Fruit Ripening & Ethylene Management Workshop Cold Storage Disorders of Fruits and Vegetables Why use cold storage? Shelf-life is inversely proportional to respiration (colder temp slower respiration longer shelf-life). Respiration is halved for every 1 C (18 F) drop in temperature (i.e., a Q 1 of 2). Low temperatures reduce microbial growth. Mikal E. Saltveit Mann Laboratory, Department of Plant Sciences University of California, Davis Low temperatures reduce water loss (air at 1 C can hold twice as much water as at C) Effect of temperatures on strawberries Chronological vs Physiological Holding for 48 h at different temperatures (5 or C) Sound Soft Rotted 1. 48 h at 5 C (41 F) 95 4 1 2. 24 h each at 5 C (41 F) and C (68 F) 3. 2 x 12 h periods at 5 C (41 F) and C (68 F) Fruit condition (%) 76 1 14 7 7 23 4. 48 h at C (68 F) 44 1 55 Chronological time is time measured on your watch uniform intervals independent of external conditions. Physiological time is time measured on tissue changes intervals depend on rate of physiological changes (e.g., rate of respiration). For example: An hour in the dentist chair is not the same perceived length of time as one hour engaged in an enjoyable activity like listening to this lecture. Plants are poikilothermic (i.e., ectothermic) and are unable to maintain a tissue temperature above ambient, while we are homoeothermic and are able to maintain a relatively constant internal body temperature irrespective of the external temperature. Our biological clock ticks as a relative uniform rate, while the plant s biological clock ticks at rates dependent on its temperature. Slide from Marita Cantwell Mitchell et al., 1996 Handling Strawberries, UC Publ. 2442 Page 1
Respiration and Temperature Life, Temperature, and the Q 1 Respiration can be expressed as the rate of CO 2 or heat produced Respiration (mg CO2/kg h) 3 1 Asparagus spears 5 1 15 25 Temperature ( C) Globe artichokes Head lettuce Heat production (1, BTU/Ton day) 5 4 3 1 Q 1 = 1/4 = 2.5 4 Brussels sprouts Shasta strawberry Wickson plum 5 1 15 25 3 Temperature ( C) Organisms can survive and even thrive as long as there is liquid water (even < C). Life can exists at extreme temperatures (-273 to > 1 C), but production agriculture tries to maintain crops within a narrow range of temperatures for optimal growth. Temperature sensitivity is predicted by the concept of the Temperature Quotient or Q 1, which compares the rate of a process at one temperature to its rate at a higher or lower temperature (usually a difference of 1 C). Q 1 = (rate at temperature T + 1) / (rate at temperature T) For example, the Q 1 = Rate at 15 C / Rate at 5 C Photochemical and physical processes (e.g., heat loss) have a Q 1 of about 1. That means that temperature changes (within the biological range) do not effect the rate of the reaction. On the other hand, biological processes (enzymatic reactions, biochemical reactions, metabolism, etc.) have a Q 1 values between 2-3, while some complicated processes (e.g., respiration of cut flowers, broccoli) have much higher values (4-6). Chronological vs. Physiological Time The Temperature window Many developmental processes, such as germination, flowering, and ripening are controlled through internal or external signals. Internal signals include the passage of a given amount of chronological time (e.g., calendar days, absolute time) or physiological time (e.g., degree days, perceived time). How many days of shelf-life would be lost if strawberries (Q 1 = 2.5) were held for 24 h at 15 C instead of at 5 C? One day of chronological time would have passed, but for the fruit, 2.5 days of physiological time would have passed; so 1.5 day would have been lost. Socrative poll room number: FRUITRIPENING Freezing Chilling Heat stress -5 5 1 15 25 35 Page 2
Outside the Optimum High temperature - scald, sunburn Non-freezing Temperatures - chilling sensitivity Sub-zero temperatures - frost injury, freezing 5 1 15 25 35 4 Problems with cold storage Excessive water loss can occur even at low temperatures because of low relative humidity produce by mechanical refrigeration. Temperatures below 32 F ( C) can cause freezing (depends on the tissue s sugar content). Non-freezing temperatures below about 5 F (1 C) can cause chilling injury. 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: CO 2, C 2 H 4 Excessive water loss can occur even at low temperatures because of low relative humidity. Coil temperature = C (32 F) Room temperature = 5 C (41 F) Air at C and 1% RH will become air at 7% RH when warmed to 5 C 1% RH at the 5 C surface of the commodity 1% RH at the C surface of the coils -1-5 5 1 15 25 7% Relative humidity (%) 1 8 6 5 4 3 3. -1-5 5 1 15 25 3 35 4 45 5 55 6 Dry bulb temperature ( C).28.24..16.12 Specific humidity (kg/kg) 45 4 35 3 25 15.8 Vapor pressure 1 deficit driving water loss.4 5 Water vapor pressure (millibars) 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 first form 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. Page 3
Ice Crystal Formation Sugars and Freezing Susceptibility 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! Why does fruit tissue close to the pit freeze first, and not tissue in the flesh near the surface? Core tissues freeze before outer flesh tissues because of differences in soluble solids content within the fruit. A growing ice crystal can t puncture a cell. Once water freezes, the volume of the ice actually decreases as it cools Freezing point depression; dependent on the number of molecules or ions and their ability to organize water. Examples of Freezing Injury Examples of Freezing Injury Why did it freeze on just those edges? Tissue collapse Water logging White patches Water logging Page 4
Most Fruits and Vegetables are chilling tolerant But many are chilling sensitive Chilling Injury Chilling injury is a physiological disorder; however, symptom development can involve increases susceptibility to specific microbes. Chilling injury develops after exposure to non-freezing temperatures below a critical temperature. The severity of the injury depends on the temperature and duration of exposure. The critical or threshold temperature varies with: crop, cultivar, growing conditions, pre-treatments, etc. Symptoms usually develop upon return to room temperature. The post-chilling environment can mitigate or accentuate symptom development. Chilling and Shelf-life The shelf-life of non-chilling sensitive commodities steadily increases as the temperature declines to the freezing point of the tissue. In contrast, while the shelf-life of chilling sensitive commodities also steadily increases as the temperature declines, it rapidly decreases as the temperature falls below the threshold temperature. Percent of possible shelf-life 1 8 6 4 Level of chilling injury Shelf-life of non-chilling sensitive commodity Threshold for chilling 5 1 15 25 3 Temperature ( C) Shelf-life of chilling sensitive commodity 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 1 C (5 F) Page 5
Ripening of Tomato Fruit Symptoms of Chilling Injury Just right Chilling injury High temperature injury Tomato fruit after 2 weeks at C. Softening and synthesis of pigments and aromas may be abnormal at cold (chilling injury) or high temperatures. Internal and external tissue browning Symptoms of Chilling Injury Symptoms of Chilling Injury Grapefruit Bell pepper Dracaena String beans Tissue necrosis followed by attack by specific pathogens Surface pitting & discoloration Cucumber Page 6
Chilling of Bananas Chilling of Bananas Control Chilled Chilled Control Browning of inner side of peel Skin discoloration, Failure to ripen Chilled bananas develop a dull gray smoke appearance upon ripening. Control (left two) and chilled (right two) bananas Chilling of Pineapples Internal Breakdown of Stone Fruit ü Mealiness (soft but not juicy) ü Flesh Browning ü Lack of Flavor ü Failure to Ripen Endogenous Brown Spot Internal Breakdown is induced at intermediate storage temperatures (2-8 C, 1-6 weeks) 1.7 C (3 F) 7.2 C (45 F) 2.5 C (4 F) 5.5 C (42 F) Crisosto et al., 1999. Susceptibility to chilling injury of peach, nectarine, and plum cultivars in California. HortScience 34(6): 1116-1118. Crisosto et al. 4. Increasing Blackamber (Prunus salicina Lindell) plum consumer acceptance. Postharvest Biol. Tech 34: 237-234. Lurie, S and C. Crisosto. 5. Chilling injury in peach and nectarine. Postharvest Biol. Tech. 37: 195-8. 3 C (38 F) Page 7
Susceptibility Varies with Cultivar Variation in Chilling Sensitivity Plum market life (in weeks) held at 2 storage temperatures based on chilling injury symptom development Crisosto et al., 8. Adv. Hort. Sci. (Italy) 22(3): 1-4. Category Cultivar C 5 C A Betty Anne 5 5 October Sun 5 5 Flavor Rich 5 5 Joanne Red 5 5 B Angeleno 5 3 Fortune 5 3 Hiromi Red 5 3 Black Amber 5 2 Purple Majesty 5 3 C Show Time 4 2 Friar 4 2 Earliqueen 3 2 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 temperature conditioning. Conditioning or Delayed Cooling (1% Mealy) 5ºC Days Conditioning at C for 2 days prevents mealiness from developing during days of cold storage at 5 C (% Mealy) ºC 48 Hours + 5ºC Days (43% Mealy) ºC Days Crisosto, et al. 4. Controlled delayed cooling extends peach market life. HortTechnology 14:99-14. Preventing Chilling Injury Are we preventing injury or symptom development? Don t chill Controlled atmospheres Conditioning High temperature pre-treatment Low temperature treatment (above threshold) Intermittent warming Page 8