Glucose, fructose, and sucrose - how different sugars ferment

[TimT]

As folks on this forum will know (because sometimes I just don't shut up about it), we have bees, and over the years we've had them I've noticed quite a lot of variability in between the honeys produced from month to month. Well, duh. No surprises there, obviously, especially since we live in the burbs and there's a wide variety of plants for our bees to forage from at any one time. But I'll ramble on a bit more about each batch of honey as it will allow me to frame my question a bit better.

The first batch I can't remember much about, but I made a fairly straightforward mead from it that's come into its own about the start of this year. The honey fermented cleanly, gave the mead a strong spirituous vibe with a lingering sweetness that puts it into a dessert wine territory.

The second batch of honey was quite different; dark and, frankly, a bit festy. It came from the oldest combs, the ones that we'd got from when we did the hive split. It's possible the area it came from had a lot of gums flowering, too (as opposed to where we are, full of fruit trees and European flower varieties). It created mead of a dark amber to almost-brown colour that again fermented to a kind of spirity liquor, again with a lingering sweetness. I mixed some of this batch of honey with plum juice to create quite a nice melomel that is now developing the distinctive toffeeish mead odours. I think the plum juice has mellowed out the flavour of the straight honey ferment quite nicely.

The third (latest) batch of honey, different again. We collected this lot during autumn, and it tended to be quite finely coloured and smooth - much cleaner than the previous batch. Like other batches of honey, it candied over winter, though the texture is quite even right through. From this honey, I made a bochet (quite flavoursome, though it's quite spicey and the yeast may have just been a bit lazy, so it's left a residual sweetness), and a rose-hip mead. The rose-hip mead fermented right out - there is almost no detectable sweetness in the flavour at all. What you are left with is a bit of spiciness and tannin from the rose-hips (though not much), and possibly some spiciness and body from the honey. It's quite quaffable, but has hardly any appreciable character; some of this, however, will develop over time.

All this has got me thinking, though.

Honey is generally quite fermentable - about 95 per cent of the sugars are fermentable, I've read. The main sugars are glucose, fructose, and sucrose. Now, I'm wondering, what the differences are between these sugars, and especially, how differently will they ferment? Do some ferment more cleanly than others? What about the residual elements they leave behind? I'm especially interested to find out how I can improve on my rose-hip mead for next time: two methods occur to me; one, use a high amount of honey (or a low-attenuating yeast) so I'm certain to be left with some residual sweetness? Just chuck in a lot of spice on the assumption that a neutral honey will accommodate spiciness more? (The bochet is one of my more successful meads).

Another, related question is: is there a rule of thumb you can use to guess at the glucose/fructose/sucrose content of any particular honey? (Our friend/'mentor/loveable local mad scientist A. did tell us one method at a previous bee group meeting but I'm afraid his method has gone right out of my head).

Any contributions to this topic will be greatly appreciated

 

[verysupple]

I don't know much about honey so I'll go with what you said about it being made mostly of glucose, fructose and sucrose. Yeast can fully ferment all of those - glucose and fructose are monosacharides and are no problem and the yeast use invertase to spilt sucrose into a glucose and a fructose.

Now, my organic chemistry isn't very good, but I'm pretty sure the products of fermenting glucose, fructose and sucrose are all the same in the end. There's just more steps to get there for sucrose compared to glucose for example.

But that's just the basic science. In the real world there may be taste differences so I'll let others chime in on that.

EDIT: It may be the remaining ~5 % that isn't glucose, fructose and sucrose that gives you all the character. The "spirity liquor" may simply be because the yeast go nuts on all the simple sugars and the temp is too high giving more of the higher molecular weight alcohols.

 

[Feldon]

The main sugars are glucose, fructose, and sucrose. Now, I'm wondering, what the differences are between these sugars, and especially, how differently will they ferment? Do some ferment more cleanly than others? What about the residual elements they leave behind?

Fructose, sucrose and glucose are closely related - only the addition or removal of a water molecule is what makes them different.

Glucose is the simplest sugar and is broken down (by enzymes) to alcohol and CO2. Fructose and Sucrose need to be first broken down to form glucose, and in the process give off other chemicals.

 

[TimT]

One of the weird things about all wines is that after they ferment out, they may taste quite dry and/or unsubtle for the first few months of their life. Then a strange thing happens. Often they seem to become *more* sweet again. Maybe residual sugars begin to show through as the powerful but unstable chemicals left by the yeast drop away; maybe the sweetness is partially an illusion created by odours the ageing wine will throw off. This certainly happens with meads too. So the yeast converting the glucose, fructose and sucrose to sugar is only part of the story; it's what happens afterwards to what is left afterwards that must give the mead its character.

 

[MHB]

There are four simple sugars, Glucose (the most common) Fructose, Galactose and Mannose (you’re not likely to see much of this one). All four are isomers (same bits put together differently) of the same molecule and can be completely fermented to 2alcohol and 2CO2.
Yeast metabolises simple sugars the same way, they are transported across the cell wall and acted on internally and waste products (Alc & CO2) are excreted back into solution.
Where it gets more interesting is when yeast encounters a disaccharide (2 simple sugars joined up)
In Brewing the most common disaccharide is Maltose (Glucose-Glucose) it is metabolised the same way as Glucose and Fructose (taken into the yeast cell and broken down)
Sucrose (Glucose-Fructose) can’t be imported into the cell so the yeast releases Invertase (an enzyme that breaks the Sucrose into Glucose and Fructose) the Glucose and Fructose can then be imported and metabolised.
Lactose (Glucose-Galactose) can be neither inverted nor metabolised, so it just hangs around.
Honey being made from nectar (mostly sucrose, fructose, glucose and water) is collected by bees; they add salivary Invertase to it to stop it from crystallising in the comb – they just aren’t up to handling rock sugar.
Nectar starts out about 55% Sucrose, 25% Glucose and 20% Fructose, by the time the bees are finished with it the Sucrose content is usually under 2%
When we try to ferment honey there is still enough Sucrose in it to trigger the release of Invertase. I have been thinking that to some extent the release of Invertase may account for the cindery twang a lot of home brewers and mead makers encounter (speculative, needs testing).
Making sure your must has a good supply of nutrient Nitrogen and plenty of organic acids (Malic, Citric, Tartaric Tannic...) will minimise the maturation time, reduce the spirit twang and make the mead ready to drink much sooner.
Personally I don’t see the point in making dry mead, it always tastes like crappy Chardonnay to me, so I tend to work on sweet meads, using a highly attenuateive yeast (usually Wyeast Dry Mead) and feeding it till it drowns in alcohol at around 18-19% and is still sweet (knickerdropper par-excellence), with good nutrient and acid additions it’s usually ready to drink in three months, stunning at six and keeps improving for 2-3 years.
Mark

 

[technobabble66]

Keen to know more about this.

@TimT - have you tried a "DME + [saccharide]" series of mini-fermentations to compare the different sugars? Something like 50% DME and 50% of each chosen sugar plus some yeast nutrient.
I did something similar with a variety of fermentables to determine what would leave the most residual sweetness. But probably the best thing i got out of it was a much better understanding of the flavour input of each of those fermentables. For example, i'm now much better at picking when astringency comes from too much crystal - try an 80% crystal mini-batch and you'll see what i mean.

@MHB - so you're suggesting that the only fermentable that adds the "cider" twang is sucrose? I though fructose did that also.

 

[MHB]

I don't believe that its Fructose, that's a simple sugar that yeast can handle no problem
I don't even think its the Sucrose, but am speculating that it might be the way yeast reacts to Sucrose (releases a flood of Invertase) 1/ that enzyme is then floating around in your beer looking for something to do 2/ the release of invertase takes energy and resources from the yeast that would otherwise by used in the "normal" process of converting sugars into energy - it must have some effect on the way the yeast behaves, the result is the typical cindery twang.
Wine yeast evolved to eat sucrose and plays by a whole different set of rules.
Mark

 

[TimT]

That's fascinating and very helpful Mark. Some mead makers do seem to be especially generous with the additions of yeast nutrient (at several stages in the ferment to ensure yeast health). This helps to explain why.

 

[technobabble66]

Hey MHB,
yeah, i'd assume the differences in what the yeast is doing would be fairly minor in the grand scheme of things, however given the sensitivity of our taste sensation and the the melding together of hundreds of minor and major flavour components to create our favourite beverage, i kinda guess it might take only a fairly minor deviation (or change in the balance) in the biochemistry of the yeast's processes.
Subsequently i wouldn't be surprised if the presence of a small amount of invertase produced a noticeable change in flavour. Similarly, i'd wonder if there is a noticeable flavour difference resulting from the different biochemical pathways used to break down the 2 different monosaccharides.

All theorising as i don't have any experience fermenting with fructose.
As i said, v interested in this saccharide stuff and keen to be educated, Mark :lol:

 

[Airgead]

Wine yeast evolved to eat sucrose and plays by a whole different set of rules.
Mark

Actually, I suspect that all yeast started eating sucrose as wild yeast is ubiquitous in the environment and typically lives on fruit which are high in both sucrose and fructose. I suspect that beer yeast has had its ability to properly metabolise sucrose bred out over the generations by giving it a very sucrose poor diet.

I use wine yeast in my meads and have never experienced "cidery twang" (even if I skimp on nutrients, which I did experiment with a while back as a way to do sweet meads... lesson - don't very unpredictable).

Cheers
Dave

 

[MHB]

Airgead - yes I pretty much agree.
People forget how much we and yeast have co-evolves each other, with a generation time measured in hours or days if you apply selective pressure for a couple of thousand years you are bound to change the yeast a bunch.
The word Saccharomyces literally means Sugar Eating, so there is not much doubt about its dietary preferences.

I'm a big lover of sweet mead, its all in the presentation, served with desert or a nice cheese and fruit plate it goes down a treat.
Mark

 

[Airgead]

I'm a big lover of sweet mead, its all in the presentation, served with desert or a nice cheese and fruit plate it goes down a treat.
Mark

Love the sweet mead as well. I was experimenting with making them by intentionally limiting nutrients to force a stalled ferment.Not so great. Very unpredictable. Sometimes it would stall but couldn't predict when, sometimes it would dribble on for months, sometimes it looked like it had stalled but the bottled went foom....

these days i either kill it with alcohol (like my liver) or use sorbitol.

 

[TimT]

Be interested in hearing more about the fermenting practices of wine yeast as well. I ferment most of my meads over a wine yeast (up to this point, usually champagne yeast). My bochet was a different story, did that one over an ale yeast.

I hear you about sweet mead v dry mead. I share your preference too. I guess when you ferment a fruit wine, eg, a standard grape wine, you get a whole lot of other stuff from the fruit as well: tannins from the skin, nutrients, etc. Not to mention the possibility of some natural souring/fermenting from wild yeast or bacteria even before you add your own yeast. And all that adds character to the final wine.

Honey, though, as you say, is quite different: it's essentially concentrated sugar, and a lot of the qualities of the plant have been subtracted from the honey before you get to work with it. (Side note: I do wonder, though, if the bacteria that naturally occurs in a hive and actually helps to preserve the honey doesn't add some stuff of interest to the final mead?)

So maybe it really is the weird, residual sugars - the 5 per cent or so that is unfermentable or difficult to ferment, the sugars that *aren't* sucrose, glucose, or fructose - that gives mead a lot of its character? And they're the sugars worth looking at in detail?

 

[MHB]

Been doing a bit of reading up on "the Cidery Tang" most of the information I can find puts it down to Acetaldehyde which is produced during the early stages of fermentation and not cleaned up later by the yeast for a number of reasons, under-pitching, yeast health, low nutrient, oxidisation or bacterial action... all the usual suspects.

I find this a very useful guide to faults, Author gives permission to use
Complete Beer Fault Guide

View attachment Complete_Beer_Fault_Guide.pdf