6. Effect of processing on nutritional value

Contents - Previous - Next

Root crops are not easily digested in their natural state and should be cooked before they are eaten. Cooking improves their digestibility, promotes palatability and improves their keeping quality as well as making the roots safer to eat. The heat used during cooking can be dry heat as in baking in an oven or over an open fire, or wet heat as when boiling, steaming or frying. Heat helps to sterilize the food by killing harmful bacteria and other microorganisms, and it increases the availability of nutrients. Proteins are denatured by heat. In this form they are more easily digested by proteolytic enzymes; cellulosic cell walls that cannot be broken down by monogastric animals like man are broken down, and some anti-nutritional factors such as enzyme inhibitors are inactivated. However, processing may reduce the nutritional value of some root crops as a result of losses and changes in major nutrients, including proteins, carbohydrates, minerals and vitamins.

Nutrients may be lost during cooking in two ways. First, by degradation, which can occur by destruction or by other chemical changes such as oxidation, and secondly by leaching into the cooking medium. Vitamins are susceptible to both processes while minerals are affected only by leaching. Free amino-acids could also be leached or may react with sugars to form complexes. Starches may be hydrolysed to sugars. The percentage loss will depend partly on the cooking temperature and on whether the food is prepared by boiling, baking or roasting. Baking losses may appear deceptively low if expressed on a fresh weight basis, due to the concentration of nutrients by loss of water. However, less damage is done by baking than by canning or drum drying (Purcell and Walter, 1982).

The first step in processing any root crop is usually peeling. This may remove nutrients if it is not done carefully. Cooking losses can be reduced by retaining the skin to minimize leaching and to protect the nutrients. It is sometimes advisable to peel after boiling, and to make use of the cooking water in order to conserve water-soluble nutrients.

Vitamin C is the most thermolabile vitamin and is also easily leached into cooking water or canning syrup. Elkins (1979) reported complete retention of vitamin C in freshly canned sweet potato but the vitamin content dropped to 60 percent of its original value after storage for 18 months. The concentration of the canning syrup did not affect vitamin retention (Arthur and McLemore, 1957). Air drying of thin slices of sweet potato leads to only slight losses of vitamin C.

Boiling may result in a 20 to 30 percent loss of vitamin C from unpeeled roots and tubers as shown in Table 6.1. If peeled before boiling the loss may be much higher, up to 40 percent. Swaminathan and Gangwar (1961) estimated that 10 to 21 percent of the loss is due to leaching into the cooking water and the rest to destruction by heat. Baking losses of vitamin C in unpeeled potato are about the same as in boiling but roasting results in higher losses, while making into crisps seems to be slightly better in terms of vitamin retention. Frying results in the loss of 50 to 56 percent compared to 20-28 percent on boiling unpeeled (RoyChoudhuri et al., 1963). Streghtoff et al. (1946) reported a 28 percent loss during baking and only a 13 percent loss when boiled after peeling. The difference may be that the higher temperature of baking leads to greater destruction of the vitamin. As much as 95 percent of the vitamin C is retained when yam is cooked with the skin on but this is reduced to 65 percent if it is cooked after peeling; 93 percent is retained on frying and 85 percent on roasting. (Coursey and Aidoo, 1966).

Up to 40 to 60 percent of the vitamin C content of potato can be lost on storage (Sweeny et al., 1969; Augustin et al., 1978; Faulks et al., 1982) depending on the temperature. Storage for 30 weeks at 5° or 10-C resulted in a loss of 72 percent and 78 percent respectively (Yamaguchi et al., 1960); and 49 percent loss at 8.5 months (Roine et al., 1955). On the other hand storing for 12 weeks at a tropical humid temperature of 16- or 28 C and 55 percent and 60 percent relative humidity, respectively, resulted in some sprouting and softening of the potato, followed by an increase in the vitamin C content from the initial value of 8.2 mg to 10.1 mg and 10.5 mg per 100 g respectively. This indicates that storage losses of vitamin C from potato are less in humid tropical conditions than in dry temperate conditions (Linnemann et al., 1985).

TABLE 6.1 - Composition of potato, cassava and plantain cooked by different methods (per 100 9)

TABLE 6.1 (cont.) - Composition of potato, cassava and plantain cooked by different methods (per 100 9)

Vitamin A is fat-soluble and thermo-stable so it will not normally be degraded by cooking. During studies on the canning of sweet potato, Arthur and MeLemore (1957) found no effect on the vitamin A content of the product due to syrup concentration, 0 to 35 percent sucrose, to cooking time, SO to 90 minutes, or to the peeling conditions. However, Elkins (1979) reported about 14 percent loss of vitamin A activity after processing sweet potato but no additional loss over 18 months, while other investigators have reported a 20 to 25 percent loss of vitamin A activity on cooking. This is probably because of the destruction of the beta-carotene. The main reaction that could take place during the canning of sweet potato is the isomerization of beta-carotene, to neobeta-carotene leading to a reduction in the vitamin A activity from 95 to 91 percent. The loss will be greater with increasing temperature (Panalaks and Murray, 1970). Losses of carotene and the development of off-flavours occur when sweet potato is stored at an ambient oxygen concentration at which antioxidants are not effective. About 20 to 40 percent of the carotene could be destroyed in the first 30 days from autoxidation. (Deobald and McLemore, 1964). At the same time autoxidation of the lipids, which are highly unsaturated, may occur leading to the development of the off-flavours.

Some of the reported losses in the B-group of vitamins are inconsistent because of differences in the heat lability of the vitamins. Thiamine is thermolabile, but boiling unpeeled potatoes reduced their thiamine content by only 23 percent, drying unpeeled potato led to a loss of only 20 percent while frying after peeling gave a 55 to 65 percent loss (Hentschel, 1969). Riboflavin and niacin are heat-stable and so there is complete retention of these nutrients on boiling, roasting or frying potatoes, although some leaching losses may occur (Finglas and Faulks, 1985). The effect of cooking on the food value of boiled, steamed and baked taro (cocoyam) is shown in Table 6.2. In the ease of pyridoxine there is a 98 percent retention of this vitamin on boiling potatoes, the loss being higher with peeled than unpeeled potatoes (August in et al., 1978). However, no loss was reported on baking, roasting or frying, probably due to the concentration of nutrient through loss of water (Finglas and Faulks, 1985). Complete retention of thiamine and nicotinic acid in canned sweet potato has been reported, even after storage for 18 months (Elkins, 1979).

TABLE 6.2 - Effect cooking on composition of taro (cocoyam) (results calculated on fresh weight basis)¹

¹Results from five corms of cultivar Samoa were averaged. standard deviations in parentheses differences marked with one asterisk arc significant at P<0.05. those with two asterisks at P<0.01. Other results not included in the table are protein 9.6 (1.5), fat 0.5 (0.3), raffinose 0.3 (0.1) g/kg^-1; Fe 7.9 (1.8), Cu 2.0 (0.7) mg/kg^-1.
²The value of 655 was the moisture content at harvest in Fiji, the moisture content before cooking in Canberra was 582 (17) g/kg-1.
Source: Bradbury & Holloway, 1988.

Storage has variable effects on different members of the vitamin B group. In potatoes stored at 5° or 10° C, 30 to 50 percent of their thiamine content is lost after six or seven months. There was a significant increase in pyridoxine level, 154 percent and 86 percent respectively for two varieties of potato kept for six months at 4.5°C (Page and Hanning, 1963).

Raw potato starch is undigestible but digestibility increases with cooking time to 75 percent after 15 minutes and to 90 percent after 40 minutes (Hellendoorn et al., 1975). When the whole tuber is baked, as with sweet potato, virtually all the starch is hydrolysed to dextrin and sugars, mainly maltose. The concentration of reducing sugars is low, probably because of the Maillard reaction with lysine.

Baking may decrease the amount of pectin in roots and the degree of esterification, thereby decreasing their dietary fibre content, but this is not nutritionally significant.

The major change in amino-acids that occurs on cooking is the MailIard reaction which makes lysine unavailable, thereby reducing the nutritive value of the roots. Loss of free amino-acids also takes place through leaching (Meredith and Dull, 1979). When sweet potato was canned in 30 percent sucrose or water, the concentrations of essential amino-acids as a percentage of the original were 70 and 58 percent respectively, aromatic amino-acids 69 and 48 percent and sulphur amino-acids 86 and 60 percent respectively. Purcell and Walter (1982) noted a significant reduction in the lysine and methionine content of sweet potato on canning, which may probably be due partly to leaching.

Boiling does not appreciably reduce the total nitrogen content of potato except for some loss owing to peeling. There is a 0.8 percent loss in the boiled, unpeeled tuber compared to a loss of 6.5 percent in the peeled tuber (Herrera, 1979). Nitrogen loss on roasting is also very small, apart from loss of lysine, with losses being greater in frying than baking.

Minerals are usually lost through being leached into syrup during canning, most especially with potassium, calcium and magnesium (Lopez et al. 1980); though the minerals can be completely retained if the tubers are vacuum-packed (Elkins, 1979). The iron content of canned sweet potato increased threefold after 18 months of storage and was obviously derived from the metal can. The leaching loss on boiling potatoes could be minimized if the skin were retained, as reported by True et al. (1979) who found over 90 percent retention when potato was boiled for 14 minutes with the skin on. There are no leaching losses in the case of copper and zinc and so they present no problem (Finglas and Faulks, 1985).

In some traditional processing an appreciable amount of protein could be lost. For example in the preparation of chuño blanco the protein content of the potato is reduced from 2.1 percent to 1.9 percent (Table 6.3). Some of the loss is because of removal in the exudate, but most of it takes place during the soaking in water, about 50 percent. Most of the vitamins are also lost in the process. There is a 90 percent loss in vitamin B1,75 percent in B2 and less than 50 percent of the niacin is retained. Papa seca, vitamins the best. There is an increase in the iron, calcium and phosphorus content in all the preparations (Table 6.3) because of the increased concentration of the product.

In the preparation Of gari (Table 6.5) over one-third of the protein is lost, with greater losses for fufu and lafun (Oke, 1968). Most of the minerals are also reduced appreciably, except iron, which is increased, probably owing to the use of an iron pot for frying the product (Table 6.6). When yam is boiled, steamed or baked, the dietary fibre content rises because starch is modified and some minerals are lost, particularly phosphorus and potassium. (see Table 6.4.) Processing makes some difference to the percentage of nutrients that sweet potato will provide, as shown in Table 6.8. The 6.6 percent increase in the maltose content of sweet potato on cooking is not typical for other root crops, which presumably contain less amylases (Tamale and Bradbury, 1985).

TABLE 6.3 - Composition of raw potato, chuño and papa seca (per 1009)

Source: Woolfe, 1987.

TABLE 6.4 - Effect of cooking on composition of yam (results calculated on fresh weight basis¹)

¹Results of five tubers of cultivar Da 10 were avenged standard deviations given in perentheses differences marked with one asterisk arc significant al P<0.05 those marked with two asterisks are significant at P<0.01. Other results not included above are protein 17.8 (3.9), fat 0.6 (0.5) raffinose 0.4 (0.3) g/kg^-1; Fe 6.5 (3.9) Cu 1.7 (0.3) mg/kg^-1.
²The value of 766 was the moisture content at harvest in Western Samoa; the moisture content before cooking in Canberra was 752 (16) g/kg^-1.
Source: Bradbury & Holloway 1988

TABLE 6.5 - Proximate analysis of cassava and Its products (percentage of dry maker)

Source: Oke, 1968.

TABLE 6.6 - Minor elements In cassava and its products In Nigeria

Source: Oke, 1968

TABLE 6.7 - Effect of cooking on composition of sweet potato (fresh weight basis)

¹Other results are crude protein 17.7 (2.4) g/kg^-1, Fe 7.0 (2.6), Ca 2.2 (0.6) mg/kg^-1. Standard deviations given in paretheses.
²Results of five tubers were averaged from three tubers of 83003-15, one tunber of each of 83003-13 and Hawaii. Differences showing one asterisk indicate a significant (P<0.05) change on cooking, two agterisks indicate P<0.01.
³The value of 684 for moisture was that on harvesting in Tonga: the moisture content before cooking in Canberra was 634 (30) g/kg^-1.
⁴Total sugar: control 34.5, boiled 98.5, steamed 104.4, baked 101.5 g/kg^-1.
Source: Bradbury and Holloway, 1988.

TABLE 6.8 - Percentages of adult recommended daily allowances provided by 100 g servings of processed potato products¹

¹Unless otherwise indicated, calculated from figures for processed potato products given in Table 6.1 as percentages of RDAs given by Passmore et al. (1974).
²As percentage of USA recommended daily allowance.
³Domestic preparation.
⁴A 33.3 g serving, considered to be a more realistic estimate of a single serving of chips.
Source: Woolfe, 1987.

Contents - Previous - Next