Food testing basics: composition, contaminants and microbiology

Food testing answers three questions. Is the food safe? Is it what the label says it is? Does it meet the legal standard and the customer's specification? A food laboratory may be in-house at a factory, a commercial contract lab, or a government enforcement lab.

Unlike pharmaceutical QC, food testing rarely has a single monograph per product. Instead the lab works to official method collections (AOAC Official Methods, ISO and EN standards, the FSSAI Manual of Methods of Analysis of Foods, Codex methods) and checks results against limits in food law — FSSAI regulations in India, EU regulations in Europe, FDA rules in the US, and Codex standards internationally.

Proximate analysis splits a food into five or six broad fractions that add up to about 100 g per 100 g. It is the basis of the nutrition panel and of most compositional standards (for example minimum protein in a dairy product or maximum moisture in flour).

• Moisture: loss in mass on drying in an oven under set conditions (temperature, time, sometimes vacuum). Water activity (aw) is different — it measures how 'available' the water is to microbes, not how much there is.
• Ash: residue after burning off organic matter in a muffle furnace at about 525–550 °C. It estimates total minerals.
• Protein: total nitrogen by Kjeldahl (or Dumas combustion) × a nitrogen-to-protein factor — 6.25 for most foods, 6.38 for dairy, 5.7 for wheat products.
• Fat: solvent extraction (Soxhlet) after acid hydrolysis where fat is bound, or Gerber / Röse-Gottlieb for milk.
• Crude or dietary fibre: residue after acid and alkali digestion (crude fibre) or enzymatic–gravimetric methods (dietary fibre).
• Carbohydrate: usually calculated 'by difference' = 100 − (moisture + protein + fat + ash + fibre).

Energy is calculated, not measured: commonly about 4 kcal/g for protein and carbohydrate, 9 kcal/g for fat, 2 kcal/g for fibre and 7 kcal/g for alcohol (17, 37, 8 and 29 kJ/g). Check the factors your labelling law uses.

Contaminants are substances not added on purpose — they come from the environment, processing or storage. Residues are what remains of substances used deliberately, such as pesticides on crops or veterinary drugs in animals. Both are controlled by maximum levels (MLs) or maximum residue limits (MRLs) in food law.

Concentrations are very low — µg/kg for mycotoxins, mg/kg or less for metals and pesticides — so these tests need clean-up steps, sensitive and selective detectors, matrix-matched calibration and strict blank control.

• A residue of a pesticide with no specific MRL may be judged against a default limit (0.01 mg/kg in the EU), so a lab must report reliably down to that level.
• Enforcement decisions usually allow for measurement uncertainty — a result is non-compliant only if it exceeds the limit beyond reasonable doubt.
• For some contaminants the toxic form matters: inorganic arsenic in rice, methylmercury in fish. Total-element results are used as a screen and speciation follows if needed.

Food additives — preservatives, sweeteners, colours, antioxidants, emulsifiers — are permitted only in listed food categories and up to stated maximum levels (Codex GSFA, FSSAI Food Additives Regulations, EU additives regulation). The lab checks two things: that the additive is allowed in that food, and that the amount is within the limit and declared on the label.

Most water-soluble additives are measured by reversed-phase HPLC with UV or diode-array detection. One run can often measure benzoic acid, sorbic acid, caffeine, saccharin and acesulfame-K together.

• Express preservatives as the free acid unless the regulation says otherwise (sodium benzoate × 0.8474 = benzoic acid).
• Remember natural occurrence: some fruits, spices and fermented foods contain benzoic acid naturally.

Food microbiology tests fall into two groups. Indicator and hygiene tests count organisms — aerobic plate count, Enterobacteriaceae, coliforms, E. coli, yeasts and moulds — and show how well the food was handled and processed. Pathogen tests look for organisms that cause illness — Salmonella, Listeria monocytogenes, E. coli O157, Bacillus cereus, Staphylococcus aureus.

Counts are reported as colony-forming units per gram (CFU/g). Pathogen tests are usually presence/absence in a stated mass (for example 'not detected in 25 g') and need pre-enrichment, selective enrichment, plating on selective agars and confirmation.

• Common reference methods: ISO 4833-1 (aerobic count), ISO 6579-1 (Salmonella), ISO 11290-1 (Listeria), ISO 21527 (yeasts and moulds), ISO 4832 and ISO 16649 (coliforms and E. coli), and the FDA Bacteriological Analytical Manual (BAM).
• Microbiological criteria often use sampling plans with n, c, m and M — for example n = 5 samples, of which c = 2 may fall between m and M but none above M.
• Media must pass growth promotion checks, and incubators and autoclaves must be qualified and monitored.

A food lab usually tests a few grams that must represent a whole lot of tonnes. For many tests the sampling and sample preparation contribute more uncertainty than the instrument. This is especially true for aflatoxins, which occur in a few heavily contaminated kernels.

1. 1Follow the official sampling plan for the test (number of incremental samples, aggregate sample mass) — for example the sampling rules attached to contaminant regulations.
2. 2Label, seal and record the chain of custody. Keep a retained (reference) portion for possible dispute or repeat testing.
3. 3Transport and store under conditions that do not change the result: chilled for microbiology (test quickly), frozen for pesticides, protected from light for mycotoxins and some vitamins.
4. 4Homogenise the whole laboratory sample before taking the test portion. Grind, mill (cryogenically for pesticides), or blend.
5. 5Take the test portion from the homogenate and weigh it accurately. For microbiology, work aseptically.

Most food testing labs that issue reports to regulators or customers are accredited to ISO/IEC 17025, 'General requirements for the competence of testing and calibration laboratories'. In India, accreditation is given by NABL; FSSAI recognises NABL-accredited labs for official food testing. Other countries have their own accreditation bodies (UKAS, A2LA, DAkkS and so on).

Accreditation is granted per method and matrix (the 'scope'), not for the whole lab in general.

• Method validation or verification: recovery, precision, LOD/LOQ, linearity and selectivity for each matrix.
• Measurement uncertainty estimated for each quantitative test.
• Metrological traceability: calibrated balances, thermometers and pipettes; certified reference materials and standards.
• Quality control in every batch: blanks, spikes, duplicates, CRMs and control charts.
• Proficiency testing (e.g. FAPAS, LGC or national schemes) to compare performance with other labs.
• Impartiality, confidentiality, document control, records and complaint handling.

Composition results feed the nutrition panel. Typical declarations per 100 g (or 100 mL) include energy, protein, carbohydrate, total sugars, fat, saturated fat, trans fat, sodium (or salt) and sometimes fibre. Codex guidelines on nutrition labelling, FSSAI Labelling and Display Regulations and EU food information rules set what must be declared and how.

Declared values are averages, so regulators allow tolerances around them. Values may come from analysis, from recipe calculation using food composition tables, or from both. Analysis is needed to confirm claims such as 'high protein' or 'low fat'.