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March 06
Sugar additions to boost ABV

​Question:

I’m reading on the crabtree and chaptalization. I have been adding raw sugar to fermented beer in my brewery for many years with great results, but really instable result. I started to look at the crabtree effect as an explanation, but both subject are greatly unexplained on the internet which makes it difficult to understand. Do you have any reading to suggest or some experience you had with adding raw sugar or any opinion of what are the underlying mechanism that occurs in the crabtree effect and chaptalization? From what I understand both are opposite, but both occurs, without anyone knowing at what range.

Answer:

The “crabtree effect” has to do with the yeast’s inability to produce respiratory enzymes in high concentration sugar solutions even when oxygen is present.  So if you are adding sugar to a working fermentation and it picks up air it will still “aerobically ferment” the sugar and produce ethanol.  This is why when growing baker’s yeast they avoid alcohol production by keeping the sugar concentrations low and fed in a controlled manner along with aeration.   Chaptalization is the addition of sugar to boost the initial gravity at the start of fermentation or during fermentation.  This practice is used by winemakers on musts that are not high enough in Brix to get the alcohol concentration they are looking for and also by brewers who want to build up the fermentable extract of their starting wort to make very high alcohol beers.  The sugars used for chaptalization can be anything that is fermentable, ie. Sucrose, Belgian candi-sugar (sucrose), glucose, high fructose corn syrup, molasses, honey, etc and the sugars can be fed into the fermentation as it proceeds without additional aeration.  As sugar concentrations increases the yeast may start to produce interesting flavor compounds like fruity esters and solventy higher alcohols, this will be affected by the temperature of the fermentation, the concentrations of the sugars added and the yeast itself.

I asked Chris Boulton of Univ of Nottingham and yeast expert about your question and here is his more detailed response:

Regarding your question I would not disagree with anything you have said.  There are two Crabtree effects (long and short term).  The former seems to operate at the enzyme level such that at high sugar concentrations the respective Km values for the pyruvate dehydrogenase and pyruvate decarboxylase are such that the majority of the carbon flow goes to acetaldehyde (and ethanol) and not acetyl-CoA.  The longer term effect has the same gross effect but it operates via repression of the genes involving carbon flow through pyruvate dehydrogenase part of the TCA cycle and the respiratory electron transport chain.  These effects have the same gross outputs and as you say are independent of oxygen and reflect the fact that with a surplus of sugar the yeast can generate more than enough ATP via glycolysis to satisfy its energy requirements.  The result though is that you get a lot less biomass formed so as you also say the fed-batch fermentations make sure this doesn't happen by feeding in the sugar at an exponential rate balanced with growth rate and they supply a lot of oxygen.  This results in a massive increase in biomass yield.  As an approach to increase abv in brewing fermentations feeding sugar in batches avoids the possibility of arrest of fermentation via osmotic shock.  The sugars are taken up in order so if you feed with sucrose or glucose this will delay the uptake of maltose.  If you feed a more complex sugar source (plus other nutrients) this will have differing effects.  The flavour changes probably reflect the fact that the added sugar changes the C:N ratio (and C to other nutrients as well).  The effect is to change the concentrations and spectrum of precursors to pathways such as those leading to higher alcohols.

Essentially if you are adding sugar the fermentation do this in a steady feed as the fermentation is proceeding.

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