Measurement
Quality measurement methods
Every quality attribute has to be observed somehow, and every method has a range where it is trustworthy and a range where it is not. This section covers the instruments and methods, the principle each works on, and — the part that decides whether a number means anything — where each one misleads.
A number is only as good as the method and the sample
Two things routinely invalidate a post-harvest measurement before the instrument is even at fault. The first is sampling: a reading describes the sample, and only describes the lot if the sample genuinely represents it. The second is calibration: many field instruments are indirect — they measure something correlated with the property of interest and convert it using a commodity-specific calibration. An electronic moisture meter does not weigh water; it responds to electrical properties and converts. Move to a different commodity, a different temperature, or an uneven lot, and the conversion degrades.
That is why reference methods exist. Oven-drying is slow and destructive, but it is what fast meters are calibrated against — and knowing which of the two you are holding is the difference between a measurement and a guess.
What these pages do not give you
No thresholds, no pass/fail values, and no operating procedures. Acceptable values belong to the applicable standard or contract, and procedures belong to the method’s own documentation. See quality attributes for what is being measured and why.
Measurement methods A–Z
Each entry covers what the method measures, the principle it works on, sampling requirements, whether it is destructive, its reference method, and where it is unreliable.
18 entries
- Quality measurement
Colorimeter
A colorimeter — also called a chroma meter or tristimulus colorimeter, with a spectrophotometer as a related but distinct instrument — measures the light reflected from a produce surface under a specified illuminant and reports it as coordinates in a defined colour space. The instrument is precise about the exact spot it reads and silent about everything colour is commonly used as a proxy for: ripeness, internal condition, and eating quality.
- Quality measurement
Electronic Moisture Meter
An electronic moisture meter infers grain moisture from an electrical property of the sample — capacitance or conductance — using a stored, commodity-specific calibration built against the oven-drying reference method. It is a calibration, not a direct measurement, and it is only right for the commodity and conditions its calibration was built for.
- Quality measurement
Ethylene Measurement
Ethylene measurement quantifies ethylene gas either produced by a commodity, sampled from the headspace of a sealed enclosure, or present in a store's atmosphere, sampled at a defined point. A fruit's production rate and a store's ambient concentration are genuinely different quantities that are routinely and consequentially confused: production is a property of the commodity, while store concentration also depends on load, free volume, ventilation, and any active scrubbing.
- Quality measurement
Falling Number Test
The falling number test times how quickly a stirrer falls through a hot paste made from ground grain and water, reported as total elapsed seconds. Because active alpha-amylase thins the paste, a LOWER falling number means MORE enzyme activity and greater sprout damage — the inverse of what the name suggests, and the test's most common misreading.
- Quality measurement
Germination Test
The germination test places a counted sample of seeds under prescribed substrate, temperature, moisture, and light conditions for a prescribed period, then classifies the resulting seedlings and reports the proportion that are normal. It measures performance under near-ideal conditions, so it routinely overstates field emergence and does not measure vigour.
- Quality measurement
Grain Probe Sampling
Grain probe sampling draws increments from a bulk with a trier, spear, or pneumatic probe — or, for moving grain, cuts the stream with a diverter sampler — and composites and divides them to a working sample. Sampling determines what value a lot is ultimately assigned, and sampling error usually exceeds the error of whatever instrument later analyses the sample.
- Quality measurement
Near-Infrared Spectroscopy
Near-infrared spectroscopy predicts moisture, protein, oil, starch, kernel hardness, dry matter, and soluble solids from how a sample absorbs near-infrared light, using a multivariate calibration model built against primary reference methods. It does not measure any of these constituents directly — it is a correlation, entirely dependent on the quality of the reference analyses it was trained against and the similarity of new samples to its training set.
- Quality measurement
Oven-Drying Method
The oven-drying method determines moisture content by weighing a sample, drying it under prescribed conditions, and calculating the mass lost as moisture. It is the reference method every faster moisture technique is calibrated against, but it measures mass loss, not water specifically, and its result is only valid alongside the official method that produced it.
- Quality measurement
Penetrometer
A penetrometer, also called a fruit pressure tester or firmness tester, measures firmness by driving a probe of a specified diameter into pared flesh to a specified depth and recording the peak force. It is a destructive, instrument-dependent measurement: the probe geometry, penetration depth, and fruit temperature are all part of the result, and the reading tells you about resistance to a probe, not about eating texture.
- Quality measurement
Refractometer
A refractometer, also called a Brix meter, determines soluble solids content by measuring how strongly expressed juice bends light and reporting that refractive index on the degrees-Brix scale, which is calibrated against pure sucrose solutions rather than real fruit juice. It is the precision instrument of post-harvest quality measurement and also its most commonly misread one: a Brix reading is not a sugar assay, not a sweetness score, and not a universal maturity verdict.
- Quality measurement
Respiration Measurement
Respiration measurement quantifies how fast a commodity consumes oxygen or evolves carbon dioxide, using either a sealed closed system that tracks gas accumulation over time or a flow-through system that tracks the difference between inlet and outlet gas. The measurement disturbs the very process it observes, is strongly temperature-dependent, and reports a rate of reserve depletion rather than a remaining shelf life — it must be read as a snapshot of current metabolic activity, not a forecast.
- Quality measurement
Sieve Analysis
Sieve analysis separates a grain sample by passing it over sieves of prescribed aperture size and shape, weighing what is retained and what passes through. What counts as "broken," "dockage," or a size class is defined entirely by the sieve prescribed in the applicable standard, so results from different standards are not comparable, and the method sorts by size and shape only — never by identity or condition.
- Quality measurement
Temperature Probe
A temperature probe or fixed cable system reads grain temperature and its trend over time. It measures none of the things it is used to detect — mould and insects — but is used because respiring grain, growing fungi, and feeding insects all generate heat that a poorly conductive bulk retains, making a rising trend a practical, if lagging and localised, surveillance proxy.
- Quality measurement
Test-Weight Apparatus
Test-weight apparatus fills a container of defined volume from a defined height and funnel, strikes the surface level, and weighs the contents to report mass per unit volume. The result is defined entirely by the prescribed procedure — a different apparatus or filling method gives a different figure on the same grain, and converted figures between measurement systems are not equivalent to direct measurements.
- Quality measurement
Thousand Kernel Weight Count
Thousand kernel weight count determines the attribute by counting a defined number of whole, sound kernels — by hand or electronic counter — weighing them, and scaling and adjusting the result to a stated moisture basis. The figure is meaningless without that moisture basis, and both the counting step and the definition of "kernel" used are sources of real, hidden error.
- Quality measurement
Titratable Acidity Titration
Titratable acidity titration determines the total acid in expressed juice by neutralising it with a standardised alkali to a defined endpoint, then expressing the result as the commodity's dominant acid. The result is a laboratory convention, not a raw chemical fact: the acid it is expressed as, the endpoint detection method, and the presence of dissolved carbon dioxide all change the figure, and it must never be substituted for a pH measurement.
- Quality measurement
Visual Defect Inspection
Visual defect inspection is a trained inspector examining a drawn sample against defined criteria and reference standards, counting or weighing units into defect categories. It cannot detect mycotoxins, cannot see inside an intact unit, and identifies appearance rather than cause — a lot judged clean by eye is not a lot cleared of every hazard.
- Quality measurement
Water Activity Meter
A water activity meter measures the equilibrium relative humidity of the air in a sealed chamber containing the sample — by chilled-mirror dew point or by a capacitive or resistive sensor — and reports that value as water activity. It measures the air, not the sample directly, and it requires true equilibrium and a stated, matched temperature to be meaningful.
Sources
This article draws on the following authoritative sources. See our sources & methodology for how they are selected.
- [1]USDA ARS — Agricultural Research Service (opens in a new tab)Authoritative
USDA Agricultural Research Service (ARS)
Cited for: Measurement methods and reference procedures
- Type:
- Government agency
- Jurisdiction:
- United States
- Accessed:
- 2026-07-12
- [2]FAO — Food and Agriculture Organization (opens in a new tab)Authoritative
Food and Agriculture Organization of the United Nations (FAO)
Cited for: Post-harvest quality measurement guidance
- Type:
- Intergovernmental organization
- Jurisdiction:
- Global
- Accessed:
- 2026-07-12
- [3]AHDB — Agriculture and Horticulture Development Board (opens in a new tab)High
Agriculture and Horticulture Development Board (AHDB)
Cited for: Grain sampling and testing context
- Type:
- Government agency
- Jurisdiction:
- United Kingdom
- Accessed:
- 2026-07-12