Recommendations for Maintaining Post harvest Quality of   Grapes

دلائل إكتمال النمو Maturity Indices

يتم تحديد موعد الحصاد المناسب لمحصول العنب في كاليفورنيا علي أساس تركيز المواد الصلبة الذائبة SSC  من  14 – 17% وذلك علي حسب الصنف ومنطقة الإنتاج. وفي بعض الحالات تستخدم النسبة بين تركيز المواد الصلبة الذائبة والحموضة  (SSC/TA) في حدود  20 أو أكثر لتحديد الصلاحية للقطف في الأصناف المبكرة في مناطق الإنتاج المبكرة أما بالنسبة للأصناف الحمراء أو السوداء اللون فإن هناك حد أدني للتلوين.

دلائل الجودة Quality indices

يتحقق قبول عالي جدا من المستهلك بتوافر قيم مرتفعة لل SSC  أو  SSC/TA كنسبة وكذلك فإن صلابة الحبات بالإضافة إلي خلوها من الإصابات المرضية (التدهور) وتشقق الحبات وتلوين العنقود باللون البني والذبول والحبات الجافة أو التي بها لسعة شمس أو الإصابات الحشرية.

درجة الحرارة المثلي Optimum Temperature

يوصي بتخزين العنب علي درجة حرارة ما بين )ا –  صفر( ْم حيث أن أعلي نقطة تجمد للحبات هي -3 ْم وتختلف نقطة التجمد بإختلاف تركيز المواد الصلبة الذائبة SSC إلا أن تجمد أنسجة العنقود يمكن أن تحدث عند -2 ْم.

الرطوبة النسبية Optimum Relation Humidity

يوصي بإستخدام رطوبة نسبية 90 – 95% وسرعة هواء ما بين 6 – 12 متر (20-40) قدم في الدقيقة أثناء فترة التخزين.

معدلات التنفس Rates of Respiration

(في عناقيد العنب: أي الحبات مع العنقود وتفريعاته)

درجة الحرارة                معدل التنفس

صفر                             مل  Co2/ كجم/ ساعة

5                                        1 - 2

10                                      3 – 4

20                                    12 – 15

إن العفن الرمادي هو أخطر أمراض ما بعد الحصاد في العنب حيث يمكنه النمو علي درجات حرارة 0.5 ْم للإنتشار من حبة إلي أخري. إن العفن الرمادي يجعل الحبات بنية اللون في أول الأمر ثم يؤدي إلي إنفصال جلد الحبة عن اللحم ثم تظهر هيفات الفطر البيضاء الخيطية خارج جلد الحبة ثم تتكون كتل من الجراثيم رمادية اللون كما أن الجروح التي تحدث قرب موعد الحصاد تؤدي إلي زيادة فرص العدوي ولو أنه ليس ضروريا حدوث الجروح عندما يسود جو به رطوبة عالية. ويمكن تقليل الإصابة بالعفن الرمادي Botrytis sp.  عن طريق إزالة الحبات الجافة المصابة بهذا المرض من المواسم السابقة وكذلك الحبات المصابة من الموسم الحالي من الكرمات مع إزالة بعض الأوراق من النمو الخضري وإستخدام مبيدات ما قبل الحصاد وإزالة الحبات الواضح عليها الإصابة والحبات المتشققة أو التي بها أي إصابات ميكانيكية وذلك قبل التعبئة ثم إجراء التبريد الأولي (السريع) بطريقة سليمة وكذلك التبخير بغاز ثاني اكسيد الكبريت بمعدل 100 جزء في المليون لمدة ساعة واحدة أو بإستخدام بطانات إنتاج ثاني أكسيد الكبريت المعروفة باسم SO2Pads.

1. Introduction

1.1 The importance of post-harvest losses

Time and money are required to cultivate food products, and unless the farmer is providing food only for his own household, he automatically becomes part of the market economy: he must sell his produce, he must recover his costs, and he must make a profit.

Estimates of the post-harvest losses of food grains in the developing world from mishandling, spoilage and pest infestation are put at 25 percent; this means that one-quarter of what is produced never reaches the consumer for whom it was grown, and the effort and money required to produce it are lost-forever. Fruit, vegetables and root crops are much less hardy and are mostly quickly perishable, and if care is not taken in their harvesting, handling and transport, they will soon decay and become unfit for human consumption. Estimates of production losses in developing countries are hard to judge, but some authorities put losses of sweet potatoes, plantain, tomatoes, bananas and citrus fruit sometimes as high as SO percent, or half of what is grown. Reduction in this wastage, particularly if it can economically be avoided, would be of great significance to growers and consumers alike.  

1.2 Causes of losses vary widely

Factors affecting post-harvest food losses of perishables vary widely from place to place and become more and more complex as systems of marketing become more complex. A farmer who is growing fruit for his family's consumption probably doesn't mind if his produce has a few blemishes and bruises. If he is producing for a market at any distance from his own locality, however, he and his workers, if he has any, must have a different attitude if he hopes to get the best money return on his work.

By knowing his market, the grower can and must judge how important the requirements of appearance, maturity and flavour for his produce are. Furthermore, the grower must decide whether the investment in packaging will in fact pay for itself in increased value of the crop. It will be of no value to buy expensive containers for his produce if the field hands pitch them around and damage the contents. It is more important for the grower to change the attitude of himself and his workers toward reducing post-harvest losses than it is for him to think that buying fancy packaging will automatically solve his problems and improve his income. The farmer must give careful attention to:

• Market demand for the products he will grow; he must know the market and his buyers
• Cultivation
• Harvesting and field handling
• Packing or packaging
• Transport
• Market handling; possibly storage or refrigeration
• Sales to consumers, wholesalers or agents
• Perishability of the produce.

The following sections will discuss these among other factors. The grower must recognize that small changes in attitudes toward the prevention of post-harvest food losses may profit him more than changes in the techniques of the marketing chain, whether containers or transport improvements, and may cost him less in the long run. He must instruct his family, field workers, and others in the methods of reducing his losses.

2. Nutrition and fresh produce

2.1 The contribution of fresh produce to human nutrition

Most people eat a mixed diet of foods from plants and animals. In most societies, starchy staple foods, particularly cereal grains, are the main source of energy in the human diet. In certain areas, especially in the humid tropics, root and tuber crops, together with plantains and similar plants, are either the staple food or a supplement to cereal staples.

Fruit and vegetables are important sources of essential minerals and vitamins in the human diet. When eaten together with some root (potato, sweet potato) and leguminous (pigeon peas, beans, lentils) crops, they provide a proportion of protein requirements as well as variety in flavor and color.

2.2 Energy requirements

• Starches and sugars, formed within the plant for its own use, are used as energy foods. Starch is the main component of root and tuber crops and also of plantains and green bananas.
• Oils and fats are also energy foods. Fresh produce contains only small amounts except for avocados, which contain 15-25 percent oil.

2.3 Food for body growth and repair

• Proteins are essential to the building and repair of muscles and organs. They are needed in large amounts by growing children. Fresh produce is low in protein content, although on a dry-weight basis some root crops such as sweet potato and potato as well as leaves of several crops have protein contents approaching that of animal products. Cassava has very low protein content.
• Minerals are required for health but only in small amounts as compared with energy foods and proteins. Sodium, potassium, iron, calcium, phosphorus and many trace elements are essential. Vegetables contain significant amounts of calcium, iron and some other minerals.
• Vitamins are essential for the control of chemical reactions in the body. Fruit and vegetables, and to a lesser extent root crops, are important sources of vitamin C and other essentials. Table 1 lists the important vitamins derived from fresh produce.
• Fiber or "roughage" is found in large amounts in fresh produce. Though indigestible, it plays an important part in the function of digestion, and a diet containing high fiber content is shown by medical studies to reduce susceptibility to disease.

2.4 Loss of food value in fresh produce

The keeping and the preparation of fresh produce after harvest affects its nutritional value in several ways, for example:

• Dry-matter content (the energy supply) is reduced with time as the continuation of living processes within the produce uses up stored food reserves.
• Vitamin C content decreases with time after harvest, and little may remain after two or three days.
• Cooking partially destroys vitamins C and B1. Raw fruit and vegetables are particularly valuable provided they are grown and handled hygienically.
• Peeling may cause significant loss of food value, especially in potatoes, where the protein content is just beneath the skin.
• Water used in cooking vegetables or fruit contains the dissolved minerals and trace elements of the food and should not be thrown out but used in soups or in preparing other foods.

Further information on the food value of fresh produce can usually be obtained at national nutritional councils or departments of health.

TABLE 1. Vitamins supplied by fruit, vegetables and root crops

Source J. Srhuur, FAOR, Barbados.

3. Pre-harvest factors in produce marketing

3.1 Pre-harvest influences on post-harvest performance

The overall quality and condition of fresh produce cannot be improved after harvest. The final potential market value of his produce depends on the grower's decisions on what and when to plant and on the subsequent cultivating and harvesting practices. The adoption of good post-harvest practices described in the later sections of this manual can extend the useful post-harvest life of fruits and vegetables but only to the extent that their quality and condition at harvest permit.

Growers in general rely on their own experience and local traditions in selecting crops and in cultivation practices, but if they want or need assistance they may need to be referred to agricultural extension officers or possibly to research and development specialists of their national department of agriculture or its equivalent.

3.2 Market factors for the produce

Market factors affecting farmers' decisions on the growing of specific crops are:

• Potential purchasers for the produce: neighbors, townspeople, retailers, jobbers or middlemen, commission agents?
• quality requirements of the buyer: size, shape, maturity, appearance, perishability of the produce;
• Pricing limitations of the buyer.

A commodity can be "too good" as well as "too bad": one that greatly exceeds market requirements may not bring higher prices and thus be a waste of labour and resources.

An important limitation of most markets is that only certain varieties of a commodity are traded and others are unacceptable. In Indonesia, for example, 242 varieties of mango have been recorded by the Agricultural Seed Experiment Station in East Java, but only seven have any commercial potential beyond certain villages. The non-marketable mangos, however, constitute about 70 percent of the total production, and the local grower can effectively increase his market share only by replacing existing trees with those of the desirable varieties.

In international trade, this specification of variety is of critical importance. Countries wishing to export have little choice but to offer what will be bought by importing countries. This holds true among developing countries. For example, the Association of South East Asian Nations (ASEAN) has consciously promoted trade in fruit and vegetables, many of which are common in the various countries, but there are still distinct preferences for different cultivars between countries.

New varieties are not easily introduced into developing countries and established as profitable crops. Apart from physical conditions and cultivation practices, problems may include the overcoming of traditional human conservatism unless there are compelling economic incentives.

3.3 Influence of production practices

Pre-harvest production practices may seriously affect post-harvest returns in quality and quantity and result in the rejection or downgrading of produce at the time of sale. Some of them are:

3.3.1 Water supply (Irrigation). Growing plants need                   a continuous water supply for both photosynthesis (the process by which plants convert light to chemical energy and produce carbohydrates from carbon dioxide and water) and transpiration (the giving off by a plant of vapour containing waste products). Bad effects can be caused by:

• too much rain or irrigation, which can lead to brittle and easily damaged leafy vegetables and to increased tendency to decay;
• lack of rain or irrigation, which can lead to low juice content and thick skin in citrus fruit;
• dry conditions followed by rain or irrigation, which can give rise to growth cracks or secondary growth in potatoes or to growth cracks in tomatoes (see color section, Figure 3).

3.3.2 Soil fertility, use of fertilizers. Lack of plant foods in the soil can seriously affect the quality of fresh produce at harvest. On the other hand, too much fertilizer can harm the development and post-harvest condition of produce. Some of the effects are:

• lack of nitrogen can lead to stunted growth or to the yellow-red discoloration of leaves in green vegetables, e.g. cabbage;
• lack of potash can bring about poor fruit development and abnormal ripening;
• calcium-moisture imbalance can cause blossom-end rot in tomatoes and bitter pit in apples;
• Boron deficiency can lead to lumpiness in papaya (see color section, Figure 4); hollow stem in cabbage and cauliflower; the cracking of outer skin in beets.

These are a few of the commoner soil-nutrition problems that can be readily identified at harvest. The problem of fertilizer balance in soils and its effect on crops is complex and depends also on other conditions such as temperature, moisture, acidity of the soil and reactions among different fertilizer chemicals. Severe soil-nutrition problems need reference to specialist advice, if available.

3.3.3 Cultivation practices. Good crop husbandry is important in achieving good yields and quality of fresh produce. Certain aspects are particularly important, such as:

• Weed control-weeds are commonly alternate or alternative hosts for crop diseases and pests, and those growing in fallow land near crops are as important as those growing among the crop. Weeds also compete with crops for nutrients and soil moisture;
• Crop hygiene-decaying plant residues, dead wood, and decaying or mummified fruit are all reservoirs of infection causing post-harvest decay. Their collection and removal are crucial factors in the reduction of post-harvest losses.

3.3.4 Agricultural chemicals. These are of two types:

• Pesticides and herbicides are used as sprays or soil applications to control weeds, disease and insect pests. They are dangerous because they can damage produce by producing spray burns if used incorrectly, and they can leave poisonous residues on produce after harvest. In most countries there are laws to control the use of pesticides, which should be used only in recommended concentrations. Strict observance of the recommended delay between the last spraying and the harvesting is required in order to keep poisonous spray residues from reaching the consumer. Advice on regulations should come from extension or other agricultural department officers.
• Growth-regulating chemicals are used in the field mainly to improve the marketability of fruit in order to control the time of fruit set and to promote uniform ripening. They are of little importance to small-scale production. Their effective use requires specialist knowledge, and they are mainly applicable to large-scale commercial production.

3.4 When is fresh produce to harvest?

A critical time for growers of fruit and vegetables is the period of decision on when to harvest a crop. Normally any type of fresh produce is ready for harvest when it has developed to the ideal condition for consumption. This condition is usually referred to as harvest maturity. Confusion may arise because of the word maturity since, in the botanical sense, this refers to the time when the plant has completed its active growth (vegetative growth) and arrived at the stage of flowering and seed production (physiological maturity) as shown in Figure 3.1. Harvest maturity thus refers to the time when the "fruit" is ready to harvest and must take into account the time required to reach market and how it will be managed en route. This time lag usually means that it is harvested earlier than its ideal maturity.

3.4.1 How is harvest maturity identified?

 Most growers decide when to harvest by looking and sampling. Judgments are based on:

• Sight-color, size and shape
• Touch-texture, hardness or softness
• Smell-odour or aroma
• Taste-sweetness, sourness, bitterness
• Resonance-sound when tapped.

Experience is the best guide for this kind of assessment. Newcomers to fresh produce-growing may find that learning takes time. Harvest maturity can readily be observed in some crops: bulb onions when their green tops collapse and potatoes when the green tops die off. Other crops can be more difficult: avocados remain unripe off the tree after maturity.

Large-scale commercial growers combine observation with more sophisticated measurement:

• time-recording, from flowering to harvest;
• environmental conditions, measuring accumulated heat units during the growth period;
• physical properties, including shape, size, specific gravity, weight, skin thickness, hardness, etc.;
• chemical properties (important in fruit processing, less so in vegetables), sugar/acid ratio, soluble solids content, starch and oil content;
• Physiological characteristics, including respiration rate, acidity or alkalinity (pH).

The final decision on harvesting will take account of the current market value of the expected yield, and also the time during which the crop will remain in marketable condition. With seasonal crops, growers are often tempted to harvest too early or too late in order to benefit from higher prices at the beginning and end of the season.