ISO 1842:1991

Overview

ISO 1842:1991 specifies a potentiometric method for determining the pH of fruit and vegetable products. This second edition (1991) replaces the 1975 edition and describes the principle, required apparatus, sample preparation, test procedure, expression of results, and the contents of the test report. The method covers a wide range of matrices including juices, brines, purees, jams, syrups, dried and frozen products.

Key topics and technical requirements

• Measurement principle: potentiometric determination of the potential difference between electrodes immersed in the test portion.
• Apparatus: pH meter (scale reading to 0.05 pH or better) and electrodes - glass electrode, calomel reference electrode (stored in saturated KCl), or combined-electrode systems.
• Sample preparation: specific procedures for
  • liquid and easily filterable products (mix to homogeneity),
  • thick/semi-thick products (grind; if still thick, add equal mass of distilled water),
  • frozen products (thaw, remove stones, then treat as liquid or thick as appropriate),
  • dried products (reconstitute with 2–3× mass of distilled water and heat/mix),
  • freshly prepared products with separate solid/liquid phases (treat like semi-thick products).
• Calibration and buffers: calibrate using buffer solution(s) with pH close to the sample. ISO 1842 provides recipes and temperature information for standard buffer solutions (examples include buffers around pH 3.57, 4.00, 6.88 and guidance for pH 5.00). If the meter lacks temperature compensation, maintain the test and buffer temperature (nominally 20 °C ± 2 °C).
• Procedure and reporting:
  • perform two independent determinations on separate test portions,
  • report the arithmetic mean to at least 0.05 pH unit,
  • repeatability requirement: the absolute difference between the two results (same operator, equipment, lab) should not exceed 0.1 pH unit,
  • include all operational details and sample identification in the test report.

Applications

• Routine quality control and acidity monitoring of fruit and vegetable products (juices, jams, pickles, purees, syrups).
• Product formulation and R&D (acidulation, flavor balance).
• Shelf-life and fermentation control (fermented vegetables, brines).
• Regulatory compliance and labeling where pH or acidity affects safety, preservation, or classification.

Who uses this standard

• Food manufacturers and QC laboratories in the fruit & vegetable sector
• Contract analytical laboratories and regulatory agencies
• Product development scientists and food technologists
• Academic researchers working on food chemistry and preservation

Related standards

ISO 1842 complements other ISO standards on food analysis and sampling produced by ISO/TC 34 (Agricultural and food products). Laboratories should also follow relevant general standards for pH measurement instrumentation and calibration best practices.