Lesson 9 - Yeast Management SAQ

** A yeast handling plant requires extensive cleaning, using various detergents and sanitisers. It also requires a CIP plant to ensure the yeast vessels and pipe work remain clean to prevent the contamination of the brewery culture yeast. Do you think that if all the yeast is disinfected with acid prior to pitching, it is possible to save money by reducing the frequency and rigour of cleaning regimes of brewery yeast handling plant?

Response:

It is true that some brewers use acid washing as a means of disinfecting the pitching yeast. The process requires the pH of yeast slurries to be reduced to pH 2.2 (+/- 0.1) using a food-grade acid, usually phosphoric. The treatment must be very carefully controlled so that the temperature is held at 3 ºC (+/- 1 ºC) and the exposure time must not be longer than two hours. The addition of the acid must be controlled properly to ensure efficient mixing and prevent any ‘hot spots’.

Providing these rules are strictly followed, all will be well, and yeast microbiology can be controlled effectively. But is this true? The answer is not really.

a)    The treatment will kill potential spoilage bacteria since these are less acid tolerant than yeast. However, by definition, it will not kill spoilage yeast.

b)    The effect of the acid treatment on yeast is relative. If the yeast is stressed, it is not immune to the effects and significant viability losses may occur. For this reason, it is not advised to acid wash yeast that has a viability lower than 90%.

c)    Where acid washing is used, it is often poorly controlled. Failure of any of the process specifications given here will result in the treatment being ineffective (pH too high), or harmful to the yeast (time, temperature, too long a treatment). The last point is especially important. Acid washing is not usually terminated by neutralisation; instead, the exposure to acid conditions ends when the yeast is pitched into wort and the acid diluted in the wort. This means that the process has to be started at the right time in relation to getting the fermenter ready. A common problem is when the acid washing has started and then there is a delay in wort production. There may be a temptation to allow the acid treatment to carry on for longer but if it is a lot longer, this is a mistake.

The final discussion point is one of the philosophy of hygiene control. Prevention is always better than cure and it is always far preferable to work to ensure that hygiene standards are always very high and that contamination does not occur. It is not acceptable to let standards slip and rely on acid washing as a sticking plaster which will cover up bad yeast management practices.