PRINCIPLES OF POSTHARVEST HORTICULTURE Midterm Exam I 100 points possible NAME: KEY
POSTHARVEST DETERIORATION & LOSSES (10 points) (2 points) What historical movement(s)/event(s) mainly drove the need for improved postharvest handling practices? Urbanization, especially spurred on by the industrial revolution. In addition, increased globalization and demand for produce year-round has driven movement of produce to population centers from around the world, not just from the nearby country side production areas. (2 points) What is the most important difference between fresh and processed (e.g. dried, salted, canned) horticultural commodities in terms of postharvest handling? Fresh horticultural commodities are ALIVE! Thus, they must be handled in such a way as to maintain the health of the tissues as much as possible. (2 points) What is the most important factor than can be managed to reduce losses of fresh fruits and vegetables? TEMPERATURE! (4 Points) Assume you are a cantaloupe melon producer who sells to small, local grocery stores, but wishes to expand your sales to larger retail stores. You currently harvest the fruit into bins, accumulate the bins at an open barn near your production farms, and either load pickup trucks sent by the retailers, or yourself transports bins of fruit to other retailers who pay a fee for the service. Recently, a relatively large regional retailer has expressed interest in purchasing your melons if you first wash the fruit. What factors will you consider to ultimately decide whether or not to agree to such conditions to start selling to this retailer? The main factor to consider is if the added services you will provide will provide enough economic returns to not only pay for the extra equipment and work, while also providing a profit. Another factor to consider might be if the extra washing step will cause any deterioration or reduced shelf life that might result in rejected product at the receiver. Not usually, but design/purchase washing equipment that will be a gentile as possible and consider if your cantaloupes are particularly sensitive to mechanical injury. 2
MORPHOLOGY, STRUCTURE, GROWTH AND DEVELOPMENT (10 points) (5 points) Classify each of the following groups of horticultural crops according to the relative degree of perishability exhibited by most examples of each group as High, Medium or Low (1/2 pt each). 1. Leafy vegetables - high 2. Mature roots and tubers - low 3. Immature fruit vegetables - high 4. Immature floral vegetables high (or medium) 5. Mature seeds and nuts - low 6. Mature, unripe climacteric fruits low (or medium) 7. Ripe nonclimacteric fruits high (or medium) 8. Cut flowers and foliage - high 9. Sprouts - high 10. Immature root vegetables - medium (5 points) Define the following two terms (2 pts each) and give an example of a crop that is typically harvested when it is horticulturally mature but physiologically immature (1 pt). Horticultural maturity: The stage of development when a plant or plant part possesses the prerequisites for utilization by consumers for a particular purpose. Physiological maturity: The stage of development when a plant or plant part will continue ontogeny even if detached. Crop: sprouts, any immature fruit, floral or root vegetable, leafy and stem vegetables. 3
RESPIRATION (20 points) GIVEN: mg CO2/kg hr X 61 = kcal/mt/day Q10 = (R2/R1) exp (10/T2-T1) mg CO2 produced X 0.68 = mg sugar consumed (2 points) Rank the following in order of their anticipated shelf life (1 = longest shelf life, 4 = shortest shelf life). (½ point each) _4_ - Broccoli: respiration rate at 20C = 290 mg CO2/kg-hr _1_ - Potatoes: respiration rate at 20C = 10 mg CO2/kg-hr _3_ - Tomatoes: respiration rate at 20C = 30 mg CO2/kg-hr _2_ - Grapefruit: respiration rate at 20C = 20 mg CO2/kg-hr (4 points) What is the heat of respiration? In your definition, explain how/why it is generated. The heat of respiration is a result of inefficiencies (incomplete energy transfer) between different metabolic steps of respiration. Specifically, there are about 686 kcal of total energy within each mole of glucose, but only about 326 kcal are capture within ATP after complete oxidation of that glucose. The difference in energy is released as heat, as well as eventually the energy captured within the ATP after the energy is used to do work within the cell. Describe what impact(s) the heat of respiration has on postharvest quality and shelf life of fresh horticultural commodities and practices that must be employed to deal effectively with it. It means that extra refrigeration (or at least ventilation) will be needed to remove the heat generated through respiration. Otherwise, the product would heat up, causing rapid senescence and deterioration, and speed respiration further increasing heat generation. (4 points) Explain briefly how a plant tissue s requirement for energy to carry out metabolic processes controls the tissue s rate of respiration. We discussed this in class several times: The respiration rates of all commodities are continuously regulated based on the metabolic demands of the cells. For example, higher temperatures cause faster metabolism (i.e., faster cell maintenance processes), or after being wounded, there is a greater energy demand to supply the wound-healing metabolic processes. Greater metabolic demand => lower cellular ATP levels => stimulates respiration to replenish the pool of ATP. One key enzyme in glycolysis, phosphofructokinase (PFK), is inhibited by high levels of ATP in the cytosol. Thus, lower energy (ATP) demand within the cell results in higher 4
concentrations of ATP that inhibit PFK and slow respiration (as measured by CO2 production or O2 consumption). (5 points) Explain why CO2 production increases when plant tissues transition from aerobic respiration to anaerobic respiration. The cells need to supply sufficient ATP to support their metabolic needs, but anaerobic respiration produces much less ATP per glucose molecule than aerobic respiration. Therefore, the respiration rate (CO2 production) must be higher for anaerobic respiration compared with aerobic respiration if equal ATP production is to be maintained. You are carrying out an experiment to determine the effect of temperature on the respiration of broccoli. At 5 C their respiration rate is 40 mg CO2/kg-h, and at 10 C it is 80 mg CO2/kg-h. a) (3 points) What is the Q 10 for the respiration rate of this commodity in this temperature range? Show your work. Q10 = (R2/R1) (10/T2-T1) Q10 = (80/40) (10/10-5) Q10 = 2 2 = 4 b) (2 points) If this broccoli s shelf life is 10 days at 10 C, what would the predicted shelf life be at 5 C? State your assumptions and show your work. Between 10 o C and 5 o C with a Q10 of 4, one would expect the respiration to increase by a factor of 2, which is also given in the previous question (R1 = 40 and R2 = 80; 80/40 = 2). If respiration is halved at the lower temperature, the shelf life is doubled. Thus, the broccoli shelf life would be increased from 10 days at 10 o C to 20 days at 5 o C. 5
COMPOSITION & COMPOSITIONAL CHANGES (20 points) (10 points) Which chemical constituents contribute to each of the following types of changes taking place in a ripening fruit, for example, a banana? (There can be more than one answer for each.) 1. Color: chlorophyll, carotenoids, anthocyanins (a class of phenolics) 2. Texture (softening): carbohydrates (polysaccharides), proteins, minerals (i.e., calcium) Note: not water (that s why I specified, softening to eliminate turgor changes) 3. Taste: soluble sugars, organic & amino acids, phenolics (bitter, astringent), lipids, minerals 4. Aroma: aroma volatiles (alcohols, esters, aldehydes, etc.) 5. Decay susceptibility: lipids (cuticle), phenolics (antimicrobials and phytoalexins), organic acids (ph) (10 points) Name 10 compositional changes (increase or decrease) that would be expected to occur during postharvest storage of a particular horticultural crop (specify the chosen crop). 1. Water loss 2. Soluble sugar decrease (or increase starch to sugar) 3. Organic acid decrease (respiratory substrate or ascorbic acid oxidation/loss) 4. Aroma volatiles increase (or decrease) 5. Polysaccharides conversion to fragments or simple sugars (cell wall changes) 6. Pigment changes [chlorophyll loss (yellowing) or gain (solar greening); carotenoid and anthocyanin increases] 7. Phenolics, including browning reactions, polymerization of tannins, and phytolalexin increase 8. Vitamins losses 9. Loss of lipids (respiratory substrate) or increase (suberization/wound healing) 10. Protein (enzyme) changes ok, including amino acid increases (protein synthesis) or decreases (protein degradation) 6
ETHYLENE & PLANT HORMONES (20 points) (10 points) Explain the general mechanism by which hormones exert their effects on plants, including how this relates to the changing sensitivity of plants and plant organs to a hormone during development. Hormones act by binding to specific receptors that may differ for different hormone effects. Binding of the hormone by the receptor initiates a cascade of reactions, which ultimately results in changes in gene expression. Changing sensitivity of plants and plant organs to a hormone during development relates to synthesis of the hormone s receptors along with the downstream transporters and transcription factors, etc., which allow the plant or plant organ to respond to the hormone. (5 points) Explain how and why ethylene is used to ripen climacteric fruits. Ethylene is applied to unripe climacteric fruit to initiate and coordinate their ripening. The concentration of ethylene used is a saturating dose so that all the fruit respond maximally; less developed fruit will respond more than more developed fruit because the latter are likely to be producing ethylene on their own this is why the resulting population ripens more uniformly than if allows to ripen on their own. The ripening temperature is chosen to promote the most desirable ripening changes; humidity is kept high to reduce water loss; aeration is used to prevent excessive CO2 buildup, because CO2 is a competitive inhibitor of ethylene action. (5 points) Indicate for each of the following whether it describes climacteric or nonclimacteric behavior or is true for both types. 1. Ripening is triggered in mature specimens upon exposure to ethylene. Climacteric 2. Produces ethylene in response to wounding. Both 3. a. Respiration rate elevated upon exposure to ethylene. Both b. Respiration rate returns to pre-exposure level when ethylene is removed. Nonclim. 4. Ethylene synthesis is induced upon exposure to ethylene. Climacteric 7
TRANSPIRATION & WATER LOSS (10 points) Using the psychrometric chart below, answer the following questions. (1 point) What is the relative humidity of air with a dry-bulb temperature of 25C and a wet bulb temperature of 22C? ~77% (1 point) What is the dew point of the air in the first question? ~21 o C (2 points) If the air in the first question was warmed to 30 o C and water was neither added nor removed, what would be the new relative humidity of the air? RH would decrease to ~57%. (2 points) Would a commodity (at the same temperature of the surrounding air) lose water faster if stored in the air from the first question, or if stored in air at 0C (32F) with 50% relative humidity? Why? The commodity would lose water faster if stored in question A air than at 0C with 50% RH. The VPD between 100% (saturation) and 77% RH is greater at 25C (question A) than 50% RH at 0C. (4 points) Consider a pallet of leeks at 25C and weighing 500 kg that was left in a room with conditions in the first question overnight (14 hours). During this time period, the 8
pallet of leeks lost 2% of its original weight. What would be the VPD experienced in those conditions, and from this, calculate K for the leeks. 1. Calculate the vapor pressure deficit (VPD). Using the psychrometric chart, the VPD is approximately 7 milibars. (Note: it s ok if your VP numbers are not exactly the same as these). We usually use kpa, but this psychrometric chart happens to be in milibars. You can either convert to kpa, or continue the calculations as milibars 2. Now calculate K. Using, J = K VPD = 0.14%/h = K 7 milibars = K = 0.02 (%/h)/milibars. 9
PHYSIOLOGICAL DISORDERS (10 points) (5 points) What is the most important factor involved in the development of physiological disorders of horticultural crops during the postharvest period and why? TEMPERATURE. Most postharvest physiological disorders are temperature related. In postharvest handling of produce, the goal is always to quickly cool the products after harvest to their lowest safe temperature. For temperate crops, that means just above freezing; for subtropical and tropical crops, that means their chilling threshold temperature. Thus, susceptibility to freezing and chilling injury determine the extent to which we can use temperature to maintain the quality of produce postharvest. On the other hand, solar exposure or excessively high postharvest temperatures have to be avoided to prevent high temperature injury from occurring. (5 points) Define the term, chilling threshold temperature for horticultural crops, then explain the unusual case of stone fruit, which have a chilling threshold of about 10 C, but for which storage at 0 C is recommended. The chilling threshold temperature is the lowest safe temperature at which a chilling-sensitive product can be held without chilling injury symptoms developing, regardless of the length of storage. Stone fruit are unusual, because the symptoms of chilling injury for those fruits (internal flesh discoloration and mealy/dry texture) develop during the exposure to chilling temperatures, not requiring transfer from the chilling temperature to a warmer, non-chilling temperature to develop, like is the case with most fruits and vegetables. However, the balance between chilling injury severity and symptom development maximizes at around 5 C. That results in little or no symptom development occurring at 0 C since the low temperature inhibits the symptom-related chemical reactions. Since the fruit are usually consumed very soon after removal from refrigeration, the consumer sees no chilling injury symptoms. (This is illustrated and explained in the lecture slide #25) 10