Espesor corneal

Non-biological agents, commonly environmental factors in the field or in storage may produce many disorders with the loss of the raw materials qUality.

2.3.2. 1. Souring

Souring of harvested roots is caused by water-saturated soils for a prolonged period prior to harvest. Under low oxygen levels, the roots become asphyxiated, and ethanol and C02 is accumulated in the roots, causing high rates of decay during curing and

Chapter 2: Literature review

surviving roots undergo a significant shrinkage. Water-saturated soils also reduce carotenoid pigments, dry matter content and baking quality (Clark and Moyer, 1 988).

2.3.2.2. Storage root cracking

Storage roots can crack or split in the field during the harvest or post harvest processing. The incidence of growth cracking is highly variable. It appears to result when the

subcortical tissue expands more rapidly than the periderm, and may be a result of fluctuations in soil moisture, boron deficiency or excessive nitrogen. Sometimes cracking appears during digging. This occurs when the roots are dug from the warm, moist soil and exposed to too much cooled air.

Other cracking may appear during storage. Low temperatures increase these disorders but the cause of other splits, such as those close to the end of the root is unknown (Cl ark and Moyer, 1988).

2.3.2.3. Skinning

Skinning or surface abrasion can appear like a blemish on the periderm (Clark and Moyer, 1 988). If the damage is not deep it is not important for processing (raw material­ quality), because in most cases the roots have to be peeled when the process begins.

2.3.2.4. Sunscald

When the kumara roots are left exposed to the bright sunlight after they are dug, they are commonly damaged by sunscald. The colours of the skin change, turning to a purplish brown (Clark and Moyer, 1 988).

2.3.2.5. Chilling and freezing injury

Kumara roots are sensitive to temperatures below 1 3°C during storage. Symptoms of chilling injury include an increase in the frequency of roots with decay, flavour changes, internal loss of colours and tissues that remain hard after cooking (or hardcore).

Hardcore is an easily recognised disorder attributable to the cold exposure. Low temperature modifies pectic substances in the middle lamella (cell wall), so that the tissue remains rigid during cooking (Woolfe, 1992b;Clark and Moyer, 1 988).

2.3.2.6. Internal Breakdown (Pithiness)

Sound storage roots that have been stored for long times, feel spongy when squeezed and may become significantly lighter in weight. When one of these is cut open, the flesh

Chapter 2: Literature review

has areas of white, spongy and dry tissue, interspersed within the normal tissues. The internal breakdown or pithiness is associated with the storage of roots in a warm, low­ humidity environment for long periods (Clark and Moyer, 1988; Woolfe, 1992b).

2.4. PHYSICAL AND CHEMICAL PROPERTIES OF COOKED SWEET

POTATO

Firmness is one of the important attributes determining the qUality and marketability of canned sweet potato roots, and is dependent on the cultivar, geographical location of growth, cultivation practices, curing, storage and processing techniques. The softness increased with the chronological age of roots (even after a day delay between harvest and processing), and storage temperatures increases of 15 to 30°C (Woolfe, 1992b).

Truong et al. (1997) grouped sensory texture of cooked sweet potato into three main

factors, namely moistness-fmnness, particles and fibre. These attributes, including the mouthfeel (moistness, ease of swallow, chalkiness) and mechanical-type descriptors (cohesiveness, hardness, denseness, chewiness, adhesiveness), were highly correlated with shear stress of uniaxial compression, fracturability, hardness and gumminess of Texture ProfIle Analysis (TPA).

The colour is the other important quality attribute; to prevent the discoloration is an important objective in any process after harvest. Citric acid has been used to improve the colour of the canned product, and also combination of citric acid and calcium chloride before canning improves colour (Woolfe, 1992b).

2.4.1. Colour Changes and Browning reaction

There are many reasons for changes in colour in a fresh or in processed horticultural product. Browning reactions have been categorised as non-enzymatic or enzymatic browning.

2.4.1.1. Non enzymatic browning

In this group there are reactions such as Maillard and Strecker browning. Both impair proteinaceous nutritional value. In Maillard browning, a chemical reaction occurs

between reducing sugars (mainly D-glucose) and a free amino acid or amino groups of a protein chain (lysine). The Strecker reaction is a degradation of the aminoacids to

aldehydes, ammonia and carbon dioxide (BeMiller and Whistler, 1996; Damodaran, 1996).

Caramelisation is another non-enzymatic reaction. This occurs when carbohydrates (especially sucrose and reducing sugars) are heated without nitrogen-containing

Chapter 2: Literature review

compounds present. In general, thermolisis causes dehydration of the sugar molecule with introduction of double bonds and formation of anhydro rings. These double bonds absorb light and produce the colour. Caramelisation is facilitated by small amounts of acids and by some salts (BeMiller and Whistler, 1 996).

2.4. 1.2. Enzymatic browning

Enzymes are a group of proteins that catalyse many reactions using different substrates. There are three important enzymes responsible for the chemicals alterations of pigments in horticultural products; lipoxygenase, chlophyllase and polyphenol oxidase (Whitaker,

1 996).

Lipoxidase has six important effects; bleaching of wheat and soybean flour, formation of disulphite bonds in gluten during dough formation, destruction of chlorophyll and carotenes, development of oxidative off flavours and aromas, oxidative damage to compounds like proteins and vitamins and oxidation of essential fatty acids (linoleic, linolenic and arachidonic) (Whitaker, 1 996).

Chlorophyllase enzymes hydrolyse chlorophyll during storage of raw plant foods (Whitaker, 1 996).

Polyphenol oxidase catalyses different reactions with a large number of phenols. One of these is the catechol forming 0-Benzoquinone. This compound is unstable and

undergoes further non-enzyme-catalysed oxidation by O2 and polymerisation to give melanin. This is responsible for the undesirable brown discoloration in many fruits, and vegetables and deterioration of colour in juice (Whitaker, 1 996).

The propensity of any given tissue to browning varies from one cultivar of a crop to another, due to variations in enzyme content, and the kinds and amounts of phenolic substrate present. Different methods are used to minimise this reaction such as,

exclusion of oxygen, application of acidulants, heat inactivation of the enzyme and the use of inhibitors (sulphites) (Haard and Chism, 1 996).

2.4. 1.3. Post cooking darkening reaction

2.4.1.3.1. Definition

Post cooking or after cooking darkening reaction is a change in colour that in potato is expressed as a steel blue-grey colour after cooking. It is caused by the formation of an iron-chlorogenic acid complex (Mann and De Lambert, 1 989).

2.4. 1.3.2. Factors involved

Chlorogenic acid and isochlorogenic acid are members of a group of phenolic acids that are involved in colour formation. The majority of the phenolic compounds are formed 2-14

Chapter 2: Literature review

from quinic acid and caffeic acid (Figure 2-2). In sweet potato isochlorogenic acid is the

predominant acid (Walter Jr. and Purcell, 1 980; Mann and De Lambert, 1 989; Ma et aI. ,

1 992; Woolfe, 1 992b).

Smith (1 977) thoroughly reviewed after cooking darkening in potatoes. Some

researchers (eg. Muneta 1959), found correlations between post cooking darkening and aqueous iron levels in potatoes. Other researchers did not fmd such relationships, suggesting that other factors such as citric and phosphoric acid levels complicate the extent of colour formation. Other researchers have investigated the differences in iron and darkening with position in potato tubers (eg. Wurster and Smith 1 965). Generally after cooking darkening occurs the most in potato at the stem end where the iron levels are greatest. Similarly chlorogenic acid levels in potatoes are higher at the stem end, falling to lower levels towards apical end (Hughes 1958). Although there have been some correlations reported between blackening and chlorgenic acid, the best correlation was reported with the chlorogenic acid to citric acid ratio (Hughes and Swain 1 962).

There are many other factors involved in the darkening process. One of these is the temperature. The discoloration occurs when the kumara roots are subject to elevated temperatures not high enough to denature enzymes but sufficient to disrupt cellular organisation and thus cause polyphenol oxidase to react with other compounds (Walter Jr. and Purcell, 1980; Mann and De Lambert, 1989).

Ma et al., (1 992), working with frozen sweet potato, found that the prevention of enzymatic darkening by blanching is the result of the reduction of the polyphenolic oxidase activity but not a reduction in phenol level.

OH Quinic acid OH Chlorogenic acid Caffeic acid OH Isochlorogenic acid

Figure 2 - 2 Phenol i c compounds f ound in sweet potato

(Wool f e , 1 9 9 2 b )

Chapler 2: Literature review

Methods to prevent darkening in potato include the use of chelating agents such as EDTA and sodium acid pyrophosphate (SAPP). It is thought that the mechanism for their effectiveness is the sequestering of iron in the tubers, thereby making it

unavailable to reactor with the chlorogenic acid (Smith 1977).

Other approaches to avoid darkening are, producing cultivars with low discoloration potential, and some changes in the processing such as; additive use, pre-peeling heating, longer lye-peeling time (Walter Jr. and Purcell, 1 980).

Muneta and Kaisaki ( 1 985) also reported the formation of a complex of ascorbic acid plus ferrous iron (Fe2+) which is a purple pigment similar to the after cooking darkening colour in potatoes, although the ascorbate-Fe2+ complex is much less stable than the

chlorogenic acid-Fe3+ complex discussed above. In both cases the colour formation

could be avoided by removal of iron from the system using sequestering agents such as SAPP, or elimination of oxygen.