Quality deterioration of fresh produce respiration ethylene senescence and breaking of dormancy

7.3.1 Introduction

Many factors can lead to loss of quality in fresh produce, hence the common description of these products as 'perishable'. Some of these factors are part of the life cycle of living produce, that is, over-ripening of fruits or sprouting in root and bulb crops. Others are a consequence of the act of harvesting. Once severed from the mother plant, the plant organ is deprived of its source of water, nutrients and antisenescent hormones. As a consequence normal factors such as transpiration and respiration lead ultimately to water loss and senescence of the product. The growth of pathogens or physical damage will cause direct loss of product quality through their visual impact but both also stimulate senescence. Furthermore, the storage environment will play a highly significant role in determining the speed of all quality changes.

7.3.2 Respiration

Fruits and vegetables are living commodities and their rate of respiration is of key importance to maintenance of quality. It has been commonly observed that the greater the respiration rate of a product, the shorter the shelf-life. Immature products such as peas and beans tend to have much higher respiration rates and short shelf-lives caused by natural senescence whereas the opposite is true for mature storage organs such as potatoes and onions.

Respiration is the metabolic process by which cells convert energy from one type of chemical structure into another form more useful to the cell for driving metabolic reactions. Under normal circumstances, fresh produce undergoes aerobic respiration, during which oxygen and glucose is consumed while carbon dioxide, water and heat are produced (Kays, 1991). In non-storage tissues, for example in leafy crops such as lettuce or spinach or immature flower crops such as broccoli, there is little by way of energy reserves and hence excessive respiration will eventually lead to metabolic collapse. Cell membranes will break down and allow the contents to leak out. Saprophytic bacteria may grow in these tissues and give rise to off-odours. Visible symptoms of tissue collapse and yellowing caused by senescence breakdown of chlorophyll in the chloroplasts may appear. Without adequate cooling, respiratory heat will further stimulate respiration leading to even more rapid deterioration.

Certain types of fruits (known as climacteric) can be harvested unripe and ripened artificially at a later stage (e.g. avocados, bananas, mangoes, tomatoes).

During ripening, the respiration of these fruits increases dramatically over a short period of time (Biale, 1960). Without carefultemperaturecontrol,thefruitwill rapidly over-ripen and senesce leading to internal tissue breakdown and the production of volatiles characteristic of the over-ripe fruit. Failure to control respiratory heat also can increase water loss from the produce. Furthermore, the increased warmth and moisture levels, whichcandevelopinstorage,arehighly conducive for the development ofbacterialandfungalinfections.

7.3.3 Ethylene

Ethylene is a plant hormone that plays akeyroleintheripeningandsenescence of fruits and vegetables (Reid, 1992). Allplantcellsproduce lowlevelsofethyl-ene; however, anything that causes stress to the plant tissues will stimulate ethylene synthesis. Stressors may includeexcessivewaterloss, physicaldamage or pathogenic attack. Climacteric fruits produce high levels of ethylene during initiation of ripening and the hormone isbelievedtostimulate andcoordinatethe physiological and biochemical changes which occur during ripening. Exposure to exogenous ethylene can lead to an accelerationofmaturationandsenescence,for example, green vegetables lose their chlorophyllmorerapidly, thickenedfibrescan develop in asparagus, premature ripeningcanoccurinunripefruitsandcabbages and cauliflowers can lose their leaves through accelerated leaf abscission.

7.3.4 Senescence

Senescence is the natural ageing of the planttissuesandisstimulatedbythepres-ence of ethylene and anything else that speedsuprespiration ratesasdescribed above. Senescence ultimately affects all aspectsofquality,endinginthedeathof the product. Some senescence changes can specifically affect certain types of fresh produce processing, for example, changes to the chemical and physical structure of the cell wall (Jimenez et al., 1997). Although in fresh produce, texture is highly dependent on cell turgor (see section 7.4 below), the integrity of the cell wall is important to the texture of some processed products (Femenia et al., 1998). In some fruits and vegetables (e.g. apples and tomatoes), the breakdown of intercellular adhesion between cells leads to a condition known as mealiness which is generally perceived as a loss in textural quality (Van der Valk and Donkers, 1994). In potatoes, so-called senescence sweetening is where, over time, storage starch is gradually converted to sugars. Concentrations of reducing sugars of greater than 0.1% in potato tissues being processed into chips and crisps can lead to browning or blackening of the product during the cooking process (Van der Plas, 1987).

7.3.5 Breaking of dormancy

Root, tuber and bulb crops have a natural dormancy period that can be considerably extended under suitable storage conditions. Storage and shelf-life is often limited by the breaking of dormancy. Most commonly this is seen as the growth of sprouts, for example, in onions or potatoes. Under high moisture conditions, the development of roots may also occur. Neither sprouts nor roots are acceptable in marketed produce (Schouten, 1987). Although roots and shoots can be trimmed off during processing, the internal quality of the produce generally deteriorates during the breaking of dormancy owing, for example, to the conversion of stored starch into sugars that are transported to the growing points.

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