Microbiological Testing Protocol
Document Code: HJ-QA-020
Version: 2.2
Effective Date: 2026-03-15
Prepared by: Microbiological Laboratory, Hongji Agriculture Co., Ltd.
1. Introduction
Microbiological safety is a non-negotiable requirement for all potato flake and potato powder shipments. Hongji Agriculture operates a dedicated microbiological laboratory that performs routine testing of finished products, in-process samples, raw materials, environmental swabs, and water. This protocol defines the standard test methods, acceptance criteria, sampling frequencies, and corrective actions for all microbiological parameters.
The microbiological quality program is designed to meet the requirements of ISO 22000:2018, FSSC 22000, Codex Alimentarius, and the specific microbiological criteria of importing countries (EU, USA, Japan, Southeast Asia, and others).
2. Laboratory Facilities and Capabilities
| Facility Feature |
Specification |
| Classification |
ISO Class 7 (Grade C) clean room for sample preparation |
| Biosafety Level |
BSL-2 (for pathogen testing: Salmonella, S. aureus) |
| Key Equipment |
Autoclave (vertical, 120 L), laminar flow hood (Class II), incubators (various temps), stomacher, colony counter, real-time PCR (Bio-Rad CFX96) |
| Staff |
Dedicated microbiologist and trained analysts |
| Accreditation |
Testing methods follow GB 4789 national standards; third-party verification by accredited laboratories |
3. Standard Test Methods and Acceptance Criteria
3.1 Aerobic Plate Count (APC) / Total Viable Count (TVC)
Total aerobic mesophilic bacteria indicating general hygiene and process effectiveness.
| Parameter |
Specification |
| Method |
AOAC 966.23 / ISO 4833-1 |
| Medium |
Plate Count Agar (PCA) |
| Incubation |
30 ± 1°C for 72 ± 3 hours |
| Sample Prep |
25 g sample + 225 mL 0.1% peptone water; stomache for 60 s; serial dilutions |
| Inoculation |
Pour plate (1 mL per plate, duplicate per dilution) |
| Counting |
All red/white colonies; report as CFU/g |
Coliform bacteria indicate post-process contamination or inadequate sanitation.
| Parameter |
Specification |
| Method |
AOAC 966.24 / ISO 4832 |
| Medium |
Violet Red Bile Agar (VRBA) |
| Incubation |
37 ± 1°C for 24 ± 2 hours |
| Confirmation |
Gas production in brilliant green lactose bile broth (BGLB) |
| Reporting |
CFU/g (confirmed coliforms) |
3.3 Escherichia coli
The presence of E. coli indicates fecal contamination and is a critical food safety indicator.
| Parameter |
Specification |
| Method |
AOAC 2003.01 / ISO 16649-3 (MPN) |
| Medium |
Tryptone Bile X-glucuronide (TBX) agar |
| Incubation |
44 ± 1°C for 24 ± 2 hours |
| Confirmation |
Indole test (Kovac's reagent) on presumptive colonies |
| Reporting |
CFU/g (TBX agar) or MPN/g (alternative MPN method) |
3.4 Salmonella
Pathogen — absence required in all finished products.
| Parameter |
Specification |
| Method |
AOAC 2011.03 / ISO 6579-1 (Horizontal method) |
| Pre-Enrichment |
25 g sample in 225 mL Buffered Peptone Water (BPW); 37°C, 18 ± 2 h |
| Selective Enrichment |
Rappaport-Vassiliadis (RVS) broth, 41.5°C, 24 ± 3 h + Muller-Kauffmann Tetrathionate (MKTTn) broth, 37°C, 24 ± 3 h |
| Selective Plating |
XLD agar + BGA (Brilliant Green Agar); 37°C, 24 ± 3 h |
| Confirmation |
Biochemical (TSI, LIA, urea) + serological (polyvalent O antisera) |
| Alternative |
Real-time PCR (AOAC-RI 031001) for screening; confirm positive by culture |
| Target |
Absent in 25 g |
3.5 Staphylococcus aureus
Coagulase-positive staphylococci indicate handling contamination or inadequate thermal processing.
| Parameter |
Specification |
| Method |
AOAC 2001.05 / ISO 6888-1 |
| Medium |
Baird-Parker (BP) agar with egg yolk tellurite emulsion |
| Incubation |
37°C, 24–48 h (check at 24h, re-incubate to 48h if negative) |
| Confirmation |
Coagulase test (rabbit plasma); DNase test |
| Reporting |
CFU/g |
3.6 Yeasts and Molds
High levels indicate environmental contamination, prolonged storage, or moisture ingress.
| Parameter |
Specification |
| Method |
AOAC 997.02 / ISO 21527-1 (for dry products, aw ≤ 0.95) |
| Medium |
Dichloran 18% Glycerol Agar (DG18) |
| Incubation |
25 ± 1°C for 5 days (yeasts); 7 days (molds) |
| Identification |
Morphological (lactophenol cotton blue mount for molds) |
| Reporting |
CFU/g (yeasts and molds reported separately) |
Spore-forming bacteria survive the drum drying process and can cause spoilage in certain customer applications.
| Parameter |
Specification |
| Method |
ISO 7932 (B. cereus) / ISO 13720 (total mesophilic spore-formers) |
| Medium |
Mannitol Egg Yolk Polymyxin (MYP) agar |
| Incubation |
30°C, 24 ± 2 h |
| Pre-treatment (spores) |
Heat sample at 80°C for 10 min to kill vegetative cells |
| Reporting |
CFU/g (B. cereus) / Spores/g (spore count) |
4. Microbiological Limits Summary Table
| Parameter |
Method Reference |
n |
c |
m |
M |
Interpretation |
| Aerobic Plate Count (APC) |
ISO 4833-1 |
5 |
2 |
1.0 × 10⁴ |
5.0 × 10⁴ |
m = limit; >m but ≤M: marginal (investigate); >M: reject |
| Coliforms |
ISO 4832 |
5 |
2 |
1.0 × 10² |
5.0 × 10² |
Same 3-class scheme |
| Escherichia coli |
ISO 16649-3 |
5 |
1 |
10 |
1.0 × 10² |
≤m = acceptable; >M = reject |
| Salmonella |
ISO 6579-1 |
10 |
0 |
0/25g |
— |
2-class: 0 positive in 10 samples of 25 g each |
| Staphylococcus aureus |
ISO 6888-1 |
5 |
1 |
10² |
10³ |
Same 3-class scheme |
| Yeasts |
ISO 21527-1 |
5 |
2 |
1.0 × 10² |
5.0 × 10² |
Same 3-class scheme |
| Molds |
ISO 21527-1 |
5 |
2 |
1.0 × 10² |
5.0 × 10² |
Same 3-class scheme |
| Bacillus cereus |
ISO 7932 |
5 |
1 |
10² |
10³ |
Same 3-class scheme |
| Total Spore-Formers (Mesophilic) |
ISO 13720 |
5 |
2 |
1.0 × 10³ |
5.0 × 10³ |
Same 3-class scheme |
Where: n = number of sample units tested; c = maximum allowable number of marginal-quality samples; m = microbiological limit (CFU/g) considered acceptable; M = CFU/g value above which the lot is rejected.
5. Sampling Frequency
5.1 Finished Product Sampling
| Product Type |
Test |
Frequency |
| Potato Flakes / Powder |
APC, Coliforms, Yeasts & Molds |
Every production lot (minimum 1 per shift per line) |
| Potato Flakes / Powder |
Salmonella |
Composite of 10 sub-samples per day; pool for 5-day test (or test each lot if requested by customer) |
| Potato Flakes / Powder |
S. aureus, B. cereus, Spore-formers |
Every 5th lot per grade |
| Potato Flakes / Powder |
Full micro panel |
Monthly (one composite per product grade) |
5.2 Environmental Monitoring
| Sampling Point |
Test |
Frequency |
| Drying drum surface swab |
APC, Enterobacteriaceae |
After CIP (daily) |
| Conveyor belts (contact) |
APC, E. coli |
Daily (after sanitation) |
| Packaging hopper interior swab |
APC, Yeasts & Molds |
Weekly |
| Floor drains (5 locations) |
Listeria (species level) |
Weekly |
| Air settling plates (3 locations) |
APC, Yeasts & Molds |
Weekly |
| Personnel hand swabs |
APC, S. aureus |
Monthly (random) |
5.3 Raw Material Testing
| Material |
Test |
Frequency |
| Incoming potatoes |
APC, Yeasts, Molds (surface rinse) |
Monthly composite |
| Potable water (inlet) |
APC, Coliforms, E. coli |
Weekly |
| Production water (at use point) |
APC, Coliforms |
Daily |
| Packaging materials |
Surface APC, Salmonella |
Per incoming lot (certificate review + random confirm) |
6. Sample Handling and Transport
| Step |
Requirement |
| Sampling Tools |
Sterile, disposable (single-use) scoops, spatulas, gloves |
| Sample Containers |
Sterile Whirl-Pak bags (110 mL or 710 mL) or sterile wide-mouth bottles |
| Sample Labeling |
Product name, lot number, production date/time, sampler initials, test required |
| Transport Temperature |
≤4°C (ice packs in insulated cooler) |
| Time from Sampling to Lab Arrival |
≤2 hours |
| Maximum Holding Time Before Test |
24 hours (refrigerated) for APC/coliforms; 72 hours for pathogens |
7. Corrective Actions for Positive Results
7.1 Action Plan for OOS Results
| Finding |
Immediate Action |
Investigation |
Follow-Up |
| APC > M |
Hold lot; retest duplicate; if confirmed, downgrade or destroy |
Review CCP-02 drying temp/time, sanitation records, raw material quality |
CAPA within 5 days |
| Salmonella positive (any sample) |
Immediate quarantine of entire day's production; notify QA Director and COO within 1 hour |
Full root cause investigation (processing line, ingre-dients, environment, personnel); 100% testing of all affected lots; recall evaluation |
CAPA within 3 days; full sanitation of line; re-test 3 consecutive negative lots before resuming |
| E. coli > M |
Hold lot; investigate sanitation, water system, personnel hygiene |
Source tracing; increased environmental monitoring |
CAPA within 5 days |
| Mold > M |
Hold lot; review storage humidity and packaging integrity |
Warehouse inspection (check for moisture damage) |
CAPA within 7 days |
7.2 Alert Limits vs. Action Limits
Hongji operates a two-tier system with internal alert limits stricter than the official action limits:
| Parameter |
Alert Limit (Internal) |
Action Limit (Release) |
| APC |
5.0 × 10³ CFU/g |
5.0 × 10⁴ CFU/g |
| Coliforms |
50 CFU/g |
5.0 × 10² CFU/g |
| Yeasts |
50 CFU/g |
5.0 × 10² CFU/g |
| Molds |
50 CFU/g |
5.0 × 10² CFU/g |
When an alert limit is exceeded, an investigation is initiated. Product may still be released but with quality flagging and continued monitoring.
8. Proficiency Testing and Method Validation
| Parameter |
Proficiency Scheme |
Frequency |
12-Month Avg. z-Score |
| APC |
FAPAS / BIPEA |
Quarterly |
1.5 |
| Coliforms |
FAPAS / BIPEA |
Quarterly |
1.2 |
| Salmonella |
FAPAS (qualitative) |
Semi-annual |
100% correct |
| E. coli |
FAPAS / BIPEA |
Semi-annual |
1.1 |
| Yeasts & Molds |
BIPEA |
Semi-annual |
1.7 |
9. Special Customer Requirements
| Request |
Capability |
Lead Time |
| Listeria monocytogenes testing (for frozen potato product customers) |
ISO 11290-1; Real-time PCR screening |
72 hours |
| Cronobacter sakazakii (for infant food applications) |
ISO 22964 |
5 days |
| Clostridium perfringens |
ISO 7937 |
5 days |
| Thermophilic spore count |
Custom method (100°C/30 min pre-treatment) |
5 days |
| Custom composite sampling plan |
Per customer agreement |
Request during contract negotiation |
10. References
- ISO 4833-1: Microbiology of the food chain — Horizontal method for enumeration of microorganisms — Part 1: Colony count at 30°C by the pour plate technique
- ISO 4832: Horizontal method for the enumeration of coliforms — Colony-count technique
- ISO 6579-1: Horizontal method for the detection, enumeration and serotyping of Salmonella
- ISO 6888-1: Horizontal method for the enumeration of coagulase-positive staphylococci (S. aureus)
- ISO 16649-3: Horizontal method for the enumeration of beta-glucuronidase-positive E. coli
- ISO 21527-1: Horizontal method for the enumeration of yeasts and moulds — Part 1: Colony count technique in products with water activity greater than 0.95
- ISO 7932: Horizontal method for the enumeration of presumptive Bacillus cereus
- AOAC International: Official Methods of Analysis (current edition)
- EU Regulation 2073/2005: Microbiological criteria for foodstuffs
- GB 29921-2021: National Food Safety Standard — Limit of Pathogenic Bacteria in Food (China)
End of Document
References
- AOAC International. Official Methods of Analysis, 21st edition. Gaithersburg, MD.
- ISO (International Organization for Standardization). Horizontal methods for food microbiology and physicochemical analysis.
- National Health Commission of China. GB National Food Safety Standards — contaminant limits and test methods.
- European Commission. Regulation (EC) No 1881/2006 — setting maximum levels for certain contaminants in foodstuffs.
- Codex Alimentarius. General Principles of Food Hygiene (CXC 1-1969) and commodity standards.
- FSSC 22000. Food Safety System Certification standard.
This document is part of the Hongji Agriculture (弘基农业) Technical Documentation Series. For more information about our vertically integrated potato supply chain — from seed breeding and cultivation to processing and global export — visit our official B2B website: https://hjpotatoflakes.com