Pasteurisation Units (PU)

One pasteurisation unit is one minute held at 60 °C. Everything else follows from converting other temperatures into that currency at a factor of ten for each 7 C°, which makes this the general F-value relation with the brewing trade's two constants — a 60 °C reference and a z-value of 7 — packed in rather than entered. Ten minutes at 67 °C is exactly one z above the reference, so it is worth ten times its own length: 100 PU. Drop seven degrees the other way, to 53 °C, and twenty PU takes two hundred minutes. That steepness is why flash pasteurisation exists at all: fifteen to thirty seconds in a plate heat exchanger at 72 °C does what a tunnel needs twenty minutes of warm water to do.

Ordinary packaged beer is treated to somewhere between 15 and 30 PU, and the margin that represents is enormous, because the organisms that actually spoil beer — lactobacillus, pediococcus — are feeble against heat in a medium already acidic, alcoholic, hopped and anaerobic. Low- and no-alcohol products have lost most of those hurdles and need considerably more. The two errors that matter are both about measurement rather than arithmetic. T is the temperature of the beer at the slowest-heating point of the pack, not the spray-water temperature and not the zone setpoint; the difference is several degrees and many minutes, and it is where both over- and under-processing come from. And a real pasteuriser accumulates PU through every zone including come-up and cool-down, so the honest total is the sum over the whole temperature history — what a PU logger travelling with the pack reports — while this page gives one segment at one temperature. Over-pasteurising is not free either: heat above 60 °C accelerates staling, so the target is a floor to clear rather than a score to maximise. None of this is a clinical or medical calculation, and beer is not a low-acid canned food; the botulinum question that governs canning does not arise here.