GB 5009.238-2016National standard for food safety - Determination of water activity in foods (English PDF)

0.60 to 0.90.

Method I Conway's Dish Diffusion Method

3.1 Reagents

Unless otherwise stated, the reagents used in this method are of analytical

grade, and water is the tertiary water specified in GB/T 6682.

3.1.1 Lithium bromide (LiBr · 2H2O).

3.1.2 Lithium chloride (LiCl · H2O).

3.1.7 Cobalt chloride (CoCl2 · 6H2O).

3.1.8 Strontium chloride (SrCl2 · 6H2O).

3.1.9 Sodium nitrate (NaNO3).

3.1.10 Sodium chloride (NaCl).

3.1.11 Potassium bromide (KBr).

3.1.12 Ammonium sulfate [(NH4)2SO4].

3.1.16 Potassium nitrate (KNO3).

3.1.17 Potassium sulfate (K2SO4).

3.2 Preparation of reagents

3.2.1 Saturated lithium bromide solution (water activity of 0.064, 25°C).

Accurately WEIGH 500g of lithium bromide at a readily soluble temperature.

ADD 200mL of hot water. COOL to form saturated solution of solid-liquid two-

phase. STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

3.2.4 Saturated potassium carbonate solution (water activity of 0.432, 25°C).

Accurately WEIGH 300g of potassium carbonate at a readily soluble

temperature. ADD 200mL of hot water. COOL to form saturated solution of solid-

liquid two-phase. STORE saturated solution in a brown reagent bottle. PLACE

at room temperature for one week before use.

3.2.7 Saturated cobalt oxide solution (water activity of 0.649, 25°C).

Accurately WEIGH 160g of cobalt oxide at a readily soluble temperature. ADD

200mL of hot water. COOL to form saturated solution of solid-liquid two-phase.

STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

3.2.8 Saturated strontium oxide solution (water activity of 0.709, 25°C).

Accurately WEIGH 200g of strontium oxide at a readily soluble temperature.

ADD 200mL of hot water. COOL to form saturated solution of solid-liquid two-

phase. STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

3.2.9 Saturated sodium nitrate solution (water activity of 0.743, 25°C).

Accurately WEIGH 260g of sodium nitrate at a readily soluble temperature.

ADD 200mL of hot water. COOL to form saturated solution of solid-liquid two-

phase. STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

3.2.12 Saturated ammonium sulfate solution (water activity of 0.810, 25°C).

Accurately WEIGH 210g of ammonium sulfate at a readily soluble temperature.

ADD 200mL of hot water. COOL to form saturated solution of solid-liquid two-

phase. STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

3.2.13 Saturated potassium chloride solution (water activity of 0.843, 25°C).

Accurately WEIGH 100g of potassium chloride at a readily soluble temperature.

ADD 200mL of hot water. COOL to form saturated solution of solid-liquid two-

phase. STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

3.2.16 Saturated potassium nitrate solution (water activity of 0.936, 25°C).

Accurately WEIGH 120g of potassium nitrate at a readily soluble temperature.

ADD 200mL of hot water. COOL to form saturated solution of solid-liquid two-

phase. STORE saturated solution in a brown reagent bottle. PLACE at room

temperature for one week before use.

4.1 Conway's dish (with frosted glass cover). SEE Figure 1.

4.3 Balance. with the sensitivity of 0.000 1g and 0.1g.

4.4 Constant temperature incubator. with an accuracy of ± 1°C.

4.5 Electro-thermostatic blast oven.

5.1 Preparation of samples

5.1.1 Powdery solid, granular solid and pasty samples

TAKE at least 200g of representative sample. MIX evenly. PLACE in an airtight

glass container.

5.1.4 Mixed samples of various masses

TAKE a representative sample that has been mixed evenly.

5.1.5 Liquid or flowing sauce samples

Directly TAKE a homogeneous sample for weighing.

5.2 Pre-determination of samples

5.2.1 Pre-processing

PLACE a sealed container, a Conway's dish and a weighing dish containing the

samples in a constant temperature incubator. MAINTAIN constant temperature

at 25°C ± 1°C. USE and DETERMINE immediately after removal.

5.2.2 Pre-determination

Respectively ADD 12.0mL of saturated lithium bromide solution, saturated

magnesium chloride solution, saturated cobalt chloride solution and saturated

potassium sulfate solution to the outer chambers of 4 Conway's dishes.

5.2.3 Calculation of pre-determination results

5.2.3.1 The increment or decrement of sample mass shall be calculated in

accordance with the Formula (1).

5.2.3.2 DRAW a two-dimensional straight-line graph. DRAW a two-

dimensional straight-line graph, by taking the numerical values of the water

activity (Aw) in selected saturated salt solution (25°C) as the x-coordinates, and

the mass increment or decrement of the sample of corresponding standard

saturated salt solution as the y-coordinates. TAKE the intercept value of the x-

coordinates, that is, the predicted value of water activity in this sample. REFER

to Figure A.1.

5.3 Determination of samples

According to the pre-determination results in Subclause 5.2.3, SELECT 3 kinds

of saturated salt solution with the water activity values of greater than and less

than the predicated value of the sample, respectively, each taking 12.0mL.

10.1 Water activity meter.

10.3 Sample dish.

11.1 Preparation of samples

Same as Subclause 5.1.

11.2 Determination of samples

11.2.1 USE saturated salt solution to calibrate the water activity meter at a

room temperature of 18°C to 25°C and a humidity of 50% to 80%.