ANNEX

METHOD OF MEASURING DRY SOLUBLE RESIDUE IN PRODUCTS PROCESSED FROM FRUIT AND VEGETABLES BY REFRACTOMETRY

(DETERMINATION OF THE BRIX-VALUE)

1.   DEFINITION

Dry soluble residue content (Brix value, determined by refractometry) means the percentage weight of sucrose in an aqueous solution of sucrose which, under given conditions, has the same refractive index as the product analysed.

2.   APPARATUS

The principal type of apparatus to be used is the Abbe-type refractometer. Alternatively, the use of a digital refractometer is permitted.

This apparatus must enable the percentage weight of sucrose to be determined to the nearest ± 0,1 %.

The refractometer must be calibrated at 20 °C by a system that enables the temperature of measurement cell to be adjusted from + 15 °C to + 25 °C with an accuracy of ± 0,5 °C.

Operating instructions for this apparatus, and in particular those dealing with calibration and light source, must be strictly followed.

3.   METHOD

3.1.   Preparation of the sample

3.1.1.   Liquid products

Mix carefully and proceed to determination.

3.1.2.   Semi-dense products, purées, fruit juices with matter in suspension

Carefully mix an average laboratory sample and then homogenize.

Strain a part of the sample through dry gauze folded in four, remove the first drops and proceed to the determination on the filtrate.

3.1.3.   Dense products (jams and jellies)

If the previously homogenized product cannot be used directly, weigh 40 g of the product to the nearest 0,01 g in a 250 ml beaker and add 100 ml of distilled water.

Boil gently for two or three minutes, stirring with a glass rod.

Cool, pour the contents of the beaker into an appropriate tared vessel using distilled water as a flushing liquid, add distilled water so as to obtain about 200 g of product, weigh it to the nearest 0,01 g, and mix the solution thoroughly.

Allow to stand for 20 minutes, then strain through a folded filter or a Büchner funnel. Make a determination on the filtrate.

3.1.4.   Frozen products

Defrost and remove stones or pips and cores.

Mix the product with the liquid formed during defrosting and proceed as in points 3.1.2 and 3.1.3 respectively.

3.1.5.   Dry products or products containing whole fruit or pieces of fruit

Cut the laboratory sample — or part of it — into small pieces, remove stones or pips and cores and mix carefully.

Weigh 10 to 20 g of the product to the nearest 0,01 g in a beaker.

Add distilled water corresponding to five times the weight of the product.

Heat in a bath of boiled water for 30 minutes stirring occasionally with a glass rod.

When cool, continue as described in point 3.1.3.

3.1.6.   Products containing alcohol

Weigh about 100 g of the sample to the nearest 0,01 g in a tared beaker.

Place the beaker in a bath of boiled water for 30 minutes, stirring occasionally with a glass rod, and add distilled water if necessary.

Where the alcohol content exceeds about 5 % mass add more distilled water and heat again in the bath of boiled water for 45 minutes.

After cooling weigh the final contents of the vessel, filter if necessary, and continue with the determination.

3.2.   Determination

The principle is the deduction of the dry soluble residue content of a product from its refractive index.

The measurement temperature shall be between 15 and 25 °C.

By using a digital refractometer the temperature shall be at 20 °C.

Bring the sample to the measurement temperature by immersing the container in a water bath at the required temperature.

Place a small sample on the lower prism of the refractometer, taking care to ensure that the sample covers the glass surface uniformly when the prisms are pressed against each other.

Measure in accordance with the operating instructions for the apparatus used.

Read the percentage weight of sucrose to the nearest 0,1 %.

Make at least two determinations on the same prepared sample.

4.   EXPRESSION OF RESULTS

Calculation and formulation

The dry soluble residue content is expressed in grams per 100 grams of the product (g/100 g). This is equivalent to a value in °Brix.

The dry soluble residue content shall be calculated as follows:

The percentage sucrose content indicated by refractometry shall be used directly.

If the reading is made at a temperature other than + 20 °C, correct as indicated in table 1.

If the measurement has been made on a diluted solution, the dry soluble residue content (M) shall be calculated using the following formula:

M' being the weight (in grams) of dry soluble residue per 100 g of product indicated by the refractometer and E the weight (in grams) of product per 100 g of solution.

The result of that calculation shall be given to one decimal place (+/- 0,1 °Brix).

Table 1

Corrections when determination is made at a temperature other than 20 °C

Temperature °C	Sucrose in grams per 100 grams of product
	5	10	15	20	30	40	50	60	70	75
	Subtract
15	0,25	0,27	0,31	0,31	0,34	0,35	0,36	0,37	0,36	0,36
16	0,21	0,23	0,27	0,27	0,29	0,31	0,31	0,32	0,31	0,23
17	0,16	0,18	0,20	0,20	0,22	0,23	0,23	0,23	0,20	0,17
18	0,11	0,12	0,14	0,15	0,16	0,16	0,15	0,12	0,12	0,09
19	0,06	0,07	0,08	0,08	0,08	0,09	0,09	0,08	0,07	0,05
	Add
21	0,06	0,07	0,07	0,07	0,07	0,07	0,07	0,07	0,07	0,07
22	0,12	0,14	0,14	0,14	0,14	0,14	0,14	0,14	0,14	0,14
23	0,18	0,20	0,20	0,21	0,21	0,21	0,21	0,22	0,22	0,22
24	0,24	0,26	0,26	0,27	0,28	0,28	0,28	0,28	0,29	0,29
25	0,30	0,32	0,32	0,34	0,36	0,36	0,36	0,36	0,36	0,37

5.   PRECISION

Details of an inter-laboratory test relating to precision data of the method performed on 8 samples are given in this point. They reflect the performance requirements for the method described in this annex. The precision data are given below in table 2.

Source of the precision data

The precision data were determined from an inter-laboratory test that was carried out in 1999/2000 with the participation of the European Customs Laboratories.

The evaluation of the precision data was performed according to ISO 5725.

Table 2

Precision data

Sample name	Number of laboratories	Mean (°Brix)	Repeatability limit r (%)	Reproducibility limit R (%)
Fruit cocktail	11	18,9	3,0	4,7
Pineapple	10	19,4	1,7	1,7
Apple compote	12	19,5	2,0	2,7
Tropical fruit	9	12,8	2,9	4,0
Strawberry jam	12	59,8	4,0	7,2
Apple juice	12	11,1	1,4	4,7
Orange juice concentrate	9	65,2	1,3	2,6
Powder orange juice	11	99,8	2,3	5,3