Lesson 4 - Clarification – Centrifugation

At the end of fermentation and maturation of beer, there may be varying quantities of yeast and tank bottoms. In the majority of cases, there is likely to be a significant proportion of beer entrained in the tank bottoms, which we can recover using a well-managed process.

As breweries are under greater pressure to reduce operating costs, breweries may look at beer recovery methods, if their brewing philosophies and standards permit. This may reduce the cost per hL to produce the beer. However, to assess whether this is financially beneficial, a brewer would need to consider the cost of centrifuge installation and its operating costs.

Let us consider a typical example. In a brewery, the tank bottom volume in the maturation tank amounts to 4% of the total annual output. From this figure, approximately 60% can be recovered in the form of beer. So, for a brewery with an output of one million hL, a total volume of 24,000 hL could be recovered. If a brewery is brewing to capacity, or near capacity, we can increase the brewery’s yield by recovering beer via centrifugation. This in turn delays the capital expenditure that would be used to install new tanks.

As with any additional processing step in the brewery, there are always risks and opportunities. How does beer recovery affect final product quality? What else should we be mindful of?

Opportunities

• Reduced beer losses. Centrifuging tank bottoms will increase the yield from maturation tanks by recovering entrained beer from the yeast.

• Beer recovery reduces biological oxygen demand (BOD) and chemical oxygen demand (COD), which in turn reduces brewery effluent charges.

• Reducing effluent loading improves helps the site to meet its sustainability targets, as the brewery is producing less waste.

• A brewery can either create or boost an income stream with the additional yeast from the maturation tank bottoms. This can be sold as a vitamin-rich dietary supplement or as animal feed.

• Beer can be recovered from these various sources in several ways, depending on the yeast’s flocculation properties. Highly flocculant yeast may create more compact tank bottoms with a higher solids content. A decanter centrifuge may be used in this situation. However, where loading is lighter from the use of a non-flocculant yeast, we could use membrane or diaphragm filter presses, or indeed other types of filters.

Risks

• Using an additional remedial process in a brewery poses a microbiological stability risk. We must hygienically design process equipment and pipework in order to minimise infection risks.

• If recovering beer, there should be a minimum residence time for maturation tank bottoms (feedstock) and recovered beer, to ensure microbiological stability and again reduce infection risks.

• Limiting dissolved oxygen (DO) content is critical to a recovered system. It reduces the risk of oxidation in process, which otherwise would create a staling character in the beer. All beer mains and equipment should be flushed with deaerated water before introducing beer into the system.

• Recovered beer should be blended back across batches to help minimise oxidation and any adverse effects upon the sensory profile. Blending back may occur anywhere from whirlpool all the way through to filter at various percentages, depending on a brewery’s blending rules.  Holding recovered beer may affect the brewery’s capacity as you will need to have a dedicated ‘recovered beer’ tank for blending. This tank would otherwise be used for beer maturation.

• Clarity and haze stability are another consideration. The need for clarity after recovery depends upon subsequent processing and blending. If the recovered beer is added to primary production beer during transfer to aging, further clarification occurs downstream. If the recovered beer is added later in the process, you may need to filter it before pasteurisation and blending.

• Other quality parameters of recovered beer may be affected. These include colour and pH. This is one of the main reasons for blending recovered beer across beer batches. Colour and pH changes are generally insignificant and any slight changes are compensated by blending.

• Flavour stability has a very significant impact on product quality. Tank bottoms will contain dead yeast. Any autolysed cells (ruptured cells) will affect flavour stability when the beer is recovered. Flavour changes include ‘yeast bite’. This creates a sharp, bitter taste with a meaty and sulphury off-flavour, which is caused by the discharge of amino acids and nucleotides present in yeast. Equally, lipids released from autolysed cells may create a meaty aroma, associated with rancid stale beer. A sensory analysis of blended beers will indicate any changes in beer characteristics and ensure the batch is ‘true to type’ before sign-off for packaging.

• Another significant risk is foam instability. Autolysed yeasts release protease enzymes from dead cells, which leak from the cell and then digest the foam positive proteins in beer. This reduces head retention, causing a rapid collapse of foam and the impression of a flat beer. 

So, we can see that the ‘opportunities’ are driven by financial benefits and the ‘risks’ are driven by quality implications. Therefore, if a beer recovery system is to be implemented successfully, a brewer must ensure the quality risks can be managed and mitigated as much as possible in order to reap the subsequent financial rewards.