The Nature of Harvested Produce Deterioration is a term applied to any changethat leads to a loss of quality such as plantphysiological changes, mechanical damage,water loss and any other form of injury to theproduct. Although now removed from the motherplant, the harvested products continues toundergo all of the living processes it did priorto being harvested.
Fresh products start to die as soon as theyharvested, yet they only maintain theirmarketability whilst alive. Postharvest horticulture is concerned withslowing down the rate of deterioration inharvested produce and maximizing shelf-life. With the correct application of the appropriatepostharvest techniques, this “dying” processcan be slowed down.
Factors Enhancing DeteriorationA harvested product is exposed to six stresses thatenhance its rate of deterioration and subsequently,reduce its shelf-life. These are : The removal of its supply of water The absence (in most instances) of photosyntheticallyactive light levels The imposition of atypical temperature regimes The mechanical injury caused by harvesting The increased susceptibility to microbial infection throughharvest and handling wounds.
Photosynthesis This is the process in green plants thatconverts the sun’s energy in the presence ofcarbon dioxide and water into carbohydrates(food), oxygen and water.CO2 + 2H2O (CH2O) + O2 + H2O Photosynthesis can only occur if there is light. The light must be of a high intensity to bephotosynthetically active.
Hence, from a postharvest perspective,photosynthesis and therefore, carbohydrate(food) production for the stops at the point ofharvest. This means any living processes that occurafter harvest are fuelled by a limited reserveof stored carbohydrates, which will bedepleted overtime. As fresh produce is eatento provide a source of carbohydrates, thedepletion of this reserve must be minimized.
Respiration Respiration is used as an indicator ofmetabolic activity in plant tissue. This physiological process breaks down thecarbohydrates produced duringphotosynthesis, in the presence of oxygen togive carbon dioxide, water and heat energy. Itdoes not require light proceed and occursday and night.C6H12O6 + 6O2 6CO2 + 6H2O + Heat
Respiration is a reversible reaction that canmove in either direction depending on theenvironmental conditions present at the time. High oxygen concentrations promote theforward direction whereas, high carbondioxide concentrations promote the backwarddirection of the reaction.
Hence, the process is easily manipulated byaltering the environmental conditions aroundthe product. This is basis of the postharvest technologiesknown as controlled atmosphere (C.A.) andmodified atmosphere (M.A.) The aim of these postharvest techniques is toslow down the rate of forward movement ofthe respiration process.
Respiration after harvest must considered asfollows : Stored carbohydrates are used as photosynthesisdoes not usually occur after harvest. Oxygen is necessary for the respiration process.A supply of oxygen must be maintained to thecells in the product if it is to remain in a livingstate.
Carbon dioxide produced. This must be removed,usually through ventilation. Water is produced. This can have an influence onthe composition and texture of the product. Respiration produces heat. In fact, there are 673joules of heat energy produced for each grammolecular weight of glucose respired. It ischaracter that causes all sorts of problems in thedistribution of harvested horticultural produce.
Temperature is by far the most importantsingle factor in postharvest quality control.The rate of respiration and hence, the rate ofheat production, depends on temperature-thehigher the temperature, the higher the rate.
Immature plant tissue has a generally higherrate of respiration than a more mature form oftissue. Hence products such as broccoli,sweet corn, asparagus, spinach, peas andcut flowers have high rates of respirations. The rate of each respiration of each of theseproducts will in turn be determined by theproduct temperature.
Classification of fruit and vegetables according totheir respiration ratesVery HighRateHigh Rate ModerateRateLow Rate Very LowRateAsparagusBroccoliMushroomSpinachSweet cornAvocadoBlueberryCut flowersGreen beanStrawberryApricotBananaCabbageCarrotCherryPeachPearTomatoApplesGarlicGrapeonionSweetpotatoNuts
There are two common patterns of respirationfound during the ripening phase of fruit.1. ClimactericThe characteristic of this respiratory patternis the significant increase is the rate ofrespiration at the onset of the ripeningphase. This is a measurable increase whichcontinuous to rise to climacteric peak beforeeasing off when the fruit attains ripeness.
Fruits which show this climacteric pattern ofrespiration include : Apples Avocado Banana Kiwifruit Mango tomato
2. Non-climactericThis pattern of respiration can have either anupward or downward trend, depending on theparticular product. However, there is nosignificant increase in the rate of respirationat the onset of ripening. Products that show anon-climacteric pattern of respiration include :strawberry, cherry, cucumber, grape, orange,lemon, pineapple.
Senescence The development of fruits and vegetables canbe divided into three major physiologicalstages after germination. These three stages are termed growth,maturation and senescence. Growth involves an increase in size and drymatter ; maturation overlaps growth andinvolves a variety of activities ; andsenescence involves a breakdown in drymatter.