Technical Forum

Sugar and alcohol - throw away your hydrometers

Introduction

Knowing how much alcohol can be obtained from a given mass of sugar allows you to make a successful wine without a hydrometer. One of the difficulties with determining the amount of sugar in a pulp must from the fruit is to prepare a pulp free sample to measure the specific gravity and then estimating the volume of the must to determine the sugar that comes from the fruit. The other problem is the amount of must lost with discarding the pulp.

An easier approach is to calculate the sugar requirements for alcohol production from first principles. That's where the table below comes in handy.

Sugar Alcohol Interconversion Table
(based on a 93% conversion rate)
Sugar kg 0.6 Alcohol L
Alcohol L 1.67 Sugar kg

The table contains easy ways for converting sugar in kg to alcohol in L or vice versa, converting alcohol in L to sugar in kg. If you have 5 kg of sugar you can obtain 3 L of alcohol ( 5 x 0.6 = 3).

If you need to know how much sugar is needed to make 2.8 L of alcohol simply multiply 2.8 by 1.67 which equals 4.68. Thus 4.68 kg of sugar will provide 2.8 L of alcohol.

The above factors, 0.6 and 1.67 have built in them the sugar alcohol conversion rate of 93%. What is this 93%? From empirical studies the conversion of sugar to alcohol is not 100%. There are some by-products from the sugar (glycerol and fusel oils); some of the sugar is fully oxidised by the yeast; some sugar is used within the rapidly growing mass of yeast. The conversion is more like 93% .

A different approach to sugar/alcohol

Decide on what level of alcohol you want; let's say we want 13.5% alcohol in 4.5 L (a demijohn) of a raspberry wine with the following ingredients. in 4.5 L

2.00 kg Raspberries 5% sugar 0.10 kg
0.50 kg Sultanas 60% sugar 0.30 kg
Sugar 100% sugar
Total Sugar 0.40 kg

Now 13.5% of 4.5 L is 0.608 L of alcohol (0.135 x 4.5 = 0.608). The question now is how much sugar do I need to make 0.608 L of alcohol. From the table we can see that 0.608 L of alcohol comes from 1.015 kg of sugar (0.608 x 1.67 = 1.015 ). We already have 0.40 kg of sugar from the fruit and sultanas so we only need to add 0.615 kg of sugar (1.015 - 0.40 = 0.615). So the recipe to make a 13.5% alcoholic wine looks like this:

2.00 kg Raspberries 5% sugar 0.10 kg
0.50 kg Sultanas 60% sugar 0.30 kg
0.615 kg Sugar 100% sugar 0.615 kg
Total Sugar 1.015 kg

(For the hydrometer users: 1.015 kg sugar in 4.5 L is 225 g/L which has a SG of about 1.085 which will make 13.6 % alcohol – Progressive Winemaking by Duncan and Acton ISBN 0 900841 p55)

After you have fermented the must to dryness you can add potassium metabisulfite and potassium sorbate to hibernate the yeast (or pasteurise it to kill the yeast or filter it to remove the yeast) and then add sugar to taste. In the European Union (EU) wine industry a:

  • dry wine will have up to 4 g/L or 0.4 g/100 mL
  • medium dry wine will have 12 g/L or 1.2 g/100 mL
  • medium sweet wine will have more than 12 g/L but less than 45 g/L or less than 4.5 g/100 mL
  • sweet wine will have not less than 45 g/L or greater than 4.5 g/100 mL

Analysing a recipe

Take the following example of a raspberry wine recipe from Leverett (Winemaking Month by Month 1986 p83) Let's examine the recipe which is said to make 4.5. L of very dry, light wine.

2.00 kg Raspberries 5% sugar 0.10 kg
0.50 kg Sultanas 60% sugar 0.30 kg
1.25 kg Sugar 100% sugar 1.25 kg
Total Sugar 1.65 kg

You can find the sugar content of ripe and dry fruit from books and the Internet. So the question is, how much alcohol can be be made from 1.65 kg of sugar? From the sugar alcohol interconversion table, 1.65 kg of sugar will yield 1.65 x 0.6 or 0.990 L of alcohol. In a 4.5 L batch this will be 22.0% v/v alcohol!!!

Now it depends on the yeast but most wine yeasts will stop working at about 14% alcohol. That's when about 0.63 L of alcohol has been made (14% of 4.5 L). The amount of sugar used to make this alcohol will be 1.05 kg (again from the interconversion table above 0.63 x 1.67 = 1.05). This leaves 0.6 kg of sugar (1.65 – 1.05 = 0.6 kg) in the raspberry wine. Now 0.6 kg or 600 g in 4.5 L is 133 g/L; a very sweet wine indeed, certainly not the very dry, light wine it is said to make in the recipe.

(For the hydrometer users: 1.65 kg in 4.5 L is 367 g/L which has a SG of about 1.140. This sugar level is so high you run the risk of dehydrating the yeast but this amount of sugar in theory could make 22.2% alcohol - Progressive Winemaking by Duncan and Acton ISBN 0 900841 p55)

So there you have it. Throw away the hydrometer and do a little arithmetic.

Appendix

This is how the conversion factors 0.6 and 1.67 were arrived at.

Sugar alcohol + carbon dioxide
1 kg   0.51 kg   0.49 kg

To convert the alcohol in to L, divide by its SG of 0.789 ( SG = M / V)
0.51 kg of alcohol equates to 0.646 L of alcohol
Now the conversion rate of alcohol from sugar is about 93% and 93 % of 0.646 is 0.6

Sugar kg 0.6 Alcohol L

1 L of alcohol has a mass of 0.789 kg (derived from its SG and SG = M / V)
0.789 kg of alcohol comes from 0.789/0.51 kg of sugar or 1.55 kg
Now the conversion rate of alcohol from sugar is about 93% so you need a little more sugar than 1.55 kg. In fact you need 1.55/0.93 which is 1.67.

Alcohol L 1.67 Sugar kg