Rapid Automatic Analysis of Acids in Juice a Discrete Analyzer

Significance of the topic

The composition and concentration of organic acids in fruit juices determine sensory attributes (taste, acidity), color stability and microbial shelf life, and serve as markers of product authenticity and raw-material quality. Rapid, reliable quantification of multiple acids is therefore critical across production stages for quality control, fraud detection (e.g., citric acid to D‑isocitric acid ratio) and regulatory compliance. Automating these analyses reduces operator time, sample handling variability and per-test reagent costs while increasing laboratory throughput and data traceability.

Objectives and study / article overview

This application note evaluates system reagents and measurement methods implemented on Thermo Scientific Gallery and Gallery Plus automated discrete analyzers for routine analysis of acids in juices. The aim is to describe measuring ranges, linearity, precision (repeatability and reproducibility), throughput and practical aspects of reagent handling and data traceability for assays including acetic, citric, D‑isocitric, L‑ascorbic (vitamin C), D‑ and L‑lactic acids and total acidity.

Methodology

The analyzers apply discrete cell technology using a combination of enzymatic, colorimetric and automated titration-based methods adapted for small total volumes (typically <300 µL per test). Key methodological features:

• Pre-programmed application menus with assay-specific protocols and automated dilution functions to span wide concentration ranges.
• Assays designed to be blanked for sample/reagent color interference when appropriate.
• Ready-to-use system reagents; some kits are supplied lyophilized or powdered for reconstitution to extend shelf life.
• Reagent vials are bar-coded (lot number, expiry) and the instrument tracks reagent lot data, warning when stock is low and performing automatic recalibration after reagent replacement.

Precision studies used at least three QC levels with ≥10 replicates per day across five days; assays were calibrated daily between batches.

Used instrumentation

Main instrumentation and consumables described:

• Thermo Scientific Gallery automated discrete analyzer.
• Thermo Scientific Gallery Plus automated discrete analyzer (higher throughput model).
• Thermo Scientific ready-to-use reagent kits for individual acids (some supplied lyophilized/powdered).
• Bar-coded reagent vials enabling on-board stability tracking, lot traceability and automated calibration.

Performance metrics reported for the Gallery Plus indicate high throughput: a complete acid test panel for 100 samples is reported in the order of tens of minutes (full panel ≈44 minutes); when testing only total acids the throughput improves (100 tests in ≈35 minutes). First results are typically available within minutes after sample insertion depending on the panel.

Main results and discussion

Linearity and measuring ranges:

• High correlation between measured and theoretical concentrations across methods with coefficients of determination typically ≥0.997 and several assays showing r² ≥0.999 (examples: L‑ascorbic acid r² ≈0.9998; acetic acid r² ≈0.9996; citric acid r² ≈0.9998).
• Representative measuring ranges (as implemented via automated dilutions):

1. Acetic acid: 0.04–3.00 g/L
2. L‑Ascorbic acid: 50–2500 mg/L
3. Citric acid: 15–5000 mg/L (reported also as 0.5–15 g/L for certain procedures)
4. D‑Isocitric acid: 10–600 mg/L
5. D‑Lactic acid: 25–1600 mg/L
6. L‑Lactic acid: 20–1600 mg/L
7. Total acids (juice, pH 8): reported ranges cover grams per liter levels used in juice analysis

Precision (repeatability and reproducibility):

• Within-run CVs were low across analytes (examples: citric acid within-run CV as low as ≈0.5%; L‑ascorbic acid within-run CVs ≈0.8–1.0%; acetic acid within-run CVs ≈0.8–1.3%).
• Total CVs (combined within- and between-run) typically remained below 3% for many analytes, with some assays showing total CVs around 1–2% and a few up to ~3% depending on concentration level.
• These precision data support high repeatability suitable for routine QA/QC monitoring.

Speed and throughput:

• Discrete cell format and automated dilution allow walk-away operation and multi-analyte panels from a single aliquot, significantly reducing operator time.
• Typical throughput examples: a full acid panel for 100 samples in roughly tens of minutes; single-analyte (total acids) mode yields faster batch throughput and first results within minutes.

Traceability and reagent handling:

• On-board reagent identification and lot/expiry tracking ensure full traceability of calibrations and results.
• Optimized reagent volumes (kits designed to deliver ~300 tests per kit on average) help lower per-test reagent cost.

Benefits and practical applications

Key practical advantages:

• Multi-analyte capability from a single sample reduces preparative steps and sample consumption.
• Automated dilutions extend dynamic range and allow accurate measurement across low and high concentration samples without extensive manual intervention.
• High linearity and low CVs support use for production QA/QC, raw-material screening, finished-product release and authenticity checks (e.g., citric/D‑isocitric ratio).
• Barcoded reagents and automated calibration support data integrity and regulatory record-keeping.
• Lower reagent volumes and walk-away automation reduce cost per test and labor demands.

Future trends and potential uses

Anticipated developments and applications where this approach integrates well:

• Deeper integration with laboratory information management systems (LIMS) for automated result reporting and batch release workflows.
• Expansion of discrete analyzer panels to include additional food-relevant analytes (sugars, preservatives) for broader compositional profiling in a single run.
• On-line or at-line process monitoring adaptations to provide near real-time control during juice processing.
• Further miniaturization and multiplexing of enzymatic/colorimetric chemistries to boost throughput and reduce reagent consumption further.
• Use of lyophilized reagents and improved storage/transport formats to extend kit shelf life for decentralized testing sites.

Conclusions

Thermo Scientific Gallery and Gallery Plus discrete analyzers provide robust, traceable and rapid methods for quantifying a range of organic acids in juice. The evaluated assays show excellent linearity (r² commonly ≥0.997), low within-run and total CVs suitable for routine quality control, and practical throughput that supports high-sample-volume laboratories. The combination of automated dilutions, barcode-tracked reagents and multi-analyte capability offers significant operational advantages in routine juice analysis, authenticity testing and production QA/QC.

References

1. Beutler H.H. Methods of Enzymatic Analysis. L‑Ascorbate and L‑Dehydroascorbate. In: Bergmeyer HU, editor. Methods of Enzymatic Analysis. 3rd ed. VCH Publishers (UK) Ltd.; 1988. Vol. VI, pp. 376–385.
2. International Federation of Fruit Juice Producers (A.I.J.N.). Compendium of International Methods of Analysis. Determination of Citric Acid (enzymatic). Method OIV‑MA‑AS3‑13‑09 : R2009.
3. IHS Engineering 360. DS/EN 1137 — Fruit and Vegetable Juices — Enzymatic determination of citric acid (citrate) content — NADH spectrometric method.