Fruit Ripening & Retail Handling Workshop. Why use cold storage?

Transcription

1 (c) Postharvest Technology Center, UC Davis Fruit Ripening & Retail Handling Workshop Cold Storage Disorders of Fruits and Vegetables Mikal E. Saltveit Mann Laboratory, Department of Plant Sciences University of California, Davis Cold storage disorders 1 Why use cold storage? Shelf-life is inversely proportional to respiration (colder temp slower respiration longer shelf-life). The rate of respiration declines about 50% for every 18 F (10 C) fall in temperature (Q 10 of 2). Low temperatures reduce most microbial growth. Low temperatures reduce water loss (air at 10 C can hold twice as much water as at 0 C) Cold storage disorders 2 Page 1

2 (c) Postharvest Technology Center, UC Davis Respiration and Temperature Respiration increases with increasing temperature The rate of increase increases faster at warmer temperature Respiration (mg CO2/kg h) Asparagus spears Temperature ( C) Globe artichokes Head lettuce Heat production (1,000 BTU/Ton day) Q 10 = 10/4 = 2.5 How can we calculate a Q 10? Brussels sprouts Shasta strawberry Wickson plum Temperature ( C) Cold storage disorders 3 Ripe Strawberries After 7 days What temperature were they stored at? A B C 0 C 5 C 10 C 32 F 41 F 50 F Slide from Marita Cantwell Cold storage disorders 4 Page 2

3 (c) Postharvest Technology Center, UC Davis Effect of handling temperatures on strawberry deterioration Treatment (48 h period) % Sound % Soft % Rotted h at 5 C (41 F) h each at 5 C (41 F) and 20 C (68 F) 3. 2 x (2 x 12 h) periods at 5 C (41 F) and 20 C (68 F) Fruit condition h at 20 C (68 F) Slide from Marita Cantwell Mitchell et al., 1996 Handling Strawberries, UC Publ Cold storage disorders 5 Outside the Temperature Window High temperature - scald, sunburn Sub-zero temperatures - frost, freezing Tropical commodities - chilling sensitivity Freezing Chilling Usually too warm to retain quality Heat stress Cold storage disorders 6 Page 3

4 (c) Postharvest Technology Center, UC Davis Ripening of Tomato Fruit Chilling injury Just right High temperature injury Tomato fruit after 2 weeks at 20 C. Softening and synthesis of pigments and aromas may be abnormal at cold (chilling injury) or high temperatures. Cold storage disorders 7 Mechanical Refrigeration The RH of saturated (100% RH) cold 32 F air drops to 70% RH when warmed to 41 F HEAT WARM AIR 41 F Refrigerant absorbs heat as it vaporizes EVAPORATOR COLD AIR 32 F Gas Liquid COMPRESSOR Fan inside cold room Fan outside cold room Insulation EXPANSION VALVE CONDENSER Hot gas Warm liquid RESERVOIR Warm liquid HOT AIR Refrigerant condenses as it looses heat WARM AIR Low pressure side High pressure side Most cooling technologies rely on a liquid to gas phase change, or on an adiabatic temperature change as a gas expands and contracts. Cold storage disorders 8 Page 4

5 (c) Postharvest Technology Center, UC Davis The saturated water content of air (SVP) increases logarithmically with increasing air temperature Relative humidity lines The SVP doubles for every 11 C rise in temperature Wet bulb temperature ( C) 25 R.H. = VP * 100 SVP Dry bulb lines 30 Relative humidity (%) atmosphere = 98 kpa = 0.10 mpa = 1.01 bars = 14.7 psi = 760 mm Hg = 33.9 ft H 2 O Wet bulb lines Saturation Vapor Pressure (SVP) Constant Vapor Pressure The Psychrometric Chart Vapor pressure (kpa) Dry bulb temperature ( C) 0 Cold storage disorders 9 Excessive water loss can occur at low relative humidity Relative humidity (%) Coil temperature = 0 C (32 F) Room temperature = 5 C (41 F) RH = 70% Wet bulb temperature ( C) Specific humidity (kg/kg) Water vapor pressure (millibars) Dry bulb temperature ( C) 0 Cold storage disorders 10 Page 5

6 (c) Postharvest Technology Center, UC Davis Problems with cold storage Temperatures below 32 F (0 C) can cause freezing (depends on the tissue s sugar content). Freezing reduces quality and shelf-life Non-freezing temperatures below ~50 F (10 C) can cause chilling injury in sensitive crops. Chilling injury may be hidden and only develop after the product is purchase by the consumer. A closed cold storage room allows accumulation of gasses: unusual aromas, CO 2, C 2 H 4 Cold storage disorders 11 Sugars and Freezing Susceptibility Why does fruit tissue close to the pit freeze first, and not tissue in the flesh? Frozen Core tissues freeze before outer flesh tissues because of differences in soluble solids (sugar) content within the fruit. Freezing point depression; dependent on the number of molecules or ions and their ability to organize water. Cold storage disorders 12 Page 6

7 (c) Postharvest Technology Center, UC Davis Examples of Freezing Injury Water logging Cold storage disorders 13 Examples of Freezing Injury Why did the melons on only two edges freeze and not all of them? Tissue collapse Water logging Cold storage disorders 14 Page 7

8 (c) Postharvest Technology Center, UC Davis Freezing Injury Water in the cell walls surrounding each plant cell contains less soluble solids (e.g., sugars) than does the cytoplasm in the cell, so ice crystals form first in the cell wall. The solution in the cell wall gets more concentrated as ice crystals form. Water moves from the cell into the cell wall to re-establish osmotic equilibrium, and the cell dehydrates. Cellular dehydration, not ice crystal formation, causes cellular injury associated with freezing. Cold storage disorders 15 Ice Crystal Formation Ice crystals form by the accretion of water molecules on the surfaces of an existing surface. No motive force can be exerted by a growing crystal. The volume of an ice crystal actually decreases as it cools Cold storage disorders 16 Page 8

9 (c) Postharvest Technology Center, UC Davis Volume change upon Freezing Water at 0 C (32 F) = g/cm 3 1 g water = cm 3 Water is most dense at 4 C (32 F) 1 g ice = cm 3 Ice at 0 C (32 F) = g/cm 3 1 g ice = cm 3 This small increase in volume in an elastic cell (unlike an iron pipe) does not cause mechanical tissue damage. WATER D = 100 D = , same weight, but now larger volume V = 4/3 r 3 Cold storage disorders 17 Most Fruits and Vegetables are chilling tolerant But many are chilling sensitive Cold storage disorders 18 Page 9

10 (c) Postharvest Technology Center, UC Davis Flowering plants originated in the tropics Flowering plants evolved in the tropics and most tropical and semi-tropical plants are chilling sensitive. Tropical, sub-tropical, and even some temperature fruit are chilling sensitive and damaged by non-freezing temperatures below 10 C (50 F) Cold storage disorders 19 Chilling Injury Chilling injury is a physiological disorder; however, symptom development can involve increases susceptibility to specific diseases. Chilling injury develops after exposure to non-freezing temperatures below a critical temperature (e.g., 50 F, 10 C). The severity of the injury depends on the temperature of exposure (below threshold), and the duration of exposure. The critical of threshold temperature varies with: crop, cultivar, growing conditions, pre-treatments, etc. Symptoms often develop after rewarming. The post-chilling environment can mitigate or accentuate symptom development. Cold storage disorders 20 Page 10

11 (c) Postharvest Technology Center, UC Davis Symptoms of Chilling Injury Internal and external tissue browning Cold storage disorders 21 Symptoms of Chilling Injury Bell pepper Grapefruit Surface pitting & discoloration Cold storage disorders 22 Page 11

12 (c) Postharvest Technology Center, UC Davis Symptoms of Chilling Injury Dracaena Cucumber Tissue necrosis followed by attack by specific pathogens String beans Cold storage disorders 23 Chilling of Bananas Control Chilled Browning of inner side of peel Skin discoloration, Failure to ripen Chilled bananas develop a dull gray smoke appearance upon ripening. Cold storage disorders 24 Page 12

13 (c) Postharvest Technology Center, UC Davis Chilling of Pineapples Endogenous Brown Spot 7.2 C (45 F) 5.5 C (42 F) 3 C (38 F) Cold storage disorders 25 Internal Breakdown of Stone Fruit Mealiness (soft but not juicy) Flesh Browning Lack of Flavor Failure to Ripen Internal Breakdown is induced by intermediate storage temperatures (2-8 C, 1-6 weeks) Crisosto et al., Susceptibility to chilling injury of peach, nectarine, and plum cultivars in California. HortScience 34(6): Crisosto et al Increasing Blackamber (Prunus salicina Lindell) plum consumer acceptance. Postharvest Biol. Tech 34: Lurie, S and C. Crisosto Chilling injury in peach and nectarine. Postharvest Biol. Tech. 37: Cold storage disorders 26 Page 13

14 (c) Postharvest Technology Center, UC Davis Conditioning or Delayed Cooling (100% Mealy) (43% Mealy) 5ºC 20 Days 0ºC 20 Days (0% Mealy) 20ºC 48 Hours + 5ºC 20 Days Crisosto, et al Controlled delayed cooling extends peach market life. HortTechnology 14: Cold storage disorders 27 Diurnal changes in Chilling Sensitivity Diurnal changes in chilling sensitivity Cool mornings, hot afternoons, carbohydrate status Tomato seedlings are very chilling sensitive in the early morning and less sensitive later in the day. Tomato fruit also exhibit diurnal changes in chilling sensitivity. Changes can be duplicated with postharvest temp conditioning. King and Reid Cold storage disorders 28 Page 14

15 (c) Postharvest Technology Center, UC Davis Preventing Chilling Injury Are we preventing injury or symptom development? Don t chill! Use resistant cultivars Conditioning High temperature pre-treatment Low temperature treatment (above threshold) Intermittent warming Controlled atmospheres Cold storage disorders 29 Page 15