DETERIORATION OF FRESH PRODUCE

The Nature of Harvested Produce Deterioration is a term applied to any change

that leads to a loss of quality such as plant physiological changes, mechanical damage, water loss and any other form of injury to the product.

Although now removed from the mother plant, the harvested products continues to undergo all of the living processes it did prior to being harvested.

Fresh products start to die as soon as they harvested, yet they only maintain their marketability whilst alive.

Postharvest horticulture is concerned with slowing down the rate of deterioration in harvested produce and maximizing shelf-life.

With the correct application of the appropriate postharvest techniques, this “dying” process can be slowed down.

Factors Enhancing Deterioration A harvested product is exposed to six stresses that

enhance 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 photosynthetically

active light levels The imposition of atypical temperature regimes The mechanical injury caused by harvesting The increased susceptibility to microbial infection through

harvest and handling wounds.

Photosynthesis

This is the process in green plants that converts the sun’s energy in the presence of carbon 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 be

photosynthetically active.

Hence, from a postharvest perspective, photosynthesis and therefore, carbohydrate (food) production for the stops at the point of harvest.

This means any living processes that occur after harvest are fuelled by a limited reserve of stored carbohydrates, which will be depleted overtime. As fresh produce is eaten to provide a source of carbohydrates, the depletion of this reserve must be minimized.

Respiration

Respiration is used as an indicator of metabolic activity in plant tissue.

This physiological process breaks down the carbohydrates produced during photosynthesis, in the presence of oxygen to give carbon dioxide, water and heat energy. It does not require light proceed and occurs day and night.

C6H12O6 + 6O2 6CO2 + 6H2O + Heat

Respiration is a reversible reaction that can move in either direction depending on the environmental conditions present at the time.

High oxygen concentrations promote the forward direction whereas, high carbon dioxide concentrations promote the backward direction of the reaction.

Hence, the process is easily manipulated by altering the environmental conditions around the product.

This is basis of the postharvest technologies known as controlled atmosphere (C.A.) and modified atmosphere (M.A.)

The aim of these postharvest techniques is to slow down the rate of forward movement of the respiration process.

Respiration after harvest must considered as follows : Stored carbohydrates are used as photosynthesis

does not usually occur after harvest. Oxygen is necessary for the respiration process.

A supply of oxygen must be maintained to the cells in the product if it is to remain in a living state.

Carbon dioxide produced. This must be removed, usually through ventilation.

Water is produced. This can have an influence on the composition and texture of the product.

Respiration produces heat. In fact, there are 673 joules of heat energy produced for each gram molecular weight of glucose respired. It is character that causes all sorts of problems in the distribution of harvested horticultural produce.

Temperature is by far the most important single factor in postharvest quality control. The rate of respiration and hence, the rate of heat production, depends on temperature-the higher the temperature, the higher the rate.

Immature plant tissue has a generally higher rate of respiration than a more mature form of tissue. Hence products such as broccoli, sweet corn, asparagus, spinach, peas and cut flowers have high rates of respirations.

The rate of each respiration of each of these products will in turn be determined by the product temperature.

Classification of fruit and vegetables according to their respiration rates

Very High Rate

High Rate Moderate Rate

Low Rate Very Low Rate

Asparagus

Broccoli

Mushroom

Spinach

Sweet corn

Avocado

Blueberry

Cut flowers

Green bean

Strawberry

Apricot

Banana

Cabbage

Carrot

Cherry

Peach

Pear

Tomato

Apples

Garlic

Grape

onion

Sweet potato

Nuts

There are two common patterns of respiration found during the ripening phase of fruit.

1. Climacteric

The characteristic of this respiratory pattern is the significant increase is the rate of respiration at the onset of the ripening phase. This is a measurable increase which continuous to rise to climacteric peak before easing off when the fruit attains ripeness.

Fruits which show this climacteric pattern of respiration include : Apples Avocado Banana Kiwifruit Mango tomato

2. Non-climacteric

This pattern of respiration can have either an upward or downward trend, depending on the particular product. However, there is no significant increase in the rate of respiration at the onset of ripening. Products that show a non-climacteric pattern of respiration include : strawberry, cherry, cucumber, grape, orange, lemon, pineapple.

Senescence

The development of fruits and vegetables can be divided into three major physiological stages after germination.

These three stages are termed growth, maturation and senescence.

Growth involves an increase in size and dry matter ; maturation overlaps growth and involves a variety of activities ; and senescence involves a breakdown in dry matter.