Shelf Life of Pasteurized Fluid Milk
There is a well-known fact in the dairy industry; milk spoils if you keep it too long. Pasteurized fluid milk products (e.g., 161°F/15 sec or 145°F/30 min) are perishable commodities that must be processed and handled with care to maintain quality up to and beyond their “code” or sell-by dates, the dates that consumers count on to ensure freshness. There is another well-known fact; changes in retail distribution and consumer expectations require that processors provide milk that lasts longer. Milk code dates used by the dairy industry 30 years ago averaged 12± days; today 18 days is common with some manufacturers stretching to 21 and beyond.
Ensuring that pasteurized milk products maintain acceptable quality over its shelf-life is a challenge. While milk is susceptible to a number of non-microbial defects (e.g., light induced), spoilage and reduced shelf-life is most often due to bacterial growth. Defects such as bitter, fruity, rancid or unclean typically become apparent when bacteria counts exceed 1-10 million per milliliter. Unless milk is improperly stored, bacteria that spoil milk are psychrotrophic or “psychrotolerant” in nature, which means they are able to grow under refrigeration. Generally, bacteria that spoil milk can be classified into two categories based on point of contamination and bacterial classification based the gram-stain reaction; gram-negative psychrotolerant bacteria that contaminate milk after the pasteurization process (post-pasteurization contamination) and gram-positive psychrotolerant bacteria that survive pasteurization.
When milk shelf-life is dramatically reduced, post-pasteurization contamination (PPC) with gram-negative psychrotolerant bacteria is most often to blame. Strains of Pseudomonas are commonly implicated, but other gram-negatives including psychrotolerant strains of coliform bacteria (i.e., Citrobacter, Enterobacter, Rahnella) may be the predominant spoilage microbe. As a rule, gram-negative bacteria do not survive pasteurization, thus when found in pasteurized milk products, some level of PPC has occurred. Preventing PPC requires effective cleaning, sanitization and preventative maintenance programs. Quality assurance programs that reduce the risk of PPC should consider:
- System/equipment construction and design to facilitate effective cleaning/sanitization.
- Cleaning/sanitization parameters with support from a reputable chemical supplier:
- effective and timely rinses, pre- and post-wash
- appropriate chemicals, temperatures, physical action and time, both manual and clean-in-place (circulation) cleaning
- proper sanitizer strength, application and coverage
- Preventive maintenance and inspection, scheduled and implemented:
- pasteurization systems, pasteurized regen section forward
- vacuum breakers, throttling valves
- pipeline gaskets, especially swing pipes
- manual valves (3-way, most butterfly designs not recommended)
- air valves; O-rings, seats, CIP pulsation
- milk pumps; gaskets, packings
- milk tanks; spray balls, agitators, agitator shafts
- fillers; bowls, sensors, valve assemblies, screens (autoclaved)
- Monitoring, corrections and records; for all preventive activities
- Training; for all personnel, all activities
One more fact; it only takes one bacterium per container to cause milk spoilage. PPC from one bad gasket can mean a bad shelf-life day. At refrigeration temperatures of 40-44°F some psychrotolerant strains can divide every 6-8 hours (i.e., generation time). At this growth rate a single bacterium in a quart of milk would grow to exceed 10 million/mL within 11 days. Keeping milk colder extends the time for bacterial numbers to hit spoilage levels (e.g., at 34-36°F, generation times might exceed 24 hours taking 30+ days to 10 million), but the goal should be to minimize PPC.
When PPC is prevented, psychrotolerant gram-positive bacteria capable of surviving pasteurization have the potential to grow and cause spoilage in milk. Some bacterial genera form dormant structures known as endospores or “spores,” that are more resistant to heat and other extremes than vegetative cells. While non-spore-forming psychrotolerant gram-positive bacteria have been isolated from pasteurized milk products, spore-forming strains appear most often. This is especially true in milks pasteurized at higher temperatures that tends to select for the more heat resistant spore-formers. While older literature lists strains of Bacillus (e.g., B. cereus, B. coagulans) as the most common psychrotolerant spore-formers (PSF) isolated from milk, bacterial nomenclature has changed and Paenibacillus species are currently considered as the predominant PSF in milk (Paenibacillus is a relatively new genera under which some Bacillus were reclassified).
Milk spoilage due to outgrowth of PSF generally occurs later in shelf-life (i.e., 14-21+ days) than is observed for gram-negative PPC. This may be due to lower contamination levels, extended lag times before growth commences and/or longer generation times. PSF are common in the dairy farm environment (i.e., soil, feeds, manure). Raw milk contamination likely occurs during milking due to soiled udders, although other farm management factors may be involved. Improperly washed milking equipment, milk tank trucks, and plant raw milk storage and handling should be considered. It is also possible that PSF and other gram-positive bacteria may occur as PPC. As higher pasteurization temperatures select for spore-formers, higher temperatures may also result in more rapid outgrowth of PSF. This was demonstrated in research from 1970-80’s and in more recent studies based on processor complaints of reduced shelf-life after following FDA recommendations to pasteurize at 177°F for 22.5 seconds or equivalent as a food defense precaution. This recommendation has since been rescinded.
As with bacteria associated with PPC, it only takes one PSF per container to eventually cause spoilage. Improving farm practices (e.g., pre-milking hygiene; cow environment; equipment cleaning) may help reduce levels of PSF in raw milk on a farm level but achieving low enough levels in the supply chain to consistently reduce the risk of PSF spoilage would be challenging. At the process level, bacterial clarification (e.g., Tetra-Pak Bactofuge or GEA Bacterial Clarifier) and microfiltration have been investigated and used to reduce spore levels in milk supplies. Efficiencies of clarification systems rely on set up and incoming spore loads. Microfiltration is effective in skim milks whereas the cream needs to be processed separately. Investment in these technologies would only be of value in systems designed to thoroughly eliminate the risk of PPC (e.g., steam sterilization and clean fillers used for UP milks), as PSF are inconsequential if gram-negative PPC occurs. As with PPC, keeping milk colder reduces the risk of PSF spoilage. All said and done, minimizing PPC is a must and at this point limiting sell-by dates (e.g., < 21 days) seems to be the best bet to control PSF spoilage and keep milk “fresh.”
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