Organisation:Massey University, Private Bag 11-222, Palmerston North, New Zealand Massey University
Author: Linus U. Opara
Edited by AGST/FAO: Danilo Mejía, PhD, FAO (Technical), Emanuela Parrucci (HTML transfer)

CHAPTER XXVI ONIONS: Post-Harvest Operation

Preface

1 Introduction

1.1 Economic and Social Impacts of Onions

1.2 World Production and Trade

1.3 Primary product

1.4 Requirements for export and quality assurance

1.5 Consumer Preferences

Preface

The post-production sector accounts for about 60% of the economic activities in most developing countries. The growing, handling, storage and marketing of onions is an important part of these economic activities. In many growing regions, it is a major source of income for rural families who sell their produce in local, regional and international markets. Onion are also grown in large quantity in developed countries and these products dominate the international market partly due to their higher quality and longer storage life which results from the application of improved post-harvest systems. The objective of this chapter is to examine the post-harvest handling and storage technology for onions, which assist in quality maintenance.

Fig. 1. Onion bulbs

1. Introduction

1.1 Economic and Social Impacts of Onions

Onions (Allium cepa L.) are important vegetable crops in most parts of the world, particularly the varieties that are grown for bulbs. In terms of global weight of vegetables produced, at nearly 28 million tonnes per annum, only tomatoes and cabbages exceed bulb onions in importance.

1.2 World Production and Trade

Onions are important crops in the tropics, which account for nearly 30% of total global production. Although some tropical countries are net importers, export potential of onions is developing in several tropical regions partly because if dried and packed properly, the bulbs can be transported for considerable distances without deteriorating. World-wide production of onions has increased during the past five years (Table 1), and global onion trade is estimated at about 3 million Mt annually, worth about US$700 million during this period.

Table 1: Global production and trade in dry onions

Source: FAOSTAT, 2000

1.3 Primary product

Onions (Allium cepa L.) are grown mainly as food materials. They are highly valued for their flavour and for their nutritional value in supplying minor constituents such as minerals and trace elements (Table 2). The bulbs are boiled and used in soups and stews, fried or eaten raw. They are also preserved in the form of pickles. Onion leaves, especially the Spring onion, are also used in salads and soup. There is a dearth of information on secondary and derived onion products.

Table 2: Contents of major nutrients and vitamins in onion bulbs (Allium cepa var. cepa) and leaves (Allium fistulosum) per 100g edible portion.

Source: (Platt, 1962; FAO, 1972

1.4 Requirements for export and quality assurance

The requirements for international quality standards for food products is presented in Table 3, and this must be interpreted and applied for each type of produce. For onions, a comprehensive interpretation has been documented (Brice et al., 1997). This interpretation applies to onions grown from cultivars of Allium cepa L. to be supplied fresh to the consumer with the exception of (a) onions with their leaves and stems still green (fresh onions), (b) onions intended for planting (basic material), and (c) onions intended for processing. Only the minimum requirements (these apply to both classes) are presented here (Table 4). The onions have to be packed in new and clean packing material, that they are saved appropriately and not subjected to external and internal changes. Both sorting and grading must be supervised by well-trained personnel in order to achieve and maintain good quality. The packages have to be designated concerning packer, kind and origin of product and trading marks.

Table 3: Summary of the Requirements for the International OECD Quality Standard

Source: (Brice et al., 1997).

Table 4: Interpretation of the OECD International Quality Standard for Minimum Requirements for Onions (Allium cepa L.)

When onions are stored and/or handled in a dry atmosphere, the outer dry skin may split and disappear. Thus small cracks in the outer dry skin, due to low air humidity, are allowed. As long as the flesh is not visible small cracks in the outer skin are not considered defects.

Also, absence of a part of the outer skin is allowed provided there are more dry skins and the flesh is not revealed.

b) Sound

The onions must not be rotten or seriously affected by disease or parasites.

c) Clean

The bulbs must be free from any impurity, which may materially alter the appearance or quality.

d) Free from any damage due to frost

Onions affected by frost rot rapidly after handling in a frozen condition and when the temperature rises above 0°C. Therefore they must not be allowed to travel.

The onion is regarded as frost-bitten when more than two of the outer layers of the flesh are affected and the flesh has a waxy appearance. A waxy appearance of the outer layers of the flesh can also be caused by mechanical handling. This slight defect does not affect the edibility of the onions. This waxy appearance of the outer layers disappears in a few days after ventilation.

e) Sufficiently dry for the intended use

At the beginning of the season, onions are frequently pulled before fully developed. Although they cannot be stored, they must be covered by a dry skin, which may be sufficiently dry for no moisture to be pressed out by the fingers, in order that any subsequent heating may be avoided. The onions must be pulled when ripe enough to meet current market standards, i.e. such that they remain sufficiently firm and do not become soft or spongy.

For onions pulled when fully developed, the expression 'sufficiently dry' means that in wet weather the bulbs may be slightly wet in view of the hygroscopic properties of their outer skin, but the leaves near the neck must be fairly dry.

f) Free from abnormal external moisture

At the time of shipment onions must show no signs of excessive moisture due to unnatural causes (such as prolonged exposure to rain) likely to impair their capacity for travel or keeping qualities. The condensation observed immediately after cold storage is not regarded as 'abnormal'.

g) Free of foreign smell or taste

This refers especially to produce that has been in poorly kept or unsuitable cold storage facilities and which may have absorbed the odour given off by other produce on the premises.

Source: (Brice et al., 1997)

1.5 Consumer Preferences

Certain varieties of onions are preferred for certain dishes. For instance, red onions are most commonly used in salads while the white and cream varieties are seldom used in salads. The Spring onions are also used mainly in salads and soup. Consumer preference is dictated by the level of onion pungency and type of food. Cultivars  that have poor storage quality are generally less pungent

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