Agriculture & Future Food Systems · Article

Measurement and moisture during storage

Bind water-activity measurements to temperature and sample conditions, then follow how packaging and humid exposure can change a stored food.

Preparation and related reading

Shared Foundations

This article helps you

Describe the conditions needed for a meaningful water-activity result.
Distinguish measurement variation from evidence of product change.
Account for water uptake without treating packaging appearance as a safety test.

Measure a defined sample

Imagine checking a dried plant ingredient before storage. A water-activity reading concerns a particular sample at a particular temperature after appropriate equilibration. Its lot, location within the lot, preparation and instrument conditions belong with the number. One reading does not describe every piece or every future condition.

A suitable method lets the sample and enclosed measurement space reach the required equilibrium. Calibration or reference checks must follow the validated method and instrument instructions. The specialist equipment chapter covers sensors; here the question is whether a recorded result actually describes the food being assessed.

Retain temperature and equilibrium

Water activity compares vapour pressures at the same temperature. A warm sample introduced into a different-temperature measurement space can produce changing conditions or condensation. Allow the method’s required equilibration and record temperature; do not interpret an unstable early display as a final value.

Temperature changes can affect aw, and the relationship depends on the actual material. There is no universal correction that turns every reading at one temperature into its value at another. Nor does a room-humidity measurement automatically equal the sample’s equilibrium humidity. Keep the comparison conditions matched before interpreting differences.

Interpret repeat measurements honestly

Suppose hypothetical replicate measurements under the same specified conditions give aw values 0.61, 0.62 and 0.63. Their mean is (0.61+0.62+0.63)/3 = 0.62. The range spans 0.02. This describes the three recorded observations; it does not quantify every source of uncertainty or the variation across the entire lot.

A later result of 0.63 cannot automatically be called a meaningful storage increase above 0.62. Sampling differences, instrument behaviour, temperature and repeatability must be considered. The shared Measurement Uncertainty record explains why a displayed decimal difference is not sufficient evidence of a real change.

Follow water uptake during storage

Dried food can reabsorb moisture from its surroundings. At a matched temperature, an exposed food with aw 0.60 and surrounding air at 75% relative humidity has a thermodynamic tendency to gain water towards equilibrium. Packaging permeability, opening, time and the food’s own moisture–activity relationship determine how that tendency develops.

This does not predict a final moisture percentage or an uptake rate. Product-specific behaviour must be measured. Pieces can also redistribute moisture among themselves, especially if their initial conditions differ. A single apparently dry surface may conceal uneven water conditions within a portion or lot.

Close an uptake account

Consider an invented 100 g portion with 90 g nonwater material and 10 g water. Assume 5 g water enters during storage and nothing else changes. It now weighs 105 g and contains 15 g water, giving 15/105×100 ≈ 14.3% wet-basis moisture. The initial value was 10%. Nonwater material remains 90 g.

The mass account establishes the stipulated uptake and changed moisture fraction. It does not determine the final aw or microbial outcome. If a real package gains mass, establish whether water uptake, another change or measurement error caused it rather than assuming the reason. Likewise, absence of visible condensation cannot prove that no water moved.

Hypothetical water-only uptake during storage
Term	Before	After
Nonwater material	90 g	90 g
Water	10 g	15 g
Total mass	100 g	105 g
Wet-basis moisture	10%	Approximately 14.3%

Track water uptake without changing dry material

Original hypothetical mass account. No uptake rate, shelf life, final aw or safety conclusion is implied. Water is the only assumed addition.

Read the diagram as text

The hypothetical portion initially contains 90 g nonwater material and 10 g water. Five grams additional water gives 105 g total with 15 g water.

Parts

100 g before storage: 90 g nonwater material plus 10 g water.
105 g after uptake: 90 g nonwater material plus 15 g water.
Approximately 14.3% moisture: 15/105×100; wet basis, not water activity.

Connections

100 g before storage → 105 g after uptake: add 5 g water
105 g after uptake → Approximately 14.3% moisture: calculate fraction

Match packaging to the storage context

The package, closure and actual storage environment form a system. A suitable moisture barrier must perform for the product, temperature, humidity, intended duration and opening pattern. Naming a packaging material or calling it airtight does not establish a measured water-vapour barrier or a verified shelf life.

Monitor appropriate product and storage properties, preserve lot identity and investigate changes. Dryness, texture and a normal appearance are useful observations but cannot establish complete food safety. The storage and safety chapters develop product-specific controls; this article shows how to connect a measurement with its conditions and follow possible moisture changes over time.

Worked examples

Summarize the three measurements

What does the hypothetical 0.61, 0.62, 0.63 record establish?

Sum: 0.61+0.62+0.63 = 1.86.
Mean: 1.86/3 = 0.62.
Range: 0.63−0.61 = 0.02; other uncertainty and lot variation remain unquantified.

The sample mean is 0.62 with a recorded range 0.02. This is not an acceptance limit or proof that a later difference is significant.

Account for moisture uptake

What follows when the stipulated 100 g portion gains 5 g water and no other material changes?

Water: 10+5 = 15 g; nonwater remains 90 g.
Total: 90+15 = 105 g.
Wet-basis moisture: 15/105×100 ≈ 14.3%.

The moisture fraction changes from 10% to approximately 14.3%; aw and food safety need separate evidence.

Check your understanding

Does room RH 75% prove an unequilibrated sample already has aw 0.75?
Show answer and reasoning
No.
The relationship concerns equilibrium at matched temperature; rate and material conditions also matter.
Can the three displayed aw values supply complete measurement uncertainty?
Show answer and reasoning
No.
They describe observed variation only; calibration, sampling, temperature and other method effects remain relevant.
Does lack of visible condensation prove that stored food has not gained moisture?
Show answer and reasoning
No.
Water can enter or redistribute without visible droplets. Appropriate product measurements are needed.

Key ideas

Keep sample identity, temperature, equilibrium and method with every result.
Separate a recorded difference from evidence of meaningful product change.
Packaging and actual storage conditions influence moisture transfer; measure the outcome.

References

Sources supporting the explanations and examples in this article.