PRINCIPLES OF POSTHARVEST HORTICULTURE Midterm Exam II 100 points possible 26 October, 2018 NAME: Key
(20 Points) POSTHARVEST PATHOLOGY (Bartz) (3 points) List three reasons why breaking the skin (cuticle and epidermis) of a commodity promotes decay. This is not asking how the skin might become broken (e.g., scratched, cut, etc.), but why breaking through this barrier promotes decay. Rupturing the epidermis allows fungi entry into tissue flesh (eliminates the physical barrier). Rupturing the epidermis provides water from the ruptured cells for pathogen spores to germinate and mycelium to grow. Rupturing the epidermis releases sugars to feed spore germination and mycelium growth and development. The injury promotes ethylene production, which promotes senescence and decay. Some said by increasing respiration and water loss. The increase in respiration from wounding is usually transient. These factors are usually relatively minor in promoting decay compared to the others, but partial credit is given. (4 points) Describe four (4) ways to reduce postharvest decay of fresh fruits and/or vegetables that do not include the use of fungicides. Any of the following: Avoid crop stress before harvest (e.g., nutritional disorders) Handle the product gently (e.g., prevent wounding) Practice good sanitation in the packinghouse incl. removing injured/cull product from the packinghouse quickly. Cool the product quickly, but remain the chilling threshold Related, maintain the cold chain during storage/transportation Sanitize recirculated water (e.g., wash) systems Handle the product quickly Curing of some commodities Avoid ethylene exposure, unless intentionally ripening Plant resistant cultivars Use of non-chemical treatments (heat, MA/CA, etc.). Some said use of essential oils, which might be consider fungicidal, but this was also accepted. (3 points) In a recirculated water system (e.g., flume or wash system) using chlorine as a sanitizer (NaOCl, CaOCl, or Cl2 gas), why does free chlorine and water ph BOTH need to be monitored to determine if concentrations are effective? Free chlorine includes both hypochlorous acid (HOCl) and hypochlorite ion (OCl - ). Lower ph (e.g., <7) results in more of the free chlorine in the hypochlorous acid form (HOCl), which is the most active against pathogens. (However, if ph drops too low, then dangerous chlorine gas (Cl2) will be released.) Higher ph (e.g., >7) results in more of the chlorine in the ion form (OCl - ), which is much less active against pathogens. 2
(4 points) List four significant fungal characteristics that would make it possible for the fungus species to be especially destructive as a postharvest pathogen. It is able to grow at low (storage) temperatures. It can directly penetrate the undamaged plant cuticle with a penetration peg and cutinase. It forms latent infections that are quiescent (and resistant to control measures) until conditions are favorable for continuation of growth. It has particularly active enzymes to vigorously degrade cell walls. It can grow with low available water. Fast growth and reproduction (sporulation). Some said long-lived structures (resistant structures), and high growth/spore production rate for which I also gave credit. (6 points) List what you think are the three (3) most important factors in reducing postharvest decay of fresh commodities and provide a convincing argument as to why they should be considered the most important factors. Rank your three factors in descending order as to their relative importance (e.g., #1 is what you consider the most important) Answers can vary, but should be reasonable and well defended. Factors of most important can include: TEMPERATURE (of course) maintaining as low as possible without developing chilling injury. Or temperatures need first for optimum curing. Preventing or minimizing injury. Creating and maintaining sanitary conditions (inoculum exclusion) both in the field and during postharvest handling. Developing resistant varieties This is more of a long-term solution that may, or may not come to fruition depending on the disease. Some did not state why the factors were important Some gave very broad factors (like the environment) that really included many factors (e.g., temperature, RH, free water, MA/CA, etc.). In these cases, the disease triangle is really what is being referred (pathogen, environment, and plant genetics related to susceptibility). Partial credit given for such answers. 3
(13 Points) MATURITY AND QUALITY STANDARDS (Brecht) (2 points) For each factor listed below, give an example of a fruit or vegetable crop for which that factor is used as a commercial maturity index. a) shape Banana, mango, etc. (any organ that fills out during maturation) b) soluble solids content Pome fruits, stone fruits, grapes, etc. (anything that accumulates soluble sugars during development) c) surface morphology Watermelon (gloss), cantaloupe (netting). d) abscission layer development Tree fruits (ease of detachment), melons, etc. (7 points) Discuss the important considerations and steps you would take in developing a harvest maturity index for a new climacteric fruit crop. Specifically, what are the likely tradeoffs between harvesting the fruit earlier versus later and what characteristics of the maturity index itself are important to allow for its practical use? Balance the desire for the highest quality product at harvest, with transportation requirements (i.e., will ripe fruit survive the journey?). Commodity attributes, such as if it is climacteric or not, will also influence the choice of maturity index. Then evaluate the commodity attributes and how they change with maturity, cultivar, season, growing region, etc. From these, choose one or more that results in a commodity of highest quality when it reaches the consumer and satisfies the following requirements: It s measurement must be relatively simple (can be performed in the field with inexpensive equipment) It s measurement should be objective (measured, not subjective) An index based on the attribute should relate to quality regardless of grower, district, or year Changes in the attribute should be predictive: a progressive change with maturation 4
(4 points) Optimal harvest maturity generally coincides with optimal quality, but climacteric fruits can be an exception to that rule. Explain why the relationship between maturity and quality is more complicated for a climacteric fruit than for a nonclimacteric crop. With nonclimacteric crops, it s simple: the optimal maturity and highest quality is at harvest, since they do not continue to mature or improve in quality once removed from the plant. With climacteric fruits, they generally continue to improve in quality on the plant as ripening commences and proceeds, but postharvest shelf life declines as harvest is delayed because ripe fruit are prone to over ripening, damage and decay. However, it is possible to harvest a climacteric fruit at physiological maturity (prior to ripening initiation) and control the onset and progression of ripening postharvest, using ethylene treatment and temperature management. That approach maximizes shelf life, but with a trade-off of lower quality compared to tree-ripe fruit. 5
(8 Points) FOOD SAFETY MIDTERM EXAM II (2 points) Why are fresh fruit growers, packers, and shippers not able to treat the produce in such a way as to eliminate potential foodborne pathogens like the citrus or apple juice manufacturers can? Because those treatments (e.g., high temperautres, strong ionizing radiation, etc.) are imposed on tissues that will no longer be living and are too extreme and would injure fresh produce, causing scalding or other blemishes/injury. (2 points) Now that the Food Safety Modernization Act (FSMA) and the resulting rules for fresh produce are in effect, other previously buyer-mandated food safety schemes (e.g., GLOBALG.A.P., Primus Labs, Safe Quality Foods, etc.) are no longer in effect. (T / F circle the correct answer) (2 points) A load of apples shipped from the U.S. to Europe arrives and the residue level of one pesticide are found to be above EU tolerances. Europe is notorious for setting low pesticide residue tolerances. In this case, the U.S. shipper can negotiate a lower price with the receiver, but still be able to sell the product in Europe. (T / F circle the correct answer) (2 points) Explain why careful handling that has long been taught as a way to maintain fresh fruit quality, is also important for food safety. This question specifically asks about careful handling, not about sanitation or innoculum. Careful handling is important for both food safety and food quality because fruit and vegetable stresses (e.g., chilling injury, peel breakdown, bruises, etc.) and/or injuries (e.g., scratches and abrasions) detract from visual appearance AND provide entry points and/or opportunities for establishment (invasion & growth) of both plant diseases (decay) and human diseases (foodborne illnesses). 6
(8 Points) QUARANTINE TREATMENTS MIDTERM EXAM II (2 points) What is the name of the federal agency in the United States that regulates quarantine treatments? USDA Animal and Plant Health Inspection Service (APHIS) (3 points) Explain why some of the approved insect quarantine treatments that are found in the APHIS Treatment Protocol Manual, while effective in killing the targeted insects, are marginal in terms of their tolerance by fruits and vegetables. Because the treatments were developed with the goal of killing the insect pests being more important than preserving the quality of the fruits and vegetables. (3 points) Why is it not necessary for an irradiation treatment to kill the targeted insects for it to be an effective quarantine treatment? Because lower irradiation energy than the lethal dose can still sterilize the insects (meaning they are unable to reproduce). 7
(12 Points) HARVESTING, HANDLING AND PACKINGHOUSE OPERATIONS (Ritenour) (2 points) During the harvest and handling procedures, give at least two reasons why is it important to remove unmarketable product as soon as possible? 1) To avoid wasting time, money and energy applying procedures to products that will eventually be discarded. 2) To reduce the potential for decay development (on infected, damaged products) and cross-contamination of other products. (4 points) What is curing? Curing is the practice of holding certain products at relatively high temperatures (30-35 C) with either high (90-100%) or low (<40%) relative humidity depending on the product. What types of crops are cured? Mainly underground storage organs like roots, tubers and bulbs. What is the difference between curing potatoes, sweetpotatoes, etc., verses onions and garlic? The purpose of curing potatoes and sweet potatoes is to promote wound healing (e.g., under warm temperatures with high RH) to protect against pathogen intrusion and water loss, while curing of onions and garlic is intended to dry the neck and outer layers of tissue (e.g., under mid to warm temperatures but low RH) to create a barrier to again protect against pathogen intrusion and water loss. What are the benefits of curing? Curing either promotes wound healing (high temperature and high humidity; potatoes, sweetpotatoes, etc.) or creates a barrier of dry tissue (high temperature/low humidity; onions and garlic); the result in both cases is that the product is more resistant to decay development. (6 points) Discuss the advantages and disadvantages of field packing (e.g., compared to packinghouse packing). How would you decide whether to field pack or packinghouse pack a new commodity? Advantages: Less material to transport and dispose; Fewer handling steps => less mechanical damage; Smaller initial start-up cost. Disadvantages: Requires large machinery in the field (soil compaction, trampled product, etc.); More dependent on weather; Requires skilled labor; Product in containers are more difficult to cool; Less control over quality; Cannot apply many postharvest treatments (e.g. waxes, fungicides, etc.). 8
How to decide? The decision on which method to use depends on the commodity and market requirements in regards to the above advantages and disadvantages. Thus, issues to consider include available start-up funds (e.g. to build a packinghouse); does the value added to a commodity by going through a packinghouse justify the costs; labor availability, costs, and skill level; accessibility of the field to large equipment; likely weather conditions during 9
Product Temperature (C) MIDTERM EXAM II (13 Points) TEMPERATURE MANAGEMENT (Ritenour) (5 points) For a commodity cooled in air, draw a typical commodity cooling curve with temperature on the y (vertical) axis and time on the x (horizontal) axis. Assume that the starting temperature is 25 o C, and the air temperature is 5 o C. Identify the initial product temperature, the air temperature, and the ½ and ⅞ cooling times. 25 (1) Initial Product Temperature 20 (3/4) Average Product Temperature Using Air 15 (1/2) 10 (1/4) 7.5 (1/8) ½ Cool Average Product Temperature Using Water 7/8 Cool 5 (1/4) 0 Air Temp 0 1 2 3 4 Hours of Cooling 5 10
(2 points) Draw in the figure above how you would expect the above cooling curve to be different if the product was cooled using water (hydrocooling)? See above figure (3 points) Explain why fruits and vegetables are cooled faster in a well-managed hydrocooler than a forced-air cooling system. It would cool quicker because of water s higher heat capacity and thermal conductivity compared to air. (3 points) Explain why the use of ice is not a particularly rapid cooling method, even though additional heat is absorbed just by the melting of the ice (conversion of solid to liquid). Although ice in contact with product gains additional cooling capabilities when the water turns from a solid (ice) to liquid (water), ice is not smooth to facility close contact with the product. In addition, as soon as the ice touching the commodity melts, then there is no longer direct contact between the product and cooling medium. Thus, poor contact greatly slows cooling. In addition, air spaces that form between the ice and product serves and an insulator between the ice and product. (13 Points) COMMERCIAL STORAGE; MA & CA (Brecht) (4 points) Describe how a mechanical refrigeration system works, being sure to mention 1) the physical principle that underlies refrigeration and 2) the three main parts that make up a refrigeration system and how they interact. Mechanical refrigeration systems work by sequentially compressing and expanding a refrigerant so that it transitions between vapor (gas) and liquid phases, causing the refrigerant to repeatedly absorb then release heat. Heat is absorbed from inside the refrigerated storage space by the refrigerant when liquid refrigerant passes through the expansion valve as it is entering the evaporator coils and changes phase from liquid to gas. The resulting gaseous refrigerant then enters the compressor where it is converted from gas phase back to liquid phase (involving work). When the high-pressure, hot, liquid refrigerant leaves the compressor, it enters the condenser, where the heat is absorbed by air or water and dumped to the outside of the refrigerated storage space. 11
(4 points) The Km = 0.1% oxygen for the rate-limiting terminal oxidase in the mitochondria in aerobic respiration (meaning the reaction is still running at ½-maximal rate with just 0.1% O2 present). So, why is it that most fruits and vegetables can t tolerate exposure to any less than about 2% oxygen in a controlled atmosphere storage environment? A concentration of 2% oxygen surrounding the fruit or vegetable results in a much lower concentration of oxygen within the plant cells. A lower oxygen concentration occurs within the plant tissue compared with the 2% outside concentration partly because there is physical resistance to diffusion through the plant tissue. A lower oxygen concentration occurs within the plant tissue compared with the 2% outside concentration also partly because the respiratory activity of the plant cells consumes oxygen, adding to the concentration gradient. (2 points) What is the difference between active versus passive modified atmosphere (MA)? Passive MA relies on the product respiration to modify the atmosphere while active MA involves an initial flushing operation to immediately establish the desired atmosphere. (3 points) For each of the following heat sources in refrigerated storage, describe what can be done to reduce its impact: Sensible or field heat Start with a cool product: harvest at night or during cooler times of the day; shade the commodity after harvest; precool the commodity prior to storage. Vital heat or heat of respiration Provide sufficient refrigeration capacity during storage to absorb vital heat; keep storage temperatures at their lowest safe (non- CI) temperature to keep respiration low; use modified or controlled atmospheres to further reduce respiration. Heat transmission across walls, floors, etc. Provide adequate insulation; paint the outside wall and roof a light color or use reflective materials that reflect (instead of absorb) sunlight and radiant heat. 12
(13 Points) TRANSPORTATION & THE DISTRIBUTION SYSTEM (Ritenour) (5 points) List five (5) things wrong with this refrigerated trailer that might make you, as a shipper of high quality fresh produce, hesitant to allow it to haul your product from Florida to California. 1. It s dirty 2. There is no return air bulkhead 3. There appears to be an open fresh air exchange that possibly can t be closed 4. The air delivery chute is torn so that cold air is not delivered to the rear of the trailer. 5. The walls are smooth rather than corrugated, not allowing air to flow down the sides of product stacked against the walls. 6. There are no T-beams in the floor; in fact, most of the floor is smooth, not facilitating air flow under the product, especially for product stacked directly on the floor. 7. Not able to truly clean and sanitize the wood sections of walls/ceiling. 13
(3 points) Identify three features of marine containers that are improvements for temperature management compared with truck trailers (1 point each)? Any of the following: -the direction of the air movement corresponds to heated air rising -refrigerated air is forced through the packages/load -better insulation in container walls -also, deeper floor channels to handle greater air volume (3 points) What kind of food products were top-air delivery trailers originally designed to carry and how does that affect the way in which fruits and vegetables should be loaded into top-air delivery trailers? Frozen foods with the cold air surrounding the product. Fruits and vegetables should be loaded so as to avoid contact between the load and the sides of the trailer so that the refrigerated air can circulate between the load and the sides Also, frozen foods did not experience internal heat generated from respiration. Fresh produce, on the other hand, generates heat from respiration so that carton venting and airflow through the load (e.g., horizontal air channels) is needed to carry away generated heat. (2 points) In contrast to top-air delivery systems mentioned in the previous question, describe the best overall method of loading a shipping container that utilizes a bottom-air deliver system? Such product should completely cover the floor space so that the air delivered at the bottom is forced up through the load. Open floor spaces allow low air resistance so that most of the cold air will bypass moving through the product, but around the product through the open floor spaces. In addition, cartons must be designed to allow vertical air movement through the load (from bottom to top). 14