Pasteurization is a thermal process that applies mild heat to foods and beverages to increase safety and extend shelf life. This technique involves heating a product to a specific temperature for a set duration, which reduces pathogenic microorganisms without significantly altering the product’s quality. Developed by French microbiologist Louis Pasteur in the 19th century, the process was first used to prevent wine and beer from souring due to microbial contamination.
The Core Safety Mechanism
The effectiveness of pasteurization is rooted in the targeted destruction of vegetative pathogenic bacteria. This thermal treatment is designed to achieve a minimum 5-log reduction, destroying 99.999% of the target microbial population. For milk, standards are calculated to eliminate Coxiella burnetii, the most heat-resistant non-spore-forming pathogen commonly found in raw milk.
The microbial destruction follows a logarithmic pattern, known as the thermal death curve, where a constant percentage of microorganisms is killed. This process also inactivates many spoilage enzymes, such as lipase, which cause rancidity and flavor defects over time. Pasteurization is not sterilization, and heat-resistant bacterial spores, such as those from Bacillus species, can survive the treatment. Therefore, the product must be refrigerated afterward to prevent the growth of these remaining spoilage organisms.
Applying Time and Temperature to Foods
Pasteurization processes are tailored to the specific heat tolerance of the target pathogen within a particular food matrix, resulting in various time and temperature combinations.
The High-Temperature Short-Time (HTST) method is the most common for liquid foods. It typically heats milk to 72°C (161°F) for 15 seconds, or juice to 71°C (160°F) for the same duration. This rapid process uses plate heat exchangers to maximize efficiency for continuous processing, resulting in a refrigerated shelf life of two to three weeks.
A less aggressive method is Low-Temperature Long-Time (LTLT), also known as batch pasteurization. This method heats products to a lower temperature, such as 63°C (145°F), but holds it there for 30 minutes.
For liquid eggs, which are susceptible to Salmonella contamination, parameters must be carefully controlled to prevent protein coagulation. Whole liquid eggs are typically heated to 60°C (140°F) for 3.5 minutes. Egg whites require a slightly lower temperature of 56°C (133°F) for the same time to avoid damaging their functional properties.
The most intensive method is Ultra-High Temperature (UHT) processing. This heats products like milk to 135°C to 140°C (275°F to 284°F) for only two to five seconds. This extreme heat combination achieves commercial sterility, resulting in a product that is shelf-stable for several months without refrigeration until the package is opened.
Nutritional and Sensory Impacts
The heat applied during pasteurization has a minimal effect on the product’s macronutrients and overall mineral content. Proteins, fats, and carbohydrates remain largely unchanged, and major minerals like calcium and phosphorus are not degraded by the thermal process. However, some heat-sensitive, water-soluble vitamins experience degradation depending on the treatment intensity.
Vitamins C (ascorbic acid) and certain B vitamins, particularly B12, are the most susceptible to heat and oxidation. Losses vary widely based on the specific method used; HTST processing causes minimal loss. More intensive processes like UHT can lead to greater losses of B-group vitamins and Vitamin C, which is also diminished by the presence of oxygen during heating.
The sensory qualities of the food are also impacted. HTST causes very little change in flavor or color. UHT processing can denature whey proteins in milk, releasing sulfur compounds that result in a noticeable “cooked” or caramelized flavor and a subtle change in color.