Food preservation

Introduction & Context

• Week 7 of course; builds on Week 5 “Food Engineering Fundamentals”
• Focus: classical & novel food processing + preservation techniques
• Learning outcomes
  • Define food preservation & explain its necessity (microbiology, safety, quality, supply-chain)
  • Explain principles that control spoilage (water activity, pH, temperature, oxygen, nutrients)
  • Compute / interpret water activity $a_w$, shelf-life; recommend preservation strategies for specific foods
  • Recognise emerging non-thermal technologies (HPP, PEF, plasma, etc.) and their industrial relevance

Rapid Recap of Week 5 Fundamentals

• Target organism for can-sterilisation: Clostridium botulinum (lethal toxin ~30 ng)
• Critical “botulinum cook” : $121.1\,^{\circ}!\text{C}$ for $2.88\,\text{min}$ (12-log reduction)
• Core SI units used in food engineering
  • Mass $\rightarrow$ kilogram (kg)
  • Length $\rightarrow$ metre (m)
  • Time $\rightarrow$ second (s)
• Heat flows hot → cold (never “coldness” moving)
• Rheology reminder: shear-thickening fluids (e.g. starch paste, D3O armour)

Shelf-Life, Safety vs Quality

• Shelf-life = interval where food remains acceptable for both safety and quality
• “Use-by” date: microbiological safety limit; legal discard after expiry
• “Best-before” date: sensory/quality optimum; food may still be safe
• Classroom poll scenarios
  • Raw chicken 7 days → safety risk (pathogens, slime)
  • Yoghurt 6 months → likely safe (low pH, probiotics) yet poorer texture/ flavour
  • Stale crackers → quality defect only
• Safety priority hierarchy
  1. Legal & ethical obligation not to harm consumers
  2. Quality drives repeat purchase but is secondary to safety

Causes of Spoilage & Hazard

1. Microbial growth (bacteria, moulds, yeasts; toxins)
2. Enzymatic reactions (browning, pectinases, lipases)
3. Non-enzymatic chemistry (lipid oxidation, Maillard, vitamin loss)
4. Physical damage (bruising, phase separation, moisture migration)

Core Preservation Principles

1 Water Activity (aw)

• Definition: $a<em>w = \dfrac{P}{P</em>0}$ where $P$ = water vapour partial pressure over food, $P_0$ = that over pure water at same T (analogous to % RH)
• Scale 0 – 1; pure water $a_w=1.00$
• Growth thresholds
  • Most bacteria > $0.90$
  • Most yeasts > $0.80$
  • Most moulds > $0.70$
  • Virtually no growth < $0.60$ (critical hurdle)
• Free vs bound water
  • Binding via solutes (salt, sugars), micelles, freezing → lowers $a_w$ without removing moisture
• Practical reduction techniques
  1. Dehydration / drying (jerky, milk powder $a_w\approx0.2$)
  2. Osmotic binding with solutes (jam ≈50 % sugar, $a<em>w\approx0.55$; honey $a</em>w\approx0.50$)
  3. Freezing (ice crystals immobilise H₂O)
• Cookie thought-experiment showed $a_w$, not % moisture, predicts spoilage rate

2 Temperature Control

• Refrigeration (0–4 °C): slows microbial metabolism & chemical kinetics (Q10 rule)
• Cold chain for milk: cow 37 °C → bulk tank 4 °C → pasteurise → rapid cool → chilled transport → retail → domestic fridge
• Freezing (< −18 °C)
  • Low T + solid water → $a_w\rightarrow0$
  • Fast freezing ≪ slow freezing; prevents large ice crystals (quality of ice-cream, lettuce integrity)

3 Heat Treatments

• Pasteurisation (e.g. HTST milk $72\,^{\circ}\text{C}, 15\,\text{s}$)
• Commercial sterilisation / retorting (botulinum cook)
• Trade-off: safety ↑ but sensory & nutrient quality ↓ (Maillard, vitamin C loss)

4 pH Control

• Critical limit $\text{pH}=4.6$ (below, C. botulinum cannot germinate)
• Methods: acid addition (vinegar, citric), fermentation (lactic acid)
• Low-acid foods (>4.6) need full botulinum cook; high-acid foods can use milder heat

5 Atmosphere & Oxygen

• Many spoilage microbes aerobic; vacuum or MAP (N₂/CO₂) suppress growth & oxidation

6 Hurdle Technology

• Combine multiple hurdles (e.g. $a_w$↓ + pH↓ + mild heat) → synergistic lethality & quality retention

Classical Preservation Methods & Examples

• Acidification / Pickling
  • Process: soak in acidic brine; often + salt/sugar (dual hurdle)
  • Foods: cucumbers, kimchi, canned beans, salad dressings
• Drying / Dehydration
  • Equipment: cabinet, tunnel, spray, freeze-dryers
  • Foods: dried fruits, beef jerky, crackers; freeze-dried candy demo
• Chilling & Freezing (cold storage, blast freezers)
• Fermentation (beneficial microbes)
  • Sauerkraut, yoghurt, wine, cheeses; out-compete pathogens + generate acids/ethanol
• Smoking
  • Heat + wood smoke phenolics (natural antimicrobials)
  • Smoked salmon, cheeses, hams
• Vacuum / Oxygen Removal
  • Vacuum-packed meats, MAP salads; slows aerobes & rancidity
• Chemical Preservatives (regulated)
  • Sorbates (dried fruit), benzoates (beverages), sulphites (wine), nitrites/nitrates (curing), antioxidants (BHA, tocopherol)

Emerging Non-Thermal (Novel) Technologies

• All aim to deliver commercial sterility with < 5 °C temperature rise → retain vitamins, colour, flavour
• Scaling, energy use & regulatory acceptance still under research; HPP most commercialised to date

Industry Practices & Consumer Myths (Case Studies)

• Milk “permeate” scandal: permeate = lactose-rich serum separated during cream standardisation; adds no water, just achieves consistent fat/protein spec
• “Half-empty” chip bags: nitrogen flush cushions product & prevents lipid oxidation (no O₂)
• Waxed apples: edible carnauba/shellac wax (from insects) reduces moisture loss, prolongs crunch; shine is secondary
• Purple vacuum-packed meat: deoxygenated myoglobin (\textit{deoxy‐Mb}); blooms red when O₂ re-enters; not spoilage

Key Numerical & Formula Summary

• Botulinum cook: $121.1^{\circ}\text{C},\; 2.88\,\text{min},\; 12\,D$
• Critical pH: $4.6$
• Critical water-activity: $a_w = 0.60$ (no growth)
• Water-activity definition: $a<em>w=\frac{P}{P</em>0}=\frac{\text{RH}}{100}$
• SI base units highlighted: kg, m, s; derived force $\text{N} = \text{kg·m·s}^{-2}$

Ethical, Regulatory & Practical Considerations

• Safety is non-negotiable; governed by FSANZ, Codex, HACCP; criminal liability for outbreaks
• Quality drives consumer acceptance; minimal processing trend spurs non-thermal R&D
• Preservative usage limited by ADI & labelling laws; consumer “clean-label” preference influencing formulation
• Energy & sustainability trade-offs (e.g. refrigeration costs vs salt load in cured meats)

Connections to Previous & Future Lectures

• Water-activity concept links to Week 5 mass-transfer & dehydration calculations
• Microbial ecology (Assoc. Prof Zhao) underpins pH/aw/temperature growth limits
• Upcoming lecture on Packaging (Prof Johannes): MAP, barrier films extend the same preservation principles
• Nutrition lecture (Week 8) will examine how processing impacts nutrient density & public health (fresh-is-best vs safety)

Study Cues & Self-Check

• Can you state three ways to reach a_w<0.60 in a fruit purée?
• How does hurdle technology explain the long shelf-life of jam without retorting?
• Calculate %RH above a cookie if $a_w=0.70$ (answer $70\%$).
• Explain why slow-frozen raspberries leak juice on thawing.
• Match each novel process (PEF, plasma, HPP) with its main energy form and typical application.