SUITABILITY OF LOCALLY AVAILABLE POTATO VARIETIES FOR CHIPPING

A study was 'conducted on eleven locally avai1ahl.e potato varieties to investigate Iheu suitability for chipping. The Lubers were stored at 80'5 RH and 26+2 ' C f i~r three months. Total sugar and dry matter contents of the fresh tubers were analyzed. Potato chips were made before storage and at one and three-montll inteivals during slurage by slicing into 1 mm tllickncss and fiying in red palm oil a t 190" C, and the chip colour was measured. Tlie effect of potassium tnetabisupllite (KMS) and hot water hlanclfing on the chip colou~ was tested. The 1.esidua1 sulfur dioxide content uf'chips was determined. Thc total sugar content increased ant1 the dry matter content decrexsctl during storage oltlle tuhers. T l ~ c hip lightness as given hy 'L'value showed a i~egativc ~.clationsliip wit11 total sugars. The coloui. of cbi'ps macle from 'Astlalic', 'Atlantic', 'Desiree' aud 'Red star' was acceptable and was nut affected hy 3-month stomgc. 'Escort', 'Grant~la', 'Monoliza', 'Prelude', 'Sita', %valdi' and 'Axa~ikn' did not produce cliips of acceptable coloul-. Hot water blanchinfi at 90-100 "C fn~. 1 m.in wit11 220 ppm KMS improved the chip culour ul'Escort', 'P1-elude', 'Sita' and 'Vivaldj' varieties. The residual s u l h r dioxide contcnI ol'tllese chips was 45 ppm. Chip colour of 'Monoliza', 'Gi.nnolaJ and 'A~snka' varieties was not improved hy the ICMS treatment.


INTRODUCTION
Potato ( S o l a~~u n~ tuberosunt L.) cultivation in Sri Lallra is confined mainly t,o the Badulla and Nuwara Eliya Districts. The extent of cultivation in 1996 was 7925 h a with the production of 1,000,755 Mt.' Both the extent of cultivation and production decreased after 1996', probably due to importation of potato tubers mainly from Paklstan and India, after removal of import restrictions. Imported potato tubers are sold a t a lower price than that of locally grown potatoes. The price of locally grown potato could not be reduced due to high cost of production. This has caused severe marketing problems to the potato farmers. Use of locally grown potato varleties to produce value-added products such as potato chips, french fries, dehydrated products, potato flour and canned potatoes lnay increase the net profits of potato cultjvation.
The leading potato processors in Sri Lanka use imported varieties for producing potato chips. Small-scale producers use varieties that are available to them a t the time of processing. As a result, the quality of chips varies tremendously, as the processing quality is a function of both physical and chemical factors of the tubers. Apart from the morphological characters such as shape, size and colour of potatoes, the quality of chips depends on the dry matter content, sugars, starch, free amino acids and phenolic compounds of the t~b e r s .~T h e desirable colour and flavour of fried chips is due t o the products of Maillard reaction between reducing sugars and amino acids." Excessive browning during frying due to high levels of reducing sugars is a major quality defect which produces an unacceptable colour and bitter taste.4 Contribution of amino acids to the chip colour is marginal since their concentration is rarely a limiting f a~t o r .~ Reducing sugar and sucrose contents in Russet Gold potato tubers have shown linear relationships with chip colour." This study was carried out to determine the suitability of eleven locally grown potpato varieties for processirlg into chips based on total sugar content and chip colour. The changes in total sugar content during storage of tubers and subsequent effect on chip colour were studied. The effect of pre-treatments such as, hot water blanching and use of potassium metabjsulphite (KMS) in blanching water on improving chip colour was also tested.
Tubers of uniform size were hand-peeled, sliced into lmm thjck~less using a stainless steel slicer (Chjba Company Limited, Japan) and washed thoroughly in water. The excess water was removed by blotting with a clean tissue paper, and the slices were fried in red palm oil at 190°C. The end point of frying was determ~ned by cessatio11 of bubbling. The c h~p color was measured by using a colour difference meter (Nippon Denshoku 2000 ZE) and expressed as the 'L' value. The 'L' value of two brands of potato chips available in the market was also determined.
The varieties that produced chips of acceptable colour were stored m perforated polythene bags at 80% RH and 26f2 "C in dark for three montl~s. The cured potato tubers and the samples withdrawn after one and three months storage were analyzed for dry matter7 and total sugars. Sugars from the tubers were extractedh, and the total sugar content was determined by the Anthrone method." Potato chips were made from the samples withdrawn dur~rlg storage, and c h~p colour was measured.
The effect of blanching and inclusion of KMS in blanching water on the chip colour of 'Aranka', 'Escort', 'Granola', 'Monoliza', 'Prelude', 'Sita' and 'Vivaldi' varieties was tested. The tubers were sliced into 1 mm thickness and subjected to four pre-treatments such as blanching a t 90-100°C for 1 min. with 50, 100, 150 and 200 ppm KMS. Blanching a t 90-100°C for 1 min. without KMS was used as the control. After each treatment, chips were prepared and the 'L'value of the chips was measured.
Residual sulfur dioxide of chips was determined according to the methods given in Sri Lanka standard 729:1985.

RESULTS
The relationship between the total sugar content and the chip colour of eleven potato varieties used in this study is presented in Figure 1. Total sugar content of 'Atlantic', 'Desiree', 'Asteric' and 'Red star' was 0.76, 2.63, 2.59 md 3.08 mglg dry weight respectively. These varieties produced chips of acceptable colour, and the 'L' value of the chips varied between 43.9 and 48.3 ( Figure 1). The variety 'Aranka' contained the highest amount of total sugars (12.37 mglg dry weight) and resulted in chips of the lowest ' I , ' value (32.3).   The relationship between the total sugar content and the chip colour of 'Atlantjc', 'Desiree', 'Asteric' and 'Red star' during storage at 80 % RH and 26f2"C for three months is presented in Figure 2. With the increase in the total sugar content of these varieties, the 'L' value of the chips decreased ( Figure 2).
The effect of blanching and i~iclusion of KMS in blanching water on the chip colour of 'Aranka', 'Escort', 'Granola', 'Monoliza', 'Prelude', 'Sita' and 'Vivaldi' is presented in Table 2. With the jncrease in concentration of KMS in the blanching water, the 'L' value of fried chips increased ( Table 2).

DISCUSSION
Colour is an important criterion for marketing potato chips, and the maintenance of the desired colour is the major problem associated with the chipping industry.") There are inherent differences among varieties with respect to chip colourl1, as indicated by the 'L' values varying from 32.2 to 48.3 for chips processed from the eleven potato varieties used in this study (Figure 1). The processors prefer varieties that are best suited to produce light colour chips. The reducing sugar levels are used to predict the suitability of potato tubers for chip processing as they are often responsible for colour de~elopment.~.~ It is well established that sucrose, fructose and glucose comprise the major soluble sugars of white p~t a t o e s .~~~~~u c r o s e , a non-reducing sugar, can also be important in chip colour development through hydrolysis during frying.14:15~1Vhus, the total sugar content could also be a good indicator of the suitability of potato tubers for chipping, as indicated by the negative correlation (r=0.93) observed between the total sugar content and the chip colour of eleven potato varieties used in this study (Figure 1). This is in agreement with the work of Rodriguez-Saona et al.,G where decrease in chip lightness with increase in both reducing sugars and sucrose content was reported. All the varieties with the total sugar content of more than 3.1 mg/ g dry weight, produced chips of dark colour as indicated by the 'L' values varying between 39.8 and 32.2 ( Figure 1). These values were lower than that of the 'L' values of two commercial brands of local and imported chips, which were 40.6 and 43.7 respectively.
A large variation in the dry matter content of 1nd.ian potato varieties is r e p~r t e d .~ Potatoes with high dry matter content are considered. suitable for chips17 as the dry matter content is associated with mealiness, cri.spness and. reduced oil uptake in the fried chips.l"hree Indian potato varieties namely 'Kufri Bahar', 'Kufri Badshai' and 'Kufri Jyoti' are reported to be unacceptable for chipping as the dry matter content is less than 17.8%.2 Based on this fact the variety Vivaldi'which contains 17.7 % dry matter (Table 1) may not be suitable for chipping.
The total sugar content of 'Atlantic', 'Desiree', 'Asteric' and 'Red star' varieties which produced chips of acceptable colour increased ( Figure 2) and the dry matter content decreased (Table 1) during storage a t 80%) RH and 26f2 "C for three months (Figure 2). This is probably due to initiation of sprouting, which was evident in this study. With the increase in the total sugar content of these varieties, the 'L' value of the chips decreased ( Figure 2). When the 'L' values were above 39.8, the colour was acceptable. However, measures need to be taken to prevent sprouting of the tubers, thereby minimizing changes in dry matter and total sugar content during storage, as the quality of chips may be affected.
Hot water blanching has been reported to be successful in removing glucose from potato slices of 'Accent' cultivar.'!' Improvement of chip colour by blanching of 'K. Jyoti' variety has also been reported." However, in this study, hot water blanching a t 90-100 "C for 1 min was not effective in improving the chip colour of 'Prelude', 'Monoliza', 'Vivaldi', 'Escort', 'Aranka', 'Granola' and 'Sita' varieties ( Table 2). This is probably due t o their high sugar content.
Sulphites are widely used t o inhibit Maillard browning." KMS is a permitted preservative in foods in Sri LankaZ2. With the increase in concentration of KMS in the blanching water, the 'L' value of fried chips increased (Table 2). KMS a t a concentration af 50 ppm was reported to be effective in improving the chip colo~~r of 'Kufri Chandramubhi' variety." However, in our study, the optimum concentration of KMS required to improve the chip colour of 'Prelude', 'Sita', 'Vivaldi' and 'Escort' varieties was 200 ppm. Even at this KMS concentration, the chip colour of 'Aranka', 'Granola' and 'Monoliza' varieties was not acceptable.
The maximum allowable level of the residual sulphur dioxide in crystallized or glaze fruits is 100 As the residual sulphur dioxide content of the fried chips was 45 ppm, hot water blanching at 90-100 " C for 1 min with 200 ppm KMS could be recommended. t o improve the chip colour of 'Prelude', 'Sita', 'Vivaldi' and 'Escort' varieties.