Serial Feeding

In my review of "Brewing Engineering" by Steven Deeds I mentioned that I had an issue with a recommendation of his that amounted to using serial feedings to get a single tube of yeast upto pitchable amounts without using a starter. This led to some discussion with Dennis of Life Fermented asking why serial feeding is bad and how it differs from sugar additions in Belgian brewing traditions. I began an answer to this question, and quickly hit the character limit for a comment. So here's the answer in full.

Just to provide a little more background, the procedure the author was recommending was to hold back a portion of your wort, such that you are pitching a tube of white labs/wyeast yeast into an appropriate volume of wort for the cell count. At a later timepoint you then add the remainder of the wort, under the (likely correct) assumption that you now have sufficient yeast for the batch.

The issue with this comes down to a few things - what drives yeast cell division (and thus, production of some off-flavours), how yeasts change to a changing fermentation environment, and how yeast process fermentables.

More below the fold...

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Cell Division, Biological Changes & Off Flavours
The first thing to start with is cell division - more specifically, what drives it and what limits it in Saccharomyces. Contrary to what many believe, yeast density it wort is not arbitrary. Rather, yeast use a number of processes (number/frequency of cell-cell contacts, formation of yeast flocks, something called "quorum sensing", etc) to get a rough idea of how many yeast are in their vicinity. At the same time the yeast are responding to nutrient and waste product levels (a partial review of these processes can be found here). As these factors converge, yeast will begin to slow their reproduction, switch to a form of metabolism which allows them to lay down glycogen (an energy store), and begin to settle - all in preparation for dormancy. High kraussen roughly matches this "transition point" in time.

So that's how yeast control their divisions and numbers, but what does that have to do with off-flavours? The answer there is simple - some (not all) off flavours (or their precursors) are produced during the rapid-growth phase of yeast - esters, acetaldehyde and fusel alcohols being the prime examples. When you add additional fresh wort to yeast you do a number of things - you dilute out both the yeast numbers and waste products, as well as provide a new source of oxygen, sugars and nutrients - with the end results of forcing the yeast back into the rapid-division cycle and the off-flavours that can produce. But that's not the end of it - that repeat transition, over such a short period of time (keep in mind that you're adding the new wort somewhere around the time the yeast are beginning their transition), is very stressful to the yeast, meaning off-flavour production can be enhanced, and the yeast you produce would also be less viable than those from a starter.

In commercial environments this kind of feeding is avoided like the plague - aside from causing whatever microorganism you are working with to behave in an entirely unpredictable fashion, it tends to amplify stress-related production issues - with beer that would be off-flavours, but it can be much more severe in other commercial contexts. The closest commercial production gets to serial feeding is batch feeding - a process in which the microorgnaism of interest is kept in a continual growth phase through the continuous removal of waste products and reintroduction of nutrients; with everything balanced such that the nutrient and waste levels remain constant. The level of technology needed to do this is quite advanced (and thus expensive), and several yeast producers have mentioned that it produces low-quality yeast even though you keep the yeast in a continually oxygenated, high-nutrient environment.

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What About Belgian (and other high-gravity) brewing?
Dennis's question that sparked this post was the following:

So what about when making a Belgian with a fair amount of candy sugar/syrup or sucrose added? Its common advice that the extra simple sugars should be added after the yeast have had a chance to plow through most of the wort sugar. This is reputed to make a cleaner, drier beer. Is this perhaps not the case with the negative consequences of serial feeding?

The answer to this question is somewhat complex. Lets tackle it in two parts - 1) why doesn't it cause the same issues as batch feeding, and 2) what does it achieve?

Why Doesn't Adding Sugar Mid-Ferment Cause the Same Issues as Batch Feeding?
The answer here is relatively simple - you're only adding fermentables, not volume or nutrients. As such, the change to the yeast environment is minimized; density-based cell detections are unaltered, as is the general nutritional state and level of waste products. So while the yeast will continue to use the added sugars, they won't be forced into another round of high-speed cell division, and therefore should not produce as many off-flavours or suffer as severe of a stress response.

What Does Late Addition of Sugars Achieve?
Dennis repeated a somewhat common myth with the late sugar additions - i.e. that it produces a cleaner beer. This is not necessarily so (although it can be an effect) - and few people would call most Belgian beers "clean". I was unable to find any research on late-sugar additions and off flavours, but yeast biology would suggest that any reduction in off-flavours would be due to the decreased period of exponential growth relative to a wort fully sugared at the onset of fermentation.

However, Dennis's second point - drier beer - is spot on, and is why late sugar additions are common in many high-gravity brewing styles. Understanding this requires a small discussion on how yeast use sugars. Simple sugars (glucose, sucrose, and maltose to a small extent) enter yeast through "glucoside permeases"; enzymes in the outer cell membrane that act as 1-way valves for simple sugars - sugars passively enter a cell, diven by diffusion. There are specialized transpoerters for maltose; some of these are simply additional 1-way valves which let maltose in via diffusion, but others use protons which co-enter the cell with the sugars, actively pumping sugar into the cell through a process called symport (allowing the cell to take up sugars faster than diffusion alone allows). This later process requires energy, as yeast must continually export protons to keep the system running. Even more complex sugars, such as maltotrise, enter via their own "valves", with transport requiring protons as an energy source.

To simplify all of that, simple sugars can be brought into a yeast cell without the cell using any energy for the import. But more complex sugars - notably, those derived from malt (maltose and maltriose) require the energy-consuming maintenance of a proton gradient for effective import, plus the cell must expend the energy to synthesize the relevant proteins to "add on" the capacity to transport these sugars.

What this means is if there are too many simple sugars present at the beginning of a ferment, the yeast will preferentially utilize those, and at the same time minimally (if at all) activate the active transporters that allow maltose and maltotriose to be effectively used. As such, adding too much sugar early means the yeast will not fully attenuate the wort - they'll eat the basic sugars, and because they cannot readily import the malt sugars, once the basic sugars are gone the yeast will begin to enter dormancy. Adding the sugar late ensures that the yeast have upregualted all of the relevant genes for complex-sugar fermentation - which includes not only the sugar transporters mentioned above, but also alpha-glucosidase; an amylase-like enzyme inside of the yeast that breaks down maltose and maltotriose into fermentable glucose.

In other words, yeast are like children - you have to make them eat their veggies first (e.g. complex malt sugars) before you give them a sugar-laden dessert.