate enzymes and maintain quality during storage.

Ready-to-eat meals: Commercial sterilisation or pasteurisation of pre-packaged meals for retail and institutional use.

What do you think? Given the trade-off between thorough microbial destruction and potential nutrient loss, how should food manufacturers decide on the right time-temperature combination for a new product? And as newer non-thermal technologies become more accessible, do you think thermal preservation will eventually take a back seat-or will it always remain the primary safety tool in food processing?

References

1. https://auctoresonline.org/article/thermal-processing-in-food-preservation-a-comprehensive-review-of-pasteurization-sterilization-and-blanching
2. https://bio.libretexts.org/Courses/Prince_Georges_Community_College/PGCC_Microbiology/10:_Control_of_Microbial_Growth/10.03:_Using_Physical_Methods_to_Control_Microorganisms
3. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/thermal-resistance
4. https://microbenotes.com/heat-treatment-of-food-preservation/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC9777713/
6. https://en.wikipedia.org/wiki/Z-value_(temperature)
7. https://www.sciencedirect.com/topics/engineering/decimal-reduction-time
8. https://www.fao.org/4/ac334e/ac334e02.htm
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC5742775/
10. https://www.interesjournals.org/articles/thermal-processing-a-key-method-in-food-preservation-112449.html