1. Basic Principles
- Water molecules vibrate (kinetic energy). The faster they vibrate, the higher their energy status and the greater their heat content (heat is a form of energy).
- Temperature as a measure of energy. We measure the energy status by measuring the temperature: the higher the energy (heat) status, the higher the temperature.
- Water phases. Water can exist in three forms or phases: solid, liquid, and gas. A change from one phase to another requires a change in energy status of the molecules.
Energy and Phase Changes
- Increasing energy:
Solid (ice) → Liquid (water) → Gas (vapour) - Decreasing energy:
Gas (vapour) → Liquid (water) → Solid (ice) - Basis for:
- Mechanical refrigeration
- Transpiration (moisture loss)
- Condensation
2. Some Physics
- As energy levels fluctuate in water molecules, they tend to evaporate or condense.
Water in Different Phases
- Liquid water
- Water vapour
The amount of water in the vapour phase is a function of temperature.
Water’s Unique Characteristic
- Van der Waals hydrogen bonds
3. Vapour Pressure and Saturation
- When the air space above the liquid is saturated with water vapour, it no longer has capacity to hold more water in the gaseous phase.
- Higher temperature: Air can hold more water vapour.
- Lower temperature: Air holds less water vapour.
- The vapour in air, being a gas, exerts pressure like any other component of the air. The more vapour there is, the higher the vapour pressure.
4. Micro Water Loss and Vapour Pressure Deficit (VPD)
- Moisture loss is driven by the vapour pressure deficit (VPD) between fruit and environment.
Source: Mitchell & Kader- High vapour pressure inside fruit
- Low vapour pressure outside
- Vapour pressure deficit:
The difference in vapour pressure between the inside of the fruit and the surrounding air. - Rate of moisture loss:
Directly related to the VPD.
5. Psychrometrics
- Temperature (°C) vs. Absolute humidity at saturation (= dew point):
Specific or absolute humidity (mixing ratio): mass of water vapour per unit mass (or volume) of air.0 5 10 15 20 25 30 35 (°C) 0 5 10 15 20 25 30 (g/kg or g/m³) - At 100% RH:
The air is saturated with water vapour. - However, the atmosphere is not always saturated.
- Relative humidity (RH):
The mass of water vapour present, expressed as a percentage of the mass of water vapour that can maximally be present at saturation at that same temperature (degree of saturation).
6. Vapour Pressure Deficit in Practice
Example 1: Orchard
- Pears harvested at 30°C, 30% RH
- Vapour pressure inside fruit (VPprod): 43 mb
- Vapour pressure in atmosphere (VPatmos): 14 mb
- VPD = 43 – 14 = 29 mbars
Example 2: Cold Room (Fruit Still Warm)
- Pears at 30°C, 30% RH, placed in cold room at 0°C, 90% RH
- VPprod: 43 mb
- VPatmos: 6.5 mb
- VPD = 43 – 6.5 = 36.5 mbars
Example 3: Cold Room (Fruit Cooled)
- Pears at 0°C, 90% RH
- VPprod: 7 mb
- VPatmos: 6.5 mb
- VPD = 7 – 6.5 = 0.5 mbars
Key Insight
- In orchard: VPD = 29 mbars
- In cold room (fruit still warm): VPD = 36.5 mbars
- Once fruit is cold: VPD = 0.5 mbars
- The driving force for moisture loss is approximately 58 times higher in warm fruit than in cold fruit
(29 mbars / 0.5 mbars = 58) - OR: 1 orchard hour = 58 cold room hours
- OR: 12 orchard hours = 1 cold room month
7. Effect of Relative Humidity During Cooling and Storage
During Cooling
- VPD (80% RH): 43 mbars – 5 mbars = 38 mbars
- VPD (95% RH): 43 mbars – 6 mbars = 37 mbars
- Conclusion: Small difference in absolute terms, and for a short duration – no issue.
During Storage
- VPD (80% RH): 6.5 mbars – 5 mbars = 1.5 mbars
- VPD (95% RH): 6.5 mbars – 6 mbars = 0.5 mbars
- Conclusion: Small difference in absolute terms, but for a long duration – big issue!
8. Graphical Representation
- Vapour pressure deficit (VPD) (hPa or mbar) = Rate of moisture loss
- Temperature (°C) vs. Relative humidity (%)
9. Raising Relative Humidity
- To raise RH from 80% to 95% in a cold room:
- 4.61 – 3.88 = 0.73 g water evaporated per m³ cold room space
- Why is RH only 80%?
Design fault or ‘cost saving’ on equipment, influencing temperature difference (TD)?
10. Effect of Cooling and Cooling Delays
- Effect of cooling on VPD:
Product at 30°C vs. product at 0°C - Effect of cooling delay on moisture loss:
Additional moisture loss due to delay in commencement of cooling and reduction of product vapour pressure (all else being equal, including the cooling capacity). - Effect of slow cooling rate on moisture loss:
Additional moisture loss due to slow cooling and slow reduction of product vapour pressure (slow cooling rate due to insufficient cooling capacity, all else being equal).
11. Sizing the Cooling System
- Formula for heat load:
kW needed = [2.1 × (A - B) × C × D] / [E × 3600]Where:
- A = pulp temperature (°C)
- B = air temperature (°C)
- C = mass of product (kg)
- D = specific heat of product (3.77 kJ/kg/°C)
- E = desired 7/8 cooling time (hours)
- How to reduce cooling time:
- Reduce heat load (less product and/or cooler incoming product and/or higher DAT)
- Increase system capacity
12. Cooling Capacity Analogy
- Swimming pool pump: 12,500 litres/hour → 200 hours to empty pool
- Irrigation pump: 125,000 litres/hour → 20 hours to empty pool
- Olympic swimming pool: 25 m × 50 m × 2 m = 2,500 m³ = 2,500,000 litres
- Key concepts:
- Heat load
- Cooling capacity
- Cooling time
13. Bottom Line
- Moisture loss is a physical phenomenon easily explained by physics.
- It is practically impossible to avoid, so the focus should be on minimising its impact.
- Primary tool: Proper refrigeration.
- Handling protocols are probably too lax regarding cooling times.
- Protocols should be adapted for problem situations (like heat waves) and problem cultivars.
14. Control Equation
Control of respiration + moisture loss + ethylene production + decay = control of temperature
15. Summary Graph
- As temperature increases, so do respiration, moisture loss, ethylene production, and decay.
End of document.