Pasteurization is a thermal process developed in the 19th century to improve food safety and extend shelf life by inactivating pathogenic and spoilage microorganisms. Unlike sterilization, it is applied below 100°C and does not eliminate all microbes, focusing instead on those harmful to health or quality. The method helps preserve the sensory and nutritional characteristics of food.

Historical Development

The origin of pasteurization has a history dating back centuries in food preservation techniques. For example, records show that wine was heated in China in 1117 to prevent spoilage, while similar methods were used in Japan in the 15th century. In the late 18th century, Italian scientist Lazzaro Spallanzani suggested that airborne microorganisms played a role in spoilage by demonstrating that boiled and sealed broth did not spoil. In 1795, French confectioner Nicolas Appert laid the foundation for pasteurization by developing the method of preserving foods by boiling them in glass jars (canning). The birth of modern pasteurization, however, occurred with Louis Pasteur's work in 1864.

While investigating the spoilage of wine and beer, Pasteur discovered that these processes were not a chemical reaction but resulted from the metabolic activity of microorganisms (especially yeasts and bacteria). He succeeded in killing pathogens and preventing acidification by heating wine to 50-60°C, naming this method “pasteurization”. Pasteur's discovery formed the basis of the "Germ Theory," which revealed that microbes cause diseases and led to discoveries in food safety and medicine.

Milk pasteurization, on the other hand, was proposed by German chemist Franz von Soxhlet in 1886 and became widespread in the early 20th century. In 1908, Chicago passed the first law mandating milk pasteurization, thus preventing milk-borne diseases such as tuberculosis, brucellosis, and typhus. Over time, along with technological advancements, high-temperature short-time (HTST), ultra-high temperature (UHT), and other methods have been developed.

Purpose of Pasteurization

The International Dairy Federation (IDF) defines pasteurization as follows: “Pasteurization is a heat treatment process compatible with minimum changes in the chemical, physical, and organoleptic properties of the product, to eliminate public health risks arising from pathogenic microorganisms associated with milk.” This definition summarizes two main objectives of pasteurization:

1. Public Health: To prevent foodborne illnesses by eliminating pathogenic microorganisms found in foods (e.g., Mycobacterium tuberculosis, Brucella abortus, Coxiella burnetii, Salmonella spp., Escherichia coli O157:H7, Listeria monocytogenes).

2. Shelf Life Extension: To increase the durability of food by inactivating spoilage-causing enzymes (e.g., lipase, protease) and vegetative microorganisms (e.g., Streptococcus spp., Lactobacillus spp.).

Pasteurization differs from sterilization, which aims to eliminate all microorganisms; bacterial spores (e.g., Clostridium botulinum spores) generally survive. Therefore, pasteurized products must be kept under appropriate storage conditions (usually at 4°C) and consumed by their expiration date.

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Types and Techniques of Pasteurization

Pasteurization is divided into different types based on the combinations of temperature and time applied. Below, the most common methods used for milk pasteurization are explained with scientific details:

• Vat Pasteurization (LTLT - Low Temperature Long Time):
• Process: Milk is heated in a closed vat at 63°C for 30 minutes and then rapidly cooled.
• Equipment: A temperature-controlled vat is used; stirring is critical for even heat distribution.
• Target: To eliminate heat-resistant pathogens such as Mycobacterium tuberculosis.
• Advantages: Suitable for small-scale businesses and cultured products like yogurt and cheese; preserves the sensory properties of the product.
• Disadvantages: It is a slow process, has low energy efficiency, and is not practical for large-volume production.
• Shelf Life: 2-3 weeks under refrigerated conditions.

• High Temperature Short Time (HTST - Flash Pasteurization):
• Process: Milk is held in a plate heat exchanger at 72°C for 15 seconds and rapidly cooled.
• Equipment: Includes a continuous flow system, plate heat exchanger, holding tube, and flow diversion valve.
• Target: Achieves a 5-log reduction by eliminating 99.9% of pathogens.
• Advantages: Energy-efficient, ideal for large-volume production, provides minimal taste change in milk.
• Disadvantages: Requires more complex equipment and is costly.
• Shelf Life: 2-3 weeks under refrigerated conditions.

• Ultra Pasteurization (UP):
• Process: Milk is heated at 138°C for 2 seconds and rapidly cooled; sterile equipment is used, but hermetic sealing is not performed.
• Advantages: Extends shelf life to 30-90 days, but requires refrigeration.
• Disadvantages: Up to 20% loss in nutrients like vitamins A, D, and E can occur; taste change is noticeable.
• Shelf Life: 30-90 days under refrigerated conditions.

• Ultra High Temperature (UHT):
• Process: Milk is heated at 135-150°C for 2-5 seconds and hermetically packaged under aseptic conditions.
• Equipment: Commercial sterile equipment and aseptic packaging system are used.
• Advantages: Destroys all microorganisms, including spores; provides a shelf life of 6-9 months at room temperature.
• Disadvantages: Loss of nutritional value (especially B vitamins), taste change, and high energy consumption.
• Shelf Life: 6-9 months unopened.

Note: For dairy products with a fat content over 10% or products with added sugar, the temperature is increased by 3°C (e.g., 75°C for HTST).

Effect on Microorganisms

Pasteurization relies on the thermal death kinetics of microorganisms. Heat denatures bacterial enzymes (e.g., proteins that enable metabolic reactions) and weakens the cell membrane, leading to an increase in internal pressure, which causes the cell to burst. Below is a list of the main microorganism groups eliminated by pasteurization:

• Acid Producers: Streptococcus spp., Lactobacillus spp., Microbacterium spp., Coliforms, Micrococcus spp.
• Gas Producers: Coliforms, Clostridium butyricum, Torula cremoris
• Ropy/Slime Fermentation: Alcaligenes viscolactis, Enterobacter aerogenes
• Proteolytic Organisms: Bacillus spp., Pseudomonas spp., Proteus spp., Streptococcus liquefaciens
• Lipolytic Organisms: Pseudomonas fluorescens, Achromobacter lipolyticum, Candida lipolytica, Penicillium spp.
• Pathogens: Mycobacterium tuberculosis, Coxiella burnetii, Salmonella spp., Listeria monocytogenes, Escherichia coli O157:H7

Bacterial spores (e.g., Clostridium botulinum) are heat-resistant and thus not eliminated by pasteurization; therefore, methods requiring higher temperatures, such as UHT, are used.

Thermal Death Kinetics

• D-Value: The time required to destroy 90% of a microorganism population at a specific temperature (e.g., the D-value for Coxiella burnetii at 72°C is approximately 0.07 minutes).

• Z-Value: The temperature increase required to change the D-value by a factor of 10 (e.g., typically 5-10°C for bacteria).

Technological Developments

Pasteurization techniques have evolved significantly in the 20th century:

• 1941: Pyrex heat-resistant glass tubes began to be used for material savings during wartime.
• 1955: The first automatic CIP (Clean-in-Place) system was installed in Ohio.
• 1956: Due to the heat resistance of Coxiella burnetii, the minimum temperature for vat pasteurization was raised from 142°F to 145°F.
• 1978: The first UHT "sterile" milk system was introduced in the USA.
• 1979: Magnetic flow meters replaced traditional timing pumps.
• Modern Innovations: Non-thermal methods (High Pressure Processing - HPP, Pulsed Electric Field - PEF, Microwave Volumetric Heating - MVH) have been developed; these methods reduce nutrient loss.

Pasteurization Parameters for Different Food Types

Pasteurization is a widely used thermal processing method aimed at ensuring microbial safety and extending the shelf life of various food products. However, since each food item differs in its physical and chemical properties, the temperature and duration of pasteurization must be specifically adjusted for each product. For example, fruit juices generally have low pH values, which naturally inhibit the growth of many pathogenic microorganisms. Nevertheless, spoilage-causing yeasts, molds, and some heat-resistant bacteria can still survive in such acidic environments. Therefore, fruit juices are typically subjected to High Temperature Short Time (HTST) pasteurization, where they are heated to temperatures between 71°C and 82°C for durations ranging from 0.3 to 90 seconds. In products like apple juice, a 5-log reduction in pathogens is targeted, using time-temperature combinations such as 71°C for 6 seconds or 77°C for 1.3 seconds. For some tropical fruit juices, longer treatments such as 85°C for 30 seconds may be preferred. In contrast, products like milk, which have a near-neutral pH and offer a more favorable environment for microbial growth, require more precise thermal treatment. HTST pasteurization at 72°C for 15 seconds is commonly used for milk, balancing microbial inactivation with the preservation of nutritional value. In conclusion, pasteurization parameters must be carefully determined based on the product’s pH, viscosity, target microorganisms, and desired shelf life.

Advantages and Disadvantages

Advantages

• Prevents diseases such as tuberculosis, diphtheria, and brucellosis by eliminating pathogens.
• Extends shelf life (2-3 weeks with HTST, 6-9 months with UHT).
• Preserves the sensory properties of food more effectively than sterilization.
• Facilitates global food trade.

Disadvantages

• Can cause vitamin (B2, C, A, D, E) and enzyme losses (e.g., up to 20% loss in UHT).
• Changes in taste and texture (especially "cooked" taste in UHT).
• Also eliminates beneficial microorganisms (probiotics like Lactobacillus).
• It may require additional preservation methods as it does not kill spores.

Nutrient Losses

While pasteurization ensures microbial safety, it can also lead to measurable losses in certain nutrients. These losses vary depending on the applied temperature, duration, and the composition of the food. Vitamin C, in particular, is one of the most heat-sensitive nutrients. In fruit juices, vitamin C loss due to pasteurization generally ranges between 10% and 50%. For instance, in orange juice subjected to pasteurization at 90 °C for 30 seconds, approximately a 40% reduction in vitamin C content has been observed. B vitamins such as folic acid (B9) and thiamine (B1) are also affected by heat, with losses typically reported between 10% and 20%. In contrast, in milk pasteurized using the HTST method (72 °C for 15 seconds), vitamin C loss averages around 20%, folic acid around 15%, and thiamine around 10%. Proteins may undergo denaturation, but the overall nutritional loss is limited to approximately 0% to 5% in terms of biological value. Fat-soluble vitamins (A, D, E, and K) are generally stable during pasteurization, showing only minor reductions in the range of 0% to 5%. Minerals (such as calcium, potassium, and magnesium), being highly resistant to heat, experience negligible losses during the process. Overall, the HTST pasteurization method is more favorable in terms of nutrient retention. However, in processes involving higher temperatures—such as UHT (110–138 °C for 2–8 seconds)—losses in sensitive vitamins become more pronounced. Considering these numerical values, pasteurization improves food safety while largely preserving nutritional quality.

Non-Thermal Pasteurization Methods: Overview and Commercial Applications

Non-thermal pasteurization methods are technologies developed to ensure microbial safety in food while minimizing the nutrient, color, and aroma losses typically caused by thermal processing. Among these methods, High Pressure Processing (HPP) and Pulsed Electric Fields (PEF) are particularly prominent.

High Pressure Processing (HPP)

HPP is based on subjecting food to pressures typically ranging from 100 to 600 MPa (megapascals) for durations from a few seconds to several minutes. This method inactivates microorganisms such as bacteria, yeasts, and molds without altering the shape or composition of the product. Since the process is usually carried out at ambient temperature, it is also referred to as “cold pasteurization.” Commercially, HPP is widely applied to fruit juices, ready-to-eat meals, deli products (such as sliced meats), guacamole, seafood, and yogurt. It has been documented to extend shelf life by 2 to 10 times and can inactivate pathogens by up to 99.9% (3–5 log reduction). Regulatory agencies such as the FDA and EFSA recognize HPP as equivalent to pasteurization for certain products.

Pulsed Electric Fields (PEF)

PEF technology involves applying short-duration (microsecond), high-voltage (10–80 kV/cm) electric pulses to the food product. These pulses cause increased permeability in microbial cell membranes (electroporation), leading to inactivation. PEF is especially suitable for liquid foods such as fruit juices, milk, and liquid eggs. The thermal effect is minimal, which helps preserve the sensory and nutritional quality of the product. In practice, electric field intensities of 20–60 kV/cm, pulse durations of 1–5 microseconds, and total treatment times of a few hundred microseconds are typical. PEF is commercially used by fruit juice brands in Europe and has demonstrated microbial reductions of up to 5 logs.

Effectiveness and Limitations

Both technologies are capable of achieving microbial inactivation levels comparable to traditional pasteurization. However, due to limited effectiveness against spore-forming bacteria, they are generally not sufficient on their own for low-acid foods. HPP is more commonly used for packaged products, while PEF is preferred for continuous processing of fluid foods. High initial investment costs, the risk of texture changes in certain products, and a lack of widespread standardization are factors that currently limit the broader adoption of these technologies.

Pasteurization is a revolutionary technique for food safety and public health. Since Louis Pasteur's discovery in the 19th century, it has ensured the safe consumption of milk, fruit juice, beer, and other foods, dramatically reducing foodborne illnesses. Different methods (Vat, HTST, UP, UHT) have been optimized according to the product type and desired shelf life. However, side effects such as nutrient loss and taste alteration highlight the limitations of pasteurization. Although modern technologies (HPP, PEF) aim to reduce these disadvantages, pasteurization continues to be one of the cornerstones of the food industry today. While scientific data proves the effectiveness of this technique, it also indicates that its use should be evaluated in a balanced manner.