Module 2: Units of density

Question 1

Density is a measure of what?

Answer 1

The relationship between mass of a substance and its volume.

Question 2

Specific gravity is a measurement of what?

Answer 2

The density of a sample relative to the density of water.

Question 3

Why can specific gravity be used as an approximate but useful indicator of sugar content?

Answer 3

In juice, dissolved solid materials (very largely sugars) determine the density and specific gravity.

Question 4

Why does the juice density in Baumé indicate a potential alcohol content that may not be reached in practice?

Answer 4

• Complete fermentation of all the juice sugars may not be possible if the alcohol content rises to a level that is sufficient to kill the yeast, typically 14-16% (v/v) of alcohol (depending upon yeast strains and other conditions).
• Even if complete fermentation is possible, it may not be desirable and the fermentation may be stopped artificially to leave an appropriate amount of residual sugar in the wine.

Question 5

What does the juice Baumé value equal?

Answer 5

Potential wine alcohol content, in %(v/v) = juice Baumé value

Question 6

What does ‘degrees bricks’ indicate on the Brix scale?

Answer 6

• The density of the sample by relating it to the density of solutions of sugar in water.
• The °Brix value indicates the mass of the sugar sucrose (common table sugar), in grams, that must be present in 100 g of a solution of it in water to obtain the density being indicated.
• For example, 100 g of a sucrose solution in water will have dissolved in it 22 g of sucrose if its density is 22° Brix.

Question 7

What does the juice Brix value equal?

Answer 7

Juice sugar content, in g/100 mL = juice °Brix value

Question 8

How do you convert °Brix to Baumé?

Answer 8

°Brix value = 1.80 x Baumé value

Question 9

What is the potential alcohol content expected from a juice of density 12.5 Bé, and what is the expected sugar concentration of this juice in units of g L-1?

Answer 9

From the relationship:
potential wine alcohol in %(v/v) = juice Baumé value,
potential alcohol content = 12.5%(v/v)
From the relationship:
°Brix value = 1.80 x Baumé value
°Brix value = 1.80 x 12.5 = 22.5 °Brix.
From the approximation:
Juice sugar content, in g/100 mL = juice °Brix value
Juice sugar content, in g/100 mL = 22.5.
Since, for any given concentration, the number of grams per litre must be ten times the number of grams per hundred millilitre:
Juice sugar content, in g L-1 = 22.5 x 10
= 225 g L-1

Question 10

How are Oechsle units related to specific gravity?

Answer 10

Oechsle = (specific gravity -1) x 1000

Question 11

To qualify for the category Trockenbeerenauslese in Germany, wine must have been made from grapes that provided a minimum ‘must weight’ (or density) of 150 Oechsle.

What Baumé and °Brix values are expected for a juice of 150 Oechsle and what potential alcohol content does this Oechsle value indicate, if the value of 150 Oechsle has 3 significant figures?

Answer 11

From the relationship:

Juice Brix value = (Oechsle value - 2.5) / 4.00

Juice Brix value = (150 - 2.5) / 4.00

= 36.875 Brix

Brix value = 1.80 x Baume value

36.875 = 1.80 x Baume value
36.875 / 1.80 = Baume value
20.486 = Be
20.5 Be and 20.5 % (v/v)

Question 12

Why can other dissolved solids contribute to errors, approximations and apparent inaccuracies when using sugar concentration units?

Answer 12

• In unripe fruit, the acid concentration is higher than at grape maturity and the sugar concentration is less than at grape maturity. Consequently, density will be a less accurate indicator of sugar concentration.
• During alcoholic fermentation, sugars are converted to ethanol. Consequently, the density becomes affected by ethanol concentration rather than sugar concentration.

Question 13

Why can fermentation contribute to errors, approximations and apparent inaccuracies when using sugar concentration units?

Answer 13

• Conversion of sugars to ethanol during alcoholic fermentation is variable in its efficiency.
• Some sugar is used for cell growth and multiplication. A fermentation that is vigorous, with much cell growth and multiplication, will convert slightly less sugar to ethanol than one which is less vigorous.
• Also, volatile loss of ethanol can occur during fermentation which will also reduce the final yield of ethanol in wine.

Question 14

What is the revised relationship between sugar concentration and the alcohol content in wine?

Answer 14

Juice sugar (g/L) / 16.8 = potential wine alcohol (%(v/v))

Question 15

The concentration of sugar in juice was found to be 235 g/L. Calculate the potential alcohol (assuming the wine is fermented to dryness – 0 g/L sugar) using:
i) the traditional relationship between juice sugar and alcohol content
ii) the revised relationship between the two components.

Answer 15

Traditional calculation:

Juice sugar (Be) = Alcohol content (%(v/v) + Residual Sugar (g/L)
here: Residual Sugar = 0 g/L, and therefore:
Juice sugar (Be) = Alcohol content (%(v/v)

Next the Juice sugar concentration needs to be converted to Bé. First we convert the g/L to °Brix and then the °Brix to Bé:

Juice sugar = 235 g/L = 235 g/1000 mL = 23.5 g/100 mL = 23.5 °Brix
Bé = °Brix / 1.80 = 23.5/1.80 = 13.1 Bé

\Therefore, as Juice sugar (Be) = Alcohol content (%(v/v), the potential alcohol in the wine will be 13.1 %(v/v) based on the traditional calculation.

Revised approach for calculation:

Juice sugar (g/L) / 16.8 = potential alcohol (%(v/v))
235 / 16.8 = potential alcohol (%(v/v))
14.0 %(v/v) = potential alcohol.