Is your spoilage microbiological or physical.
(The heat-resistant thermophiles, such as B. stearothermophilus and B. licheniformis. They can cause the ‘flat sour’ defect in UHT milk, which is characterised by acid production, but no gas production, could be an issue with non-gas forming thermoduric spore-formers.)
"If these microorganisms are producing slight levels of acid but more importantly some levels of proteolysis this could cause sufficient destabilization of the casein micelle such that on heating the product further exposure to the inversely soluble ionic calcium might result in coagulation"
It is very likely that this issue is not directly a result of the process or the packaging. It is most likely to be traced back to the initial milk supply. Some sampling of the milk supply to identify the potential source of the thermoduric spore-formers is the most likely avenue towards finding a resolution to this quality issue.
Good quality raw milk is essential for producing UHT milk with a long shelf life. In general, milk destined for UHT processing should be stored refrigerated (<5◦C) for no more than 24 h. Storage at higher temperatures and/or for longer times promotes the growth of psychrotrophic bacteria, which cause the production of lactic acid, reduction of the pH of the milk and also production of enzymes, notably proteases and lipases that can have considerable heat stability.
When the pH is reduced to ≤6.5, milk becomes unstable to heat. The UHT process destroys all vegetative bacteria and most sporeformers but does not inactivate some of the enzymes produced by psychrotrophic bacteria, such as Pseudomonas species, i.e. the most common bacterial contaminants of raw milk. Such enzymes are typically produced when the bacterial count exceeds106 cfu mL−1. If milk with such bacterial counts is UHT processed, these enzymes, particularly proteinases and lipases, can remain active in the UHT milk. Since UHT milk is usually kept at room temperature and may be stored for several months, even traces of these enzymes can produce noticeable changes, and result in bitter flavour and gelation (due to proteinases) .
Most likely the source of the high resistant spores (HRS) is the raw milk. However, biofilm formation by HRS on heat exchangers, pasteurizers, and gaskets is also very common.
Visual inspection of the production line for formation of deposits/biofilm should be checked.
Swabs should be taken from the production line to identify/exclude sources of contamination other than raw milk.
Bacteriological analysis of milk samples at all production steps especially the intermediate tank /aseptic tank prior to filling.
Milk should be held for at least 10 days after manufacturing. Samples should be stored at 30 and 55 C. During this period, any slight drop in pH (even 0.1 unit) or change in taste will indicate microbial growth.