calumscott
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I think I've got to the bottom of the whole thing...
I may have a plausible answer for the Sucrose Vs Dextrose debate but to get there I've had to chuck Maltose into the wort, so to speak.
Firstly what are they? Well they're all sugars.
Sucrose: a disaccharide comprising one Fructose and one Glucose molecule, found in and refined from sugar cane and sugar beet.
Dextrose: a monosaccharide which is a "mirror-image" or "left-handed" glucose molecule. Also known as brewing sugar.
Maltose: a disaccharide comprising two glucose molecules, found (fortuitously) in malted barley...
Fructose: a monosaccharide normally found in high concentrations in, not surprisingly, fruit. Also known as fruit sugar.
Glucose: the basic monosaccharide to which all others are compared and the most important source of energy for living things.
Yeast can produce alcohol, by fermentation - a form of anaerobic respiration, from all of these as a by product of releasing the energy they need to survive, grow and reproduce.
I believe now, having read a fair bit of stuff, that there are fundamental differences in the biochemical processes that our yeasts use to metabolise those sugars.
Mono Vs Disaccharide
The first thing you notice about sucrose is that there are two energy giving monosaccharides in each molecule. The yeast is going to have to split that apart before it can use it.
To do this it uses an enzyme (basically a protein which acts as a catalyst for the chemical reaction) called invertase or to give it its full proper name, beta-fructofuranosidase. This, on the face of it shouldn't be a problem, its a protein made in the yeast cell to break down sucrose after all. Well, yes and no. There are two types that yeast produce. One intracellular (within the cell) and one extracellular (outside the cell), yes it's made in the cell but it is pumped out through the cell wall to split the sucrose in the substrate (your beer) before being absorbed into the yeast cell as glucose or fructose. There is now a protein in your beer that you didn't mean to be there. Clearly if the yeast doesn't have to break down sucrose, it won't expend the energy producing invertase in the first place, either intra- or extracellularly.
"But Maltose is a disaccharide!" I hear you cry. It is indeed but it is a disaccharide that comprises two glucose molecules and so the bond between them is different and so a different enzyme is required to catalyse the splitting reaction. Maltase is that enzyme and it seems that there is no extracellular form, it resides within the yeast cell at all times (unless you suffer lysis or "cell bursting"). The maltose is absorbed (or rather adsorbed as it is actively forced into the cell) by the yeast and is broken down in the cell tissue to its component glucose molecules there.
So Dextrose and Maltose = no extracellular enzymes
Sucrose = extracellular invertase.
Glucose Vs Fructose
So dextrose is, at the fermentation biochemistry level, just glucose - it behaves exactly the same.
What I have found is a paper on winemaking citing various experimental sources on the action of various strains of yeast on both glucose and fructose and, what appears to have happened is that over centuries, winemakers have (just as the beer brewers have) selectively bred their yeast to perform their fermentation on the sugars in their must/wort to produce the flavour profile they want. The short version is that relative utilisation of the two sugars differs from strain to strain, each being particularly suited to grape variety and style - just the same as beer strains being suited to style.
So given that beer is traditionally made from wort containing mostly maltose (2 x glucose) it is fair to suggest that our beer yeasts are most suited (and have been selected for) their glucose performance and not their fructose performance. If the yeast is less speciallised in fructose performance it stands to reason that it will be less efficient in the process and not inconceivable that it is more likely to ferment fructose incompletely, either leaving intermediate molecules "lying around" or simply giving up and leaving fructose for other potential invaders to utilise. Those intermediate molecules by the way are pyruvate and acetylaldehide - sherry, metallic, green apple and sour flavours...
This probably also explains why winemakers experience few problems using sucrose - their yeasts are "trained" to ferment fructose well where beer yeasts will be selected for their glucose performance.
So there you have it, some amateur sleuthing on the biochemistry of glucose and fructose and sucrose and dextrose and maltose and the genetic selection of yeast.
Enjoy!
I may have a plausible answer for the Sucrose Vs Dextrose debate but to get there I've had to chuck Maltose into the wort, so to speak.
Firstly what are they? Well they're all sugars.
Sucrose: a disaccharide comprising one Fructose and one Glucose molecule, found in and refined from sugar cane and sugar beet.
Dextrose: a monosaccharide which is a "mirror-image" or "left-handed" glucose molecule. Also known as brewing sugar.
Maltose: a disaccharide comprising two glucose molecules, found (fortuitously) in malted barley...
Fructose: a monosaccharide normally found in high concentrations in, not surprisingly, fruit. Also known as fruit sugar.
Glucose: the basic monosaccharide to which all others are compared and the most important source of energy for living things.
Yeast can produce alcohol, by fermentation - a form of anaerobic respiration, from all of these as a by product of releasing the energy they need to survive, grow and reproduce.
I believe now, having read a fair bit of stuff, that there are fundamental differences in the biochemical processes that our yeasts use to metabolise those sugars.
Mono Vs Disaccharide
The first thing you notice about sucrose is that there are two energy giving monosaccharides in each molecule. The yeast is going to have to split that apart before it can use it.
To do this it uses an enzyme (basically a protein which acts as a catalyst for the chemical reaction) called invertase or to give it its full proper name, beta-fructofuranosidase. This, on the face of it shouldn't be a problem, its a protein made in the yeast cell to break down sucrose after all. Well, yes and no. There are two types that yeast produce. One intracellular (within the cell) and one extracellular (outside the cell), yes it's made in the cell but it is pumped out through the cell wall to split the sucrose in the substrate (your beer) before being absorbed into the yeast cell as glucose or fructose. There is now a protein in your beer that you didn't mean to be there. Clearly if the yeast doesn't have to break down sucrose, it won't expend the energy producing invertase in the first place, either intra- or extracellularly.
"But Maltose is a disaccharide!" I hear you cry. It is indeed but it is a disaccharide that comprises two glucose molecules and so the bond between them is different and so a different enzyme is required to catalyse the splitting reaction. Maltase is that enzyme and it seems that there is no extracellular form, it resides within the yeast cell at all times (unless you suffer lysis or "cell bursting"). The maltose is absorbed (or rather adsorbed as it is actively forced into the cell) by the yeast and is broken down in the cell tissue to its component glucose molecules there.
So Dextrose and Maltose = no extracellular enzymes
Sucrose = extracellular invertase.
Glucose Vs Fructose
So dextrose is, at the fermentation biochemistry level, just glucose - it behaves exactly the same.
What I have found is a paper on winemaking citing various experimental sources on the action of various strains of yeast on both glucose and fructose and, what appears to have happened is that over centuries, winemakers have (just as the beer brewers have) selectively bred their yeast to perform their fermentation on the sugars in their must/wort to produce the flavour profile they want. The short version is that relative utilisation of the two sugars differs from strain to strain, each being particularly suited to grape variety and style - just the same as beer strains being suited to style.
So given that beer is traditionally made from wort containing mostly maltose (2 x glucose) it is fair to suggest that our beer yeasts are most suited (and have been selected for) their glucose performance and not their fructose performance. If the yeast is less speciallised in fructose performance it stands to reason that it will be less efficient in the process and not inconceivable that it is more likely to ferment fructose incompletely, either leaving intermediate molecules "lying around" or simply giving up and leaving fructose for other potential invaders to utilise. Those intermediate molecules by the way are pyruvate and acetylaldehide - sherry, metallic, green apple and sour flavours...
This probably also explains why winemakers experience few problems using sucrose - their yeasts are "trained" to ferment fructose well where beer yeasts will be selected for their glucose performance.
So there you have it, some amateur sleuthing on the biochemistry of glucose and fructose and sucrose and dextrose and maltose and the genetic selection of yeast.
Enjoy!
