Post-Harvest Changes in Fruits and Vegetables

Post Harvest Changes in Fruits and Vegetables

This article give you detailed idea about Post Harvest Changes in Fruits and Vegetables. We have covered maturity and ripening and post harvest changes.

MATURATION AND RIPENING

As fruits and vegetables mature, the content of structural support tissue—namely cellulose, hemicellulose, and lignin gradually increases. Vegetables lose their tenderness; for example, French beans become stringy. The lignin content in vegetables increases, giving them a tough, woody texture, as seen in over-mature carrots. Vegetables should be harvested at the optimum stage of maturity to prevent toughening and loss of texture.
Physical properties are modified as very long polymers of water-insoluble protopectin are hydrolyzed and demethylated to form shorter, methylated, water-soluble pectin, which is capable of forming a gel with sugar and acid. As protopectin is converted to pectin, fruits gradually soften and develop flavor and aroma, which were previously absent in very hard green fruits.
Respiration rate is high in perishable vegetables, such as green leafy vegetables, as the uptake of oxygen and production of carbon dioxide is rapid. The respiration rate is affected by the following.

  1. Temperature: Respiration rate can be controlled by refrigeration.
  2. Stage of maturity and ripening: Respiration rate is maximum just before the fruit is fully ripe.
  3. Composition of the surrounding atmosphere: Carbon dioxide is added to extend the shelf life of fruits and vegetables. This level varies from one product to another. Ethylene gas is used to accelerate the ripening process.
  4. In fruits, starch is converted to sugar during ripening, whereas in vegetables, sugar is converted to starch.

Post-Harvest Changes in Fruits and Vegetables

Fruits and vegetables remain alive and continue to respire even after they are harvested. Most fruits and vegetables have a high percentage of moisture and retain optimum quality for only a few days. Even after harvest, enzyme activity continues, and they respire. This can be observed by keeping them in a polythene bag—drops of moisture collect inside the bag even after short storage.
Loss of moisture results in limpness and wilting. To retain quality, it is essential that fruits and vegetables be harvested at optimum maturity and ripened under controlled conditions. Fruits and vegetables should be stored at the optimum temperature, humidity, and atmosphere to control moisture loss, retain freshness, prevent microbial spoilage, and preserve the appearance, texture, flavor, and weight of the products.
During respiration, stored food is used up, affecting the ultimate quality. Flavor changes take place due to enzyme action and the conversion of sugar to starch. After fruits and vegetables are harvested, the synthesis of virtually all organic compounds stops, but physiological changes continue during storage. Selection of a proper storage area is of utmost importance and varies with the type of vegetable or fruit. Each type of vegetable or fruit has a temperature range over which it can be stored for at least a short time. Non-freezing, very low temperatures may result in chilling injuries.

CLIMACTERIC AND NON-CLIMACTERIC FRUITS

Climacteric Fruits


Fruits that are harvested when they are horticulturally mature but not ripe and have the ability to continue ripening after harvest are termed climacteric fruits. Their respiratory rate is maximum just before full ripening.

Non-Climacteric Fruits

Fruits such as pineapple, citrus fruits, and grapes do not exhibit an accelerated respiration rate after harvesting. These fruits are of the best quality if harvested when ripe.
Bulbs, roots, tubers, and seeds become relatively dormant during optimum storage, while fleshy fruits and vegetables undergo ripening after maturation and then lose palatability. Changes are seen in reference to:

  1. Weight
  2. Texture
  3. Flavor
  4. Taste


Biochemical changes seen in fruits and vegetables include:

  1. Respiration
  2. Changes in certain constituents of cell walls
  3. Conversion of sugar to starch in vegetables and starch to sugar in fruits


Fruits undergo a number of chemical changes after harvesting. Pectic substances in the cell wall are degraded by enzyme action during storage, converting pectin to pectic acid, which is more soluble. The sweetness of fruits increases. Some complex polysaccharides, such as cellulose and hemicellulose, are broken down, releasing sugars and increasing sweetness. Starch is converted to sugar by the enzyme amylase, and sucrose is broken down in green leafy vegetables and fruits by the enzyme invertase.

EXTENDING SHELF LIFE THROUGH MODIFIED ATMOSPHERE PACKAGING (MAP)

The shelf life of perishable and semi-perishable fruits and vegetables can be extended by altering the relative proportions of atmospheric gases surrounding the food. This technique, called modified atmosphere packaging (MAP), is commonly used nowadays (refer to Chapter 25 on New Trends in Food and Nutrition).
In this technique, the normal composition of air (approximately 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide) is modified to match the respiration rate of fruits and vegetables, varying from one product to another. New packaging techniques are being developed to retain freshness in fruits and vegetables. The proportion of oxygen inside the package should be lower, and the proportion of carbon dioxide should be higher than the normal composition of air. This reduces the rate of respiration and retards microbial growth, thereby preventing rot and decay during post-harvest storage.

NATURAL COLOURING PIGMENTS

Fruits and vegetables are visually appealing due to their bright and variable colors. These colors are caused by pigments present in the cell, mainly in the plastids—chloroplasts and chromoplasts—and, to some extent, in fat or water droplets in the protoplasm and vacuoles.
Pigments are classified into three major groups:

  1. Chlorophylls
  2. Carotenoids
  3. Flavonoids


Flavonoids and carotenoids present in strawberries, black grapes, melons, and pineapples add colour to our meals. To retain the attractiveness of fruits and vegetables and preserve their bright, appealing colours, it is necessary to understand the effect of different cooking processes and cooking mediums on pigments.