Tomato Growth, Ripening and Postharvest Physiology

2.1 DEVELOPMENT

Fruit anatomy moves our opinion of fruit quality. The generic term “fruit” covers a expansive range of structures, supporting and protecting the seeds, but where the diverse parts have evolved from the original fertilized flower in assorted characteristic ways. The development process of fruits can be divided into the following stages:

1. Cell Division
2. Cell Expansion
3. Maturation
4. Ripening
5. Senescence

During the early stages of development, tomato is enlarging rapidly. It is small, hard, green and stores organic acids. Tomato develops in an inside-out manner. The seeds are completely formed by the end of maturation stage. Through the process of ripening, tomato becomes soft textured, stores sugars, aroma and volatiles. Fruition of tomato initiates starch hydrolysis and is marked by increased ethylene levels as well as respiratory activity. The development of the red colour occurs due to accumulation of the Lycopene pigment. Finally the cell structure will wane, tomato will become over-ripe and becomes predisposed to pathogens.

The process of growth is largely decreed by seed development. Preharvest events affect subsequent postharvest behaviour. The genetic background essentially determines the crop response to growth and storage environments. In tomatoes the mesocarp tissues consists of large parenchyma cells and contains the main vascular network. An ovary must be encouraged for fruit development to happen. This is achieved by pollination and fertilization, which involve Gibberellins and auxins for pollination stimulus and consequent hormone generation by the fertilized ovary. These hormones are highly crucial to activating fruit development as they influence cell division (de Jong M et al, 2009).

Gibberellins get together with auxins to begin cell division. Seed cytokinins and cell division are also related as tomato seeds amass cytokinins that consequently influence cell division in surrounding pericarp tissue (Gillaspy G, et al, 1993).

2.2 CELL DIVISION AND DEVELOPMENT

Cell enlargement is not a consistent process. Cells in diverse parts of the fruit frequently expand at variable rates and in varied axes. This makes the mature fruit indicate robust gradients in cell size from surface to the heart of the fruit. Notwithstanding the complications of fruit development, certain uniformities in forms of cell division and development can be observed. These can be further followed to tissue differentiation and fruit growth. Initially in four weeks when growth is as a result of only cell division, the embryo volume remains small but the flesh volume rises rapidly. In the early stages of development, the fertilized embryo and endosperm mature and seeds start to form. The next stage begins when the pericarp restarts development and lasts till enlargement. Then it decelerates for an additional time as the fruit develops. This second phase of fruit growth is fulfilled by cell expansion. The expansion takes place in longitudinal, radial and tangential axes. Cessation of cell division is observed post anthesis. This causes a variation in the growth curve and indicates the end of the primary sigmoid phase (Figure 2.1).

2.2.1 Changes during Fruit Ripening:

(a) Changes in carbohydrate composition, resulting in sugar accumulation and increased sweetness;

(b) Change in colour;

(c) Flesh softening and textural change;

(d) Formation of aroma volatiles;

(e) Accumulation of organic acids with associated development of flavour.

2.3 FRUIT GROWTH REQUIREMENT

Growth of the fruit causes variation in various parameters such as cell wall distribution, organic acid, lipids, carbohydrates and so forth. The most essential among these in realistic terms is the carbohydrate economy. This economy addresses two agendas:

1. Growth rate, achieving maturity and fruit size.
2. Aroma, Flavour and Texture of final fruit.

The initial utilization of carbohydrate is required for cell division, expansion and tissue differentiation. The remaining carbohydrate after this stage is held as starch or sugars. Different varieties of tomatoes store different types of sugars such as hexoses or sucrose. Other nutrients flow in from varied parts of the plant such as leaves, vascular network and so forth to the rapidly enlarging fruit, usually during early summer.

The harvesting of tomato during the hotter months results in thick-skinned tomatoes and other seasons reap thin-skinned fruit. Variation in the skin of the fruit can be attributed to holding of water within the fruit in the hot weather, as tomato is largely composed of fluid component.

2.4 FIRMNESS AND COLOUR OF TOMATOES

The two most important characteristics highly valued by buyers and consumers are texture and skin colour (Tijskens et al, 1994). The susceptibility to damage increases due to ripening and softening during storage. Texture is affected by the skin strength and firmness of the flesh of the fruit (Kader et al, 1978). The extent of firmness is indicative of fruit quality (Burton, 1982) and firmness may be the final guide for consumers to purchase the produce (Gromley et al, 1978). Variability of the firmness is greatly associated with visual surface properties of tomatoes (Yang et al, 1988) which is linked to colour, shape, and tightness of skin during buying. The colour of the tomato is another major factor determining consumer choice. Determination of colour can be done instrumentally which is the most accurate (Bakker et al, 1986). This is however not the most convenient option and thus the colour chart published by the United States Department of Agriculture (USDA) may be used. Other most commonly used instruments are colour difference meter and a model of Minolta Chroma Meter (Anonymous, 1993).

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According to a study (Batu, 2004), all fruits available in the market should have firmness values above 1.45 N per mm but the Instron values of the tomato for home usage should have readings greater than 1.28 N per mm. Distinguishing values between the turning and pink stages were observed to be greater than 0.08 Minolta value. In the light red stage this value was between 0.60 and 0.95. The red stage is a slight overripe and its Minolta values were observed to be from 0.95 to 1.21.

2.5 TOMATO AGING, RIPENING AND METABOLIC CHANGES

Tomato, a climacteric fruit undergoes various changes in firmness, sweetness, acidity and changes in pigmentation during ripening. Through the process of ripening, numerous changes take place in the activities of diverse enzymes. Tomato PG, an important enzyme commonly used to study the process is observed to be synthesized de novo and accumulates as the process continues (Rhodes, 1980; Jeffery D et al, 1984). The role of ethylene in regulating fruit ripening in climacteric species is well documented (Klee et al, 2011). Analysis of proteins present in tomato at various stages revealed increases, decreases and fluctuations in the levels of many polypeptides, thus suggesting that both synthesis and degradation of proteins are involved in the ripening process (Biggs et al, 1986). An essential component of the ripening process is the stimulus to the protein synthesis. This was judged by adding radioactive amino acids as well as by incorporating protein synthesis inhibitors, which ceased the ripening (Grierson D, 1983).

2.6 CLIMATIC EFFECT ON QUALITY OF TOMATOES

2.6.1 Temperature – Complete maturity of the tomato is achieved in 6-8 weeks post pollination. Duration of maturity depends on variety and weather profile. It takes 70-75 F for optimum ripening of tomato. The ripening process may decelerate or halt, when temperatures exceed 85-90 F. This effect is observed because at temperature of 85-90 F, lycopene and carotene liable for orange to red appearance cannot be produced. Outcome being, fruit locked in mature green stage for relatively some time. The fruit when exposed to direct sunlight may also cause the temperature to rise enough to inhibit pigment formation. It may also result in a sunscald fruit. Further lower temperatures destroy the ripening enzymes. However, greater levels of Magnesium and lesser levels of Potassium can develop discoloured/uneven ripening, otherwise yellow shoulder disorder. Thus picking the fruit post the first signs of ripening and exposing them to temperatures of 70-75 F, will result in optimal development of flavour and colour.

2.6.2 Humidity – The quality of the produce is highly dependent on the relative humidity. Higher relative humidity around the plant increases Calcium (Ca) in tomatoes, and reduces shelf life due to Ca toxicity (Adams P et al, 1992). Post complete ripening of the fruit, they can be stored under conditions providing 90-95% relative humidity. This can be allied to the otherwise occurring transpirational loss, as the fruit has 90-95% moisture.

The decreased quality and quantity of the food is due to breakdown of carbohydrates, proteins and other nutrients into simpler compounds through processes of respiration, enzymatic breakdown and microbial degradation. Mostly in case of biological processes, the greater the temperature the speedier these natural degradation progressions will take place, giving rise to loss of colour, flavour, nutrients and texture changes. All of these processes are highly dependent upon temperature, which in case of India being a tropical country assume greater importance.

2.7 MATURITY STANDARDS

Optimum maturity is of principal value to ensure best quality and longest shelf life of the produce. Reid (Reid, 2002) defined maturity as “the stage of development at which a plant or plant part possesses the prerequisites for use by consumers for a particular purpose.”

2.7.1 Types of Maturity

(a) Physiological Maturity – It refers to that stage of development of fruit once highest growth and maturation have happened, to be trailed on by ripening and senescence (Wills et al, 2004).

(b)Commercial Maturity – Commercial/Horticulture maturity is that stage of development when the produce achieves the pre-requisites for selling (Reid, 2002; Wills et al, 2004). It can occur during any stage of maturation, development, ripening or senescence.

2.7.2 Effect of Maturity

The stage of maturity at which the produce is harvested plays an important role in determining the quality of the produce. The properly matured produce have better rate of ripening when compared to partially mature and immature produce. The respiration rate varies with type, stage of maturity and stage of ripening. However, the rate of respiration peaks at ripe stage compared to immature and senescent stages (Adaskaveg et al, 2002).

• Tomatoes should be harvested at mature-green stage, i.e. Green colour development at the blossom end and seeds are enclosed in jelly-like substances (Tiwari et al, 2002).
• Tomatoes can also be harvested at turning pink stage, i.e. One quarter of the surface of the tomatoes at the blossom end shown pink (Wills et al, 2004).

The United Fresh Fruit and Vegetable Association in cooperation with US Department of Agriculture and Agricultural Marketing Service Fruit and Vegetable Division, has established standards for grading of fresh tomatoes (USDA Colour Chart) . This grading system has categorized fresh tomatoes in 6 categories namely:

1. “Green” – Indicate that the surface of tomato is fully green in colour. There can be variation of light to dark in the green shade.

2. “Breakers” – Indicate that there is a definite break in colour from green to tannish-yellow, pink or red. This observance is not on more than 10 percent of the surface of tomato.

3. “Turning” – Indicates that between 10 – 30 percent, in aggregate, shows a definite change in colour from green to tannish-yellow, pink, red, or a combination thereof.

6. “Pink” – Indicates that between 30 – 60 percent, in aggregate, shows pink or red colour.
7. “Light Red” – Indicates that between 60 – 90 percent, in aggregate, shows pinkish-red or red of the surface of the tomato.
8. “Red” – Indicates that more than 90 percent of the surface of tomato, in aggregate, shows red colour.

2.8 NUTRITIVE PARAMETERS OF TOMATO

The intake of a balanced diet enriched with fresh fruits and vegetables has always been associated with health benefits and reduction of diseases (Klerk et al, 1998; WHO, 2003). This effect is mainly due to action of antioxidant compounds, which reduce oxidative damage like carotenoids, ascorbic acid, tocopherols and polyphenols (Grassmann et al, 2002; Prior et al, 2000; Wargovich, 2000).

2.8.1-carotene

Bcarotene, the precursor of vitamin A, is found abundantly in fruits and vegetables. Vitamin A is an essential part of the balance diet, and keeps eyes healthy and enhances body’s immunity to fight infections (Nandi et al, 2005).

Vitamin A deficiency is one of the principal causes of blindness among children and women in the developing nations.

2.8.2 Lycopene

Tomato is an admirable source of lycopene (Saxelby, 2002). The molecule of antioxidant is capable of decreasing or averting the oxidation of other molecules. The action of antioxidants includes, termination of chain reactions by eliminating free radical intermediates. Enhanced intake of lycopene plays an important role in decreasing the risk of prostate cancer (Lister, 2003). The carotenoids family consists of lycopene, which is fat-soluble. The insolubility of lycopene in water and its tight binding to the vegetable fibre, keeps it stable during processing. Therefore after processing, the lycopene content and its bioavailability increases.

Table 2.1: Lycopene Content in Fresh and Processed Tomatoes.

Source: (Kirstie Canene A, et al, 2005 )

Processing of tomato increases the bioavailability of lycopene. The most common processed products from tomatoes are tomato juice, dried tomatoes, tomato pickles, tomato pulp, tomato sauce and tomato ketchup (Hassan, 2006).

Reports generated from fresh cut tomato are inadequate (Lana et al, 2006) in comparison with massive literature on effects of heat processing on antioxidant activity and content in tomato (Dewanto et al, 2002; Lavelli et al, 2003; Sánchez-Moreno et al, 2006; Takeoka et al, 2001; Sahlin et al, 2004; Shi J et al, 2000).

Major quality characteristics depend on end utility of tomato (Fresh or Processed). Flavour, appearance and firmness determine the quality of the fresh fruit, however total soluble solids content, colour, pH which govern the above stated factors are useful for determining processing quality.

Ethylene biosynthesis, cell wall modifications and other changes also govern texture, firmness. These are important factors to be taken into account for organoleptic quality (Davies et al, 1981; Stevens, 1986). The plant hormone ethylene is essential for normal fruit ripening in tomato as well as other climacteric fruits. It is considered as a switch for various physical, physiological and biochemical alterations, which make tomato, appropriate for consumption.

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The fruit quality dynamically changes during the conversion of green to fully ripe stage. Apart from pH, firmness, texture and so forth various bioactive compounds also undergo changes (Serrano et al, 2008). Tomatoes are made up of 93-95% water and the rest 5-7% comprise of inorganic compounds, organic acids (citric and malic), sugars (glucose, fructose and sucrose), solids insoluble in alcohol (proteins, cellulose, pectin, polysaccharides), carotenoids and lipids which constitute the solids (Dorais et al, 2001; da Silva et al, 2008). Water loss being a leading cause of deterioration results in quantitative loss as well as loss in appearance, texture and nutritional value (Kader, 1985). The loss of water causes wilting and aging causes texture variations (Ranganna, 1997).

The harvesting of mature green tomatoes can be accounted for ripening at ambient temperature, which gives a higher total solid content than harvesting at breaker stage (Abdul-Hammed et al, 2012). Tomatoes possessing greater acid content and greater sugar have the better flavour compared to those having lower sugar and acid content (Cantwell, 1994). These correct stage harvests, which ripe in ambient temperature bear rough handling and hold the longest in storage, transport and retailer shelf (Davies et al, 1981). The main acids present in tomato are citric acid and malic acid with former being prevalent over the other (Sánchez-Moreno et al, 2006). Change in acidity is directly linked to the change in titratable acid and thus modify the flavour. The greater sugar and lesser acid the taste is more appealing (Malundo et al, 1995; Helyes et al, 2006). Generally pulps of tomato having pH less than 4.5 are more suitable, as the microbial activity deteriorating to the product is restricted (Tigchelaar, 1986). The delectableness of fruit products is significantly improved by acid content and also help maintain acid-base balance in the body (Guold et al, 1972).

The colour of tomato is due to the presence of Chlorophylls (green pigments), Carotenoids (lycopene and beta-carotene) and Xanthophylls (yellow pigment) (Ho et al, 1986). Natraceutic properties of Carotenoids are also important to human nutrition. Lycopene gives tomato the attractive red colour and represents 75-83% of total carotenoids. This carotenoid is mainly provided by tomato but the concentrations vary greatly with plant variety, degree of ripeness, time of harvest and storage conditions (Lessin et al, 1997). Lycopene is present in blood plasma and acts as a natural defence pathway as well as an antioxidant and anti-mutagenic agent.

The study by Australian Journal of Basic and Applied Sciences, concluded that tomatoes allowed to “self-ripe” on the parent plant have better quality in terms of sweetness and antioxidant nature when compared to those ripened at ambient temperatures. Also, the study stated that mature green stage harvesting is more beneficial than harvesting at breaker stage.

However in Indian commercial practice, the tomatoes are often harvested at the mature green stage to convene on post harvest durability and the capacity to endure handling. These are room ripened later. However, field-ripened fruit were shown consistently to have superior flavour quality (Prakash, 2000).