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Sugar fermentation in yeast (follow-up)

Story location: Home / science /

After last week's post about Yeast, I received a couple of questions asking for a bit of clarification. I thought it would be a good idea to post the questions and my response here.

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Question 1

In the article you wrote:

'which is the classic bakers or brewers yeast'

With that statement, it took it to mean that they are one and the same yeast but having read further it would seem not to be the case. Have I read it properly?


Sorry if I made it a bit confusing. The traditional general purpose brewers yeast is indeed the same as bakers yeast. The other 2 yeasts I mention are a bit more specialist.

Question 2

Thanks Mike. I wonder how they isolate specific yeasts, as in the pombe one?


One way is to put a dilute solution of growing culture on an agar plate, which is a kind of jelly made using the growth medium, and leave the yeast to grow for a few days. After that you pick out individual colonies, which will usually have grown from single yeast cells. There is likely to be 100s of colonies of various types depending on what is present.

You can then do whatever tests are required to identify them, which these days will often involve some kind of genetic test. If you are wanting to brew wine or beer you can make trial runs with the different colonies to see how they affect the flavour. This is probably how they ended up with the different wine yeasts where you can buy yeast 'optimized' for different types of red or white wine.

Sugar fermentation in yeast

Story location: Home / science /

This is the first in a short series of science-related posts where I will be explaining a bit of science behind some things which are of particular interest or relevence to me. I will be starting with fermentation since it is important in both baking and wine- and beer-making.

See more ....

Types of Yeast

I apologise in advance for the use of the scientific names for the yeast but I need to be precise about which types of yeast I am talking about.

There are over 1,500 different types of yeast but the one which most people will be familiar with is called Saccharomyces cerevisiae, which is the classic bakers or brewers yeast. This has been used for centuries for this purpose. This is known as a 'top fermenting' yeast since, when making beer or wine, the live yeast forms a thick layer on top of the liquid.

Another yeast used in fermentation is Saccharomyces pastorianus (sometimes called S. carlsbergensis). This is a hybrid between S. cerevisiae and a naturally occurring yeast, S. eubayanus, which has been recently isolated from populations in South America.

A third type of yeast is called Schizosaccharomyces pombe. It gets its name from the Swahili word for beer and was isolated from strains used in East Africa to brew beer from millet. Along with S. cerevisiae it is used in research as a 'model organism' to investigate things such as signalling or communication between cells.

Yeast Metabolism

Yeast are typically most efficient when using simple sugars as a food. The common everyday sugar (known as sucrose) is a 'disaccharide' which means it is composed of two simpler sugar molecules (called monosaccharides) joined together.

In the presence of oxygen, the yeast grow and multiply rapidly which leads to the rapidly forming froth on top of fermenting liquids. After the oxygen has been used up, the yeast can then get on with their primary job which is fermentation:

Glucose → 2 molecules of ethanol and 2 molecules of Carbon dioxide. In brewing it is the ethanol we want, whereas in baking it is the carbon dioxide which is needed to make bread rise (Of course if you are making champagne, cider or any other 'fizzy' drink, you also want some of the carbon dioxide too).

Another way of writing this is:

C6H12O6 → 2C2H6O + 2 CO2

Brewing yeast can normally only directly consume the simpler monosaccharides so sucrose needs to be broken down in a process called 'Inverting' (this is what the 'Inverted Sugar Syrup' means on ingredient lists on manufactured foods).

Sucrose + Water → Glucose + Fructose

The above reaction is known as hydrolysis because it involves adding water to chemically change something. It can be done by boiling a sugar syrup with citric acid, which is a technique sometimes used to speed up brewing (at least in some homebrew circles).

As a chemical reaction, this can be written as:

C12H22O11 + H2O → C6H12O6 + C6H12O6

The two C6H12O6 on the right hand side look the same but they are actually Glucose and Fructose. They contain the same atoms but arranged differently.

Glucose and Fructose
(Based on images from Wikipedia)

The yeast do not actually do this themselves but they produce an enzyme called Invertase which they release into their surroundings, where this reaction actually takes place.

Coming Soon

Future posts will cover such subjects as Astronomy, Chemistry and Biology in various combinations. Feel free to leave any comments, including requests for future articles.

Sloe Gin

Story location: Home / Blog / food_and_drink /

I made sloe gin again last year, starting with a similar recipe to the one I used earlier. I froze the sloes first which helps to split the skins and softens the fruit, which helps extract the flavour. After a few weeks in the freezer I put the sloes and sugar in a pan and warmed them through before putting everything in a jug.

When it came to straining the sloes, there was a lot more sediment than normal. I ended up with one bottle of clear sloe gin and one bottle of alcoholic sloe smoothie, which will probably work well with a mixer or fruit juice. The sloe gin was quite sweet so I poured some more gin on the sloes, without any sugar, to extract a bit more flavour. I gave this a few weeks then strained it and blended it with the first bottle. The end result was quite good but was more work than the old method.

I won't bother warming the sloes again. It results in too much sediment and is more hard work in the end, I'll stick to just freezing then infusing in gin.

Fruit Cider

Story location: Home / Blog / food_and_drink /

I made a fruit cider last month, using standard winemaking techniques but using apple juice instead of grape juice. I decided to flavour it with some 'autumn fruits' to make it a bit more interesting.

For the fruit flavours I added the syrup from one tin of blackberries and one tin of blackcurrants. I also had 200ml of of extracted juice leftover from the plum and blackberry wine I had made earlier. I added 4l of apple juice. There was no added sugar - the sugars in the juice were sufficient to give a cider of about 6.5% alcohol.

When I bottled it, the cider was tasting over-dry. I wanted a sparkling cider so when I syphoned it into plastic bottles I added 2tsp of sugar and a few grains of dried yeast. After a week the bottles were feeling nicely pressurised so we tried a bottle tonight. It had a nice crisp flavour and was just 'off dry' enough for us.

Plum and Blackberry Wine

Story location: Home / Blog / food_and_drink /

Last week I started picking blackberries. By early this week I had gathered over 1.5kg, which was enough to make a gallon of wine. I had also picked just over 1kg of plums so I decided to combine them. I used my electric juicer, poured the juice into the demijohn along with 1 litre of apple juice and 1 litre of grape juice. I also added 700g of sugar. I put the remaining pulp in a fermenting bucket with a litre of water. I will use this to top up the demijohn after I have syphoned the wine off the thick sediment which I always get when I use real fruit to make wine.

Other wine tasting notes:

Tonight I finally got around to tasting the 'berry and ginger' wine I made last year. This was made using odds and ends in the kitchen - mainly jars and tins of slightly out of date fruit in syrup. I added several tablespoons of powdered ginger and some sugar (sadly I failed to record how much I added). The bottles have been sitting in the shed since I made it. The ginger flavour is much stronger than my first attempt a couple of years ago. I think I can only have one glass in a sitting. It might be more appreciated in the winter, when a warming glass of wine would be quite welcome.

Back in february, I tasted my rhubarb wine and noticed that it was very sweet. Last year I made a carrot and orange wine which I didn't like at all - it was fairly unpleasant on tasting but was improved upon sweetening. A few weeks ago I decided to blend the two together. A bottle of carrot wine blended the same amount of rhubarb gives a decent medium white wine. The flavour is a bit nondescript. I prefer wine to be a bit drier so I'll blend the rest of the wine with a higher carrot:rhubarb ratio.

Elderflower Wine

Story location: Home / food_and_drink / wine /

I started this wine on June 1st. The recipe was based on one I found on a homebrew forum.


elderflowers: 2 handfuls
white grape juice: 1 litre
apple juice: 1 litre
sugar: 500g
lemon juice: 2 tbs
tea: 1 bag
pectin enzyme: 1tsp
bakers yeast: 1tsp
yeast nutrient: 1tsp

The juice was simply cheap supermarket cartons of 'shelf juice' rather than fresh unpasteurised 'fridge juice'.


  1. Remove and discard the stalks from the flowers.

  2. Put the elderflowers in a fermenting bucket and pour over a couple of pints of boiling water. Add a crushed campden tablet and leave for 48 hours, stirring occasionally.

  3. Strain the elderflower water into a demijohn. Add the juice and a cold cup of tea (no milk!).

  4. Dissolve the sugar in boiling water and add to the demijohn when it has cooled a bit.

  5. Add the enzyme, yeast and nutrient and leave to ferment.

  6. When it has finished fermenting, optionally clear with finings and rack into a clean demijohn. If a still wine is desired, add a campden tablet and stabiliser.

  7. (Optional) To make a sparkling wine, do not add stabiliser. Syphon into pressure bottles (e.g. plastic 1 litre lemonade bottles). Add 1 tsp sugar and a small amount of an active yeast starter. Keep in a warm room for a couple of weeks then store somewhere cool to mature.

Tasting notes

Serve chilled, this is the nicest white wine I've made so far. The flavour is light and refreshing. I attempted to make a sparkling wine by following step 7. The bottles pressurised but the resulting wine wasn't fizzy. The wine still tasted good so I consider the experiment to be a success. I still have some elderflowers in the freezer so I might try again but add slightly more sugar and yeast in step 7.

The recipe only used 500g of sugar because I was aiming for a sparkling wine with a low-ish alcohol content of around 9%. For a more traditional still wine, a higher alcohol content might be preferred so increasing the sugar content to 750g would increase the alcohol to 13-14%.

Sloe Sherry

Story location: Home / Blog / food_and_drink /

When I made the Sloe Gin, I left the gin for 3 months before filtering and bottling it. I then poured a bottle of sweet sherry onto the sloes to see how much extra flavour could be extracted from them. I didn't add any sugar, I hoped that the sweetness in the sherry was sufficient.

Today, I filtered and bottled the sherry. It doesn't have as much flavour as the gin but it's still acceptable. Next year, I think I will leave the gin for 2 months rather than 3, so there should still be some flavour left in the fruit for a 2nd 'extraction'.

I've been told that it's possible to make a rather nice 'sloe liqueur' chocolate by taking the used sloes, removing the stones and mixing the flesh with melted chocolate, then leaving to set. I was going to do that today but removing the stones was very fiddly. I tried one of the sloes and found that it was fairly dry and tasteless. Having used them to make two drinks, I had probably removed too much of the juice and flavour.

Tasting notes: Rhubarb wine

Story location: Home / Blog / food_and_drink /

The rhubarb wine has been sitting in the shed maturing for a few months so I thought it was ready to taste. It is a bit on the sweet side. I deliberately aimed to let the wine finish of slightly sweet - rhubarb needs some sugar to bring out the flavour and I thought a bone dry wine wouldn't work so well. It isn't as sweet as a dessert wine but it also doesn't have the depth of flavour of one either. It was a fairly cheap wine to make (the rhubarb was reduced at the supermarket) so it wasn't an expensive experiment. I think it's worth trying again with more rhubarb and less sugar.

Blackberry and Elderberry wine

Story location: Home / food_and_drink / wine /

I've bottled our Blackberry and Elderberry wine. I started fermenting it back in October and it's been sitting in the demijohn since then, maturing.

After filling 6 bottles there was about a glass left over so we sampled it tonight. It's fairly light flavoured, unlike my first attempt at this wine which was really full bodied. It has a reasonable fruity taste. I'll leave it in the shed for a few months to mature, to see if it improves with age.

I used 2.4kg of fruit in total. I can't remember (and didn't write down) whether I used any grape concentrate or raisins. Next time, I'll either use more fruit or supplement it with some grape or apple juice to give it more body.

Sloe Gin

Story location: Home / food_and_drink /

This is my first attempt at making Sloe Gin. The units are mixed metric/imperial because I didn't note the weight of the sloes in kg.

  • 2½ pounds Sloes (or sloe/wild plum mixture)
  • 300g white sugar
  • just over a litre of gin.

I made it in a 2½ litre jug which once contained cider. The sloes and sugar nearly filled the jug. I poured in as much gin as I could until it was nearly full.

I must remember to invert the jug a couple of times per day for the first few weeks, then every few days for the next couple of months. It should be ready to drink by Christmas.

Rhubarb Wine

Story location: Home / Blog / food_and_drink /

I bottled our rhubarb wine tonight. It was a fairly simple recipe:

  • 1.5kg Rhubarb
  • 1.25kg Sugar
  • 1 small tin of grape concentrate
  • bakers yeast

I managed to get just over 5 bottles. I'll give it a few weeks to begin to mature before tasting.

On a related note, I've ordered an electric juicer. This should make future wines easier to make. It's blackberry season at the moment and hopefully I'll have time to go and pick some soon. A friend with a grapevine has promised to give us a load of grapes, so the juicer will definitely come in handy there.

Wine Tasting Notes

Story location: Home / food_and_drink / wine /

Apricot Wine
It was drunk at room temperature rather than chilled but it was good. Quite light flavoured but you could make out the apricot taste. If I tried it again I might try more apricot or leave the dried apricots to soak for longer.

Jam Wine
Quite light in flavour. Served chilled it worked well as a mixer. Tried it with white rum (ok), gin (worked well), Tropical Sourz (worked well) and Grenadine (just seemed to make it taste sweeter).

Berry Wine
This is the strongest flavoured of our wines so far, and with (in my opinion) the right balance of sweetness and depth of flavour. Also works well in a turbo purple

Apricot Wine

Story location: Home / food_and_drink / wine /
  • 600g of dried apricots.
  • 500g pears.
  • 1kg sugar.
  • 1 small tin of white grape concentrate.

Wash the fruit in a dilute sodium metabisulphite solution, then chop and put into a large pan. Add the sugar, cover with water and bring to the boil. Turn off the heat and leave to cool.
Pour into a sterilized fermentation bin. Add 1tsp of pectin enzyme, 1tsp of yeast nutrient and the yeast. Leave for a few days, occasionally mashing the fruit to get more juice out. Strain into a demijohn, add the grape concentrate and top up to 1 gallon with water. Fit the airlock.

When the fermentation has stopped (or the wine has reached the desirable sweetness), add some wine stabiliser. The wine will need to be clarified either by adding finings or syphoning into a 2nd demijohn and leaving to settle. Or both if the wine is quite cloudy.

Homebrew Party

Story location: Home / Blog / food_and_drink /

Tonight was the party to drink the rest of the Biology Society homebrew - this was made in december and the first party (where about half of the beer got drunk) was last month.

As well as homebrew, we also tried 'turbo purple' - this was inspired by the drink served at the real ale festival, where they used strong lager, cider and blackcurrent wine. We used homebrew lager, a bottle of strongbow, and our blackberry and elderberry wine. It tasted fine.

Berry Wine

Story location: Home / food_and_drink / wine /

Berry Wine

This used the berries we collected in the autumn, along with some plums to increase the fruit level to around 3½ pounds.

  • 2½ pounds elderberries
  • ½ pound blackberries
  • ½ pound plums

The fruit was washed in a metabisulphite solution before being boiled in a pan with 4 pints of water and 1kg of sugar. The mixture was poured into a fermenting bin to stand for a few days. 1 teaspoon each of pectin enzyme, yeast nutrient and brewing yeast were added. The fruit mixture was then sieved into a demijohn, a tin of grape concentrate and water was added to take it to 1 gallon.
After a few weeks a thick sediment had formed so the liquid was syphoned into a clean demijohn and the volume was topped up with water - actually 200g sugar dissolved in water because the mixture was tasting too 'dry'.
A month later and fermentation had stopped. The wine was tasted and seemed ready for bottling. Wine stabiliser was added. After leaving to settle for a few more days, the wine was decanted into clean sterilised bottles. It's supposed to have several months to mature but I think I'll try a bottle over Christmas.

Jam Wine

This was more of an experiment. I used 3 jars of jam to provide the sugar and fruit content (plum, blackcurrant and strawberry jams), and added 2 teaspoons of pectin enzyme. No extra sugar was added and I used bakers yeast instead of brewers yeast to stop the alcohol content getting too high. Unfortunately I only got 4½ bottles out of it because so much sediment formed in the demijohn.