Post-harvest treatments are used to maintain fruit quality and prevent storage disorders.
Diphenylamine (DPA) for Superficial Scald
- Purpose: Prevents superficial scald, especially in Granny Smith, Braeburn, and Red apple varieties.
- Application: DPA is applied by shower-type drive-through drenches or by conveying bins through a dip tank.
- Concentration: 500–2500 ppm, depending on variety and scald risk.
- Residue Limits: Keep the EU maximum residue level (MRL) of 5 mg/kg in mind (varies by market). DPA breaks down during storage.
- Use lower concentrations for short-term storage, higher for long-term.
- High concentrations with short storage may exceed MRL.
Chlorine and Other Sanitisers
- Chlorine Dips: Used to disinfect fruit. Check chlorine concentration regularly, as it is volatile and binds to organic matter.
- Recommended: 75–100 ppm (35.7g/1000L water) at pH 6.5–7.0 (can be stable down to pH 5).
- Mechanism: Sodium or calcium hypochlorite forms hypochlorous acid (“free chlorine”), the main biocidal agent.
- pH Effect: Water pH outside this range reduces available free chlorine.
Reference
- Other Sanitisers: In South Africa, QAC (quaternary ammonium compound) and chlorine dioxide sanitisers (e.g., Terminator, Sporekill, DSC 1000, Oxine) are recommended over chlorine. They are non-corrosive and more effective on various surfaces.
Ozone as a Sanitiser
- Properties: Highly reactive, unstable form of oxygen (O₃), generated on-site.
- In Air: Controls post-harvest diseases, retards spore production, sanitises surfaces, and destroys ethylene. Less effective at reducing infection in wounds but slows enlargement. Penetration into packaged fruit is limited.
- In Water: Organic alternative to chemical sanitisers. Decomposes quickly, leaves no residue, and can break down pesticide residues. Stronger and faster than many disinfectants. Effective for sanitising process water and fruit surfaces, but not for wounds inoculated before treatment.
- Safety: Ozone is toxic at high levels. Workplace exposure limit is 0.1 ppm (8-hour TWA). On-site monitoring is recommended.
Fungicides
- Used with DPA, especially for Red apples and pears prone to core rot and fungal infections.
- Check regularly for allowed chemicals and MRLs (see www.dfpt.co.za).
Water Handling and Flume Systems
- Pears should preferably be handled dry, not through water flume systems.
- Flume water can be chlorinated (75–100 ppm). Chlorine concentration decreases with evaporation and organic matter, so test regularly. Chlorine is corrosive to unprotected metal parts.
Calcium Chloride Dips
- Used to prevent bitter pit (Golden Delicious, Braeburn).
- Use anhydric, 98% food-grade calcium chloride. Follow manufacturer’s mixing instructions.
- Calcium chloride and DPA can sometimes be combined, but incompatibility may cause coagulation and chemical burns, especially where fruit touches the bin floor or where water droplets form between fruit.
SmartFresh (1-MCP)
- Used for both regular atmosphere (RA) and controlled atmosphere (CA) storage to maintain eating quality during shelf life.
- Timing: Treat within 7 days of harvest. Best results when starch conversion is <40% and fruit has reached its climacteric peak. Follow cultivar-specific guidelines.
- Benefits: Controls superficial scald and eliminates the need for a chlorine drench before storage. Reduces core rot risk in Red Delicious and other varieties with wide calyx cavities.