What Is Ultra High Temperature Processing?
UHT (ultra-high temperature) processing is available in many forms. I will focus this article on the direct heating method of steam infusion and a comparison to the conventional HTST pasteurizer. The goal of any milk pasteurization process is to kill pathogenic bacteria while preserving the integrity of the product for the satisfaction of the consumer. While many opinions on milk treatment are widely available, I'll express mine for the benefit of the UHT process with aseptic filling.
In a UHT steam infusion process, milk is produced by exposing the product to high temperatures (>280ºF) in a mixture of steam and milk under pressure for about 2 seconds while maintaining sterile conditions in the processing system. This differs from the conventional HTST pasteurizer requirements of holding milk >161ºF for 15 seconds, as shown in the graphic.
HTST: Microbial Contamination and Expiration Dates
A conventional HTST pasteurizer will kill the majority of the bacteria. However, post-production testing across the shelf life of the product will show that microbial contamination will flourish, and this may surprise you. This is usually due to bacteria surviving pasteurization and post-process contamination from environmental pathogens. Post-processing contamination cannot be avoided unless specialized aseptic filling equipment is used.
Government regulations allow up to 20,000 colony-forming units per mL of milk. Microbial growth is almost always present in the milk packages produced from a conventional pasteurizer. This is the reason that a conventional pasteurizer will produce milk that will be good for only 14-21 days under refrigeration. The microbial load just gets too high after this time, and sensory degradation accelerates, and you soon have spoiled soured milk.
UHT: Microbial Contamination and Expiration Dates
The high-temperature treatment >280-290ºF will kill virtually all bacteria, yeasts, and molds that would spoil the milk. This ultra-pasteurization process is the processing step required before filling into packages. The milk is usually filled into two different formats after processing, detailed below:
- Extended shelf life (ESL) milk products (usually found in 1 quart to 1/2 gallon sizes) are packaged in an environment that poses a minor risk of post-processing contamination. Typical expiration dates of 60-90 days are given to these products, and the milk must be kept refrigerated.
- When the UHT process is coupled with aseptic filling equipment, there is little risk for post-processing environmental contamination, and the expiration date on these products can be 6-12 months at room temperature in your pantry or on the store shelves. While government regulations still allow up to 20,000 colony-forming units per mL of milk—you won't find any microbial growth, even at the expiration date. One reason they don't extend the life of these products is that the sensory characteristics slowly degrade over time, not because of product spoilage but due to bacteria.
Sensory Characteristics HTST vs. UHT
One sensory complaint of UHT processed milk is a "cooked flavor." While it can be classified as an off-flavor as compared to HTST or raw milk, I personally feel that the milk has a better flavor profile and is more appealing to the senses. It is the high temperature of the process that imparts this cooked flavor to the milk.
Packaging pasteurized HTST milk into semi-transparent milk jugs allows for the fluorescent lighting to impart an oxidized flavor into the milk, which is not an appealing flavor addition. This problem will not be encountered in aseptically packed milk as these packages are designed to be light-resistant.
Benefits and Disadvantages
UHT processed and aseptically packaged milk benefits:
- Shelf life is typically 6-12 months.
- Storage of the milk can be at ambient temperatures. Requires no refrigeration. Great for survival kits.
- Bacteria will not grow in the product until the hermetic seal on the package is broken.
- The average cost is higher.
- Sensory characteristics are different than conventionally pasteurized HTST milk. Expect a more "cooked" flavor.
A Bit About Regulatory
Any milk production plant in the United States is required to follow the lengthy federal regulations outlined in the Pasteurized Milk Ordinance. These rules include plant inspections, product testing, and pasteurizer calibrations, among other rules. Grade A milk is one of the most highly regulated conventional food products. Dairy food plants are required to implement a HACCP (Hazard Analysis and Critical Control Points) plan to protect milk products from potential process hazards.