Module 2. Post-harvest management of fruits and vegetables

Lesson 7

POST HARVEST PROCESSING-I

7.1  Storage of Fruits and Vegetables

Storage of fruits and vegetables is practiced for various reasons. It is part of orderly marketing where the storage period is usually short, to allow for accumulation of sufficient produce by the grower or group of growers to send to market. Certain fruits and vegetables are stored for longer periods of time to extend the duration of their availability. In some cases fruit and vegetables destined to be processed are first stored to even out the supply to the factory. Often the objective is to provide a constant supply of raw material for the factory for as long a time as possible. Often this type of storage is different from storage of fruits and vegetables destined for fresh market. Many types of storage systems are available for fruits and vegetables. Some of the important ones are given below.

7.1.1  Evaporative cooling and storage

In tropical countries like India deterioration in quality of fruits takes place immediately after harvest due to lack of on farm storage. To overcome this problem low cost, environmental friendly zero energy chambers also called as evaporative coolers have been developed. When water evaporates from the liquid to the vapour phase, it requires energy. This principle is used to cool stores or chambers by passing the air that is introduced into the store first through a pad of water. The degree of cooling depends on the original humidity of the air and the efficiency of the evaporating surface. These can be easily constructed at small and marginal farmers’ field and are helpful to keep temperature 15-30°C lower than atmospheric temperature. These can be used for short-term storage of fruits and vegetables at the farmers’ field.

7.1.2  Controlled and modified atmosphere storage

Normal atmospheric air contains about 78.08% of nitrogen (N₂), 21% of oxygen (O₂), 0.04% of carbon dioxide (CO₂) and remaining other gases. A reduction in the concentration of O₂ and/or an increase in CO₂ concentration of the storage atmosphere surrounding fresh fruits and vegetables reduce the rate of respiration and also inhibits microbial and insect growth. The terms “controlled atmosphere” (CA) and “modified atmosphere” (MA) mean that the atmospheric composition surrounding a perishable product is different from that of normal air. Concept of controlled atmosphere storage (CAS) of fruits and vegetables was given by J.Be’rard of France in 1821. However, commercial use of CAS grew out of the research of Kidd and West in 1917 on respiratory behaviour of fruits under modified atmospheres. It was applied commercially for the first time in 1927 for storage of fruits and vegetables. In-transit use of CA and MA has been developed much later, during 60’s only. Both CA and MA commonly involve manipulation of CO₂, O₂ and N₂ levels.

The principle of CA or MA involves the complete or partial removal of air from the storage are and its replacement with a single gas or mixture of gases by either passive or active methods. Passive modification is a slow process. It requires reactions between the fruit or vegetable and its surrounding gases to take place, and the package to play the role of a regulator. On the contrary, active modification is faster and can be achieved by gas flushing, vacuum application or by using gas scavengers / emitters. Passive MA relies on the selective permeability of the packaging materials to different gases and on product respiration and is traditionally used with fresh and minimally processed fruits and vegetables. Gas-flush MA involves the establishment of a specific gas composition within the package in a single stage during the packaging operation, by flushing with the selected gas mixture before sealing. Depending on the desired residual O₂, a vacuum operation may be needed prior to gas flushing. The gas mixture used is dependent on the type of fruit or vegetable. MA differs from CA only in how precisely gas partial pressures are controlled; CA is more precise than MA. When combined with chilling, CA or MA are increasingly important methods of maintaining high quality in processed foods during an extended shelf life. The following are some types of atmosphere-modification techniques:

a. Atmosphere Generation

Oxygen partial pressures can be reduced by purging with nitrogen, burning, using catalytic converters, or using silicone membranes. Water, lime, and molecular sieves have all been shown to be effective scrubbers for removal of CO₂. Pressurized cylinders or dry ice can be used for addition of CO₂ into the holding chamber. Carbon monoxide can also be added from pressurized cylinders.

b. Hypobaric (Low-Pressure) Systems

Hypobaric storage is a type of CA storage in which a product is held under a partial vacuum. By reducing the normal atmospheric pressure in the ambient environment around the commodity, the effective partial pressures of individual ambient gases are also lowered. For example, a one-fifth reduction in the total pressure of normal air would result in an effective oxygen partial pressure (oxygen tension) equivalent to 4% O₂. In addition to lowering the partial pressure systems allow gases to escape more rapidly. This is due to the fact that the diffusion coefficients of various gases, including ethylene and other volatiles are inversely proportional to atmospheric pressures.

c. Commodity-Modified Atmospheres

In this particular approach an actively respiring and metabolizing product reduces the O₂ an increases the CO₂ in the ambient air within a chamber in which various barriers and restrictions to gas exchange exist. Ethylene and CO₂ scrubbers are used when either of these components is considered harmful to the commodity. When a plant material is put in a CA/ MA chamber, composition of cellular environment also changes due to Devaux effect. Devaux effect defines that due to difference in concentration gradient of gases on the two sides of membrane, diffusion of gases from the higher concentration side to the lower concentration side take place till equilibrium is attained. It is caused by difference in concentration gradient of gases on the two sides of membrane. Diffusion of gases from higher concentration side to the lower concentration side takes place till equilibrium is attained. CO₂ from the tissues of plant and O₂ from the external environment to the tissues occur. Rate of diffusion of CO₂ is much faster than that of O₂. Commodity modified atmosphere can be attained by using various techniques such as wax coating, by putting plastic covers with diffusion windows, manipulation of shipping containers or vehicles, applying polythene liners in shipping containers, packaging in film wraps and using air-tight cold storage rooms. The barriers can be created by commodity itself, or using package or storage rooms.

7.1.3  Benefits of CA storage

· Very low levels of O₂ (<2%) and high CO₂ level (≥60%) can control insects.

· A considerable decrease in respiration rate, in climacteric maximum, accompanied by an expansion of both pre-climacteric and post-climacteric period.

· Reduction in effect of C₂H₄ due to its interaction with O₂ and hence delay in development and appearance of the symptoms of senescence.

· Increased flesh firmness due to inhibition of enzymes responsible for damage of cellular membrane, at high CO₂ level.

· High turgidity and hence more juicy & crisp fruits.,

· Superior sensory and nutritional quality.

· A limited degradation of chlorophyll resulting in greater colour stability.

· Some physiological disorders, such as chilling injury, spot, decay, browning, water core and scald are greatly reduced.

· Mould growth is low due to less O₂ and high CO₂.

· Longer storage life.