SOP: Determination of Equilibrium Relative Humidity by the Graphical Interpolation Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

The graphical interpolation method is based on the principle that the rate of gain or loss of moisture of a product, at any given initial moisture content and atmospheric relative humidity, increases with increasing difference between the atmospheric and the product vapour pressures. The duration of exposure is held constant in a true equilibrium (gain or loss of weight method). It is much quicker than the weight equilibrium method and gives an approximate determination of the ERH; the weight equilibrium method gives more precise data and also detailed information on storage behaviour (danger and critical points, safe margin), especially when the product shows visible changes in colour, texture, etc.

Apparatus required

• Crystallizing dishes prepared with glass covers, or desiccators (as for the weight equilibrium method)
• Petri dishes
• Analytical balance

Reagents

a. H₂SO₄ solutions: prepare 750–1,000 ml of H₂SO₄ solutions of 10, 20, 30, 40, 50, 60, 70, 80 and 90% relative humidity from the sulphuric-acid reference table at 25°C (RH 0–100%: specific gravities 1.835 down to 1.000, normalities 22.0 down to 0.0 — e.g., 50% RH / 1.335 / 11.5 N; 70% RH / 1.248 / 8.3 N; 90% RH / 1.127 / 3.9 N).

Sample preparation — Conditioning the sample to the moisture range

Prior to the determination, condition the sample to the entire range of moisture content relevant to it (e.g., for dehydrated potato, the moisture content varies from 3 to 16%):

i. Use one sample of 50–60 g as it comes out of the production line or from a package.

ii. Place another 180 to 200 g in a vacuum oven and dry at 40–45°C; remove one portion (~60 g) after 1 hr, a second after 2 hr and the last after 3 hr; store the samples at 25°C immediately after drying, in moisture-proof bottles with rubber-lined caps — these samples have lower moisture contents than the sample in i.

iii. Place 5 or 6 other portions in a large desiccator containing either water or a solution of relatively high humidity (90%) at 37°C; stir at intervals to ensure uniformity; remove each at varying intervals and immediately transfer into a bottle or tin as before, storing at 25°C.

Determine the moisture content of each of the above samples by drying in a vacuum oven at 70°C for 6 hr (fruit juice powders require longer — 16 hr heating).

Procedure — Determination of ERH

Note the tare weight of the Petri dishes; the number of dishes required for each sample equals the number of different relative humidities at which the tests are carried out

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Place 5 ± 0.01 g of the sample into each Petri dish, note the weight, and place the dishes in the humidity control units (crystallizing dish or desiccator)

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Place the units in a balance room at 25°C and note the gain or loss in weight (mg) of the samples in the dishes at intervals of 1 hr; in the same way, note the gain or loss in weight of the other samples conditioned at the different moisture contents

Plotting and interpretation

Plot a graph of weight gain/loss (mg per hour) against % relative humidity; when sufficient points have been drawn above and below the zero base line for a sample of given moisture content, draw a smooth curve; draw similar curves for the samples of different moisture contents. Each graphical interpolation curve constitutes an isotherm (a curve based on data obtained at a constant temperature). The zero base line represents no change in weight. The ERH is therefore given by the relative humidity at the point at which an isotherm cuts the zero base line. From the ERH values so obtained and the moisture contents of the samples, the humidity–moisture equilibrium curves can be drawn.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Landrock, L.B., Food Res., 22, 604 (1957) — cited in the book.