Continuous fermentation

Ken writes:

I was reading a history of New Zealand beer and brewing over the holidays and it made me realise that I know very little about one of the innovations of New Zealand Morton Coutt’s continuous fermentation system.
(My holiday reading)

(Pictures of Morton Coutts)

Continuous fermentation (CF) systems differ from traditional batch methods in that, instead of fully fermenting out one batch of beer before starting on the next one, wort is added at one end and green beer taken out at the other at a constant rate. Once up and running, CF systems can be maintained at a steady state producing good quality extremely consistent beer for months at a time.

There are various different CF system designs. CF systems are divided into open systems that allow yeast to flow out of the system and closed systems that as much as possible retain yeast in the system. I gather that it is difficult if not impossible to have a completely closed system that preserves yeast viability and remains at a steady state. This might change if the CF system exploits immobilised yeast technology (the subject of a future post).

CF systems can also be classified according to whether the fermentation conditions are the same throughout, in which case the system is said to be homogeneous, or not. I gather that heterogeneous systems are better from a brewing point of view, because the fermentation must replicate as closely as possible the results of traditional batch fermentation, if it is to produce beer that matches traditional beer in terms of quality and taste, and traditional fermentations are heterogeneous because the physical parameters of the wort change over the course of fermentation. For example, the wort sugar profile changes during the course of fermentation. Yeast consume all the available glucose in the wort before starting on the maltose. There can be a build up in maltose in a homogeneous fermentation system when yeast prefer the glucose of incoming fresh wort to the maltose of wort already in the system. The minor products of fermentation (esters and higher alcohols etc), which are so important to the characteristic taste of particular beers are very sensitive the changing physical parameters of the wort during traditional fermentation (e.g the changing oxygen level). A CF-system can be semi-heterogeneous if it has a connected serious of vessels that are maintained at different fermentation conditions (yeast concentration, gravity levels etc). Morton Coutts’ system was semi-heterogenous like this (see below).

One example of an elegant single vessel almost closed heterogenous CF system is the tower fermenter. This system was apparently briefly used by Bass in the 1970s. The tower was 8.5m high and 1m in diameter. It could fully attenuate an an ale in 3-4 hours.
(Image from Boulton and Quain, 2001)
In a typical tower fermenter, there is a gradient of yeast and a gradient of wort gravity going up the tower (making the tower fermenter a heterogenous system). The tower relied on a very flocculent yeast which formed a thick plug at the bottom of the tower. The very high concentrations of yeast at the bottom of the tower quickly consumed the available sugars in the wort. Green beer leaving a typical tower fermenter contained 0.5% yeast. The high concentration of yeast meant the beer fermented out very quickly 4 hours for an ale or 8-12 for a lager. For comparison, Budweiser has a fermentation time of 100 hours plus or minus two hours.

There are several factors that make a CF-system quicker than a traditional batch fermentation. First is the fact that once the system is up and running, the yeast is already at maximum concentration so there is no lag phase and the second is due to the very high concentration of the yeast itself.

Morton Coutts’ system consisted of a series of tanks: a wort-receiver followed by the CF system proper consisting of a hold-up vessel, a 1st fermenter, a 2nd fermenter, and a yeast separator. The wort receiver receives wort from the kettle and dilutes it to sales gravity with sterile water. The hold-up vessel blends new wort from the wort receiver one-to-one with wort recycled from the 1st fermentation vessel and yeast from the yeast separator. This achieves a few things. It lowers wort pH to 4.2, increases ethanol concentration to 2.2%ABV and introduces yeast at the exponential phase of growth. All these measures serve to minimise the risk of microbial contamination. The hold-up vessel is vigorously aerated. Apparently the yeast used is both high flocculent and has a high oxygen requirement.
(Image from Boulton and Quain 2001)

Wort moves through the system by gravity feed. Residence times can be varied to meet changes in demand from between 36 to 97 hours. Glucose and fructose are assimilated in the hold up tank and most of the rest of sugar uptake occurs in the first fermentation vessel.

The advantages and disadvantages of CF systems are summarised in the tables below.

The advantages of continuous fermentation
1. shorter fermentation time. no yeast lag phase (ferment lager from 1.044 to 1.008 in 8-12 hours -Bruens 1966)
2. improved alpha acid utilisation (less loss to yeast) 10-20%
3. very consistent product
4. less system downtime,
5. savings on cleaning
6. Tower fermentation doesn’t give green beer flavours. Conditioning needed only for colloidal stability. Bruens 1966.
7. smaller footprint and smaller actual plant than equivalent production capacity batch process would have.
8. fewer beer losses due to reduced yeast growth

Disadvantages of continuous fermentation systems
1. less scope for different products
2. less scope to respond to seasonal and short term variations in demand
3. yeast suitability restrictions for Tower fermenter (must use a highly-flocculent pure yeast culture as mixed cultures are separated out by tower)
4. difficult to maintain microbiological purity but very important to do so.
different flavour profile from batch process (obviously, if you have a brand with a recognised taste, you risk a lot changing to a new system)
5. risk of yeast culture changing overtime is greater (can’t go back to pure culture every tenth batch or so).
6. has to operate seven days a week

So why don’t more brewers employ continuous fermentation systems? The main reason in my opinion is that it is very difficult to obtain a taste match with a preexisting brand. The minor products of fermentation make a very important contribution to the overall flavour of the beer. These factors are affected by such things as the dimension and shape of the fermentation vessel, the amount of yeast growth, the amount of oxygen and nitrogenous material in the wort, the pressure inside the fermentation vessel, the temperature during fermentation and the character of the strain of yeast itself. It would be very difficult to get these details right and brewers with established brands have a lot to lose if they get them wrong. In New Zealand, there was a virtual duopoly with few imported beers, the big two could switch over to continuous fermentation and there was nothing the New Zealand consumer could do about it if they didn’t like the new taste.

Flexibility is another important consideration although perhaps it is less of an issue now than at other times. Brewers who brew a base beer at high gravity enjoy significant savings and they can differentiate the base beer into separate products using post fermentation hop extracts, varying degrees of dilution and colour additions. This much flexibility is equally available to a brewer employing continuous fermentation. The one remaining advantage of batch brewing would be the ability to significantly increase or reduce production to meet seasonal changes in demand. The continuous fermentation system has to run at much the same capacity continuously (we saw the Morton Coutts’ system can only manage the residence time of beer between 36 and 97 hours), so changes in demand have to be met by stockpiling and the like.

With CF systems there is also the worry that a microbiological contamination could result in a very costly and extended system down time. It can take as long as two weeks for a steady state to be achieved in a CF system, so emptying, cleaning and restarting the system after an infection is detected can take a long time. However, while the consequences of an infection clearly are worse for a CF system than a batch system, the chances of an infection becoming established are minimal due to modern techniques and microbiological safeguards.

REFERENCES:
Bruens, L.J. (1966) Continuous fermentation and the tower fermenter. Master Brewers’ Association of the Americas Technical Quarterly, vol 3, no. 4, pp. 248-252.
Stewart, Graham (1977) Fermentation yesterday, today and tomorrow. Master Brewers’ Association of the Americas Technical Quarterly, vol 14, no. 1 pp. 1-15
Boulton, C. & Quain, D. (2001) Brewing yeast and fermentation Blackwell Publishing.