Potato Res. 20 (1977) : 77 -84 SHORT COM rvl u N ICATION

Prevention of after-cooking darkening of irradiated potatoes

PAUL THOMAS and M. R. JOSHI

Biochemistry & Food Technology Division, Bhabha Atomic Research Centre, Bombay-400 085, India

Accepted for publication: 12 July 1976

Keywords: After-cooking blackening, after-cooking darkening, gamma rays, sprout inhibition, phenolics, carotenoids, irradiation

Summary

Potatoes of the variety "Kufri Chandramukhi" exposed to 10 krad of gamma rays for sprout inhibi- tion, tended to develop after-cooking darkening, after storage at 15~ for over three mm)ths. The darkening could be prevented or reduced (a) by pre-peeling of tubers prior to cooking: or (b) by reconditioning the potatoesby keeping them at 34-35 ~ for 4-6 days : or (c) by soaking and cooking the tubers in solutions of diaminoethane-tetra-acetic acid disodium salt (EDTA) or citric acid. Prepeeled tubers cooked in the presence of the peel tissues or added phenolics like caffeic and chlo- rogenic acids showed darkening. The results implicate that this could be due to the interaction of phenolics with Fe + + present in the tissues. The storage of irradiated tubers at low temperature also showed enhanced disappearance of carotenoids.

Introduction

After-cooking darkening of potatoes has been reported for many potato varieties grown in different parts of the world (Smith, 1959). The darkening has been attributed to the interaction of ferrous salts in the potato with chlorogenic and caffeic acids to form complexes that are oxidized to black pigments (Kiermeir & Rickerl, 1955a). Various agronomic and climactic factors (Smith et al., 1942) as well as the composi- tion of the tubers especially the content of iron, orthodiphenols, organic acids and pH are shown to influence the darkening tendency (Bate-Smith et al., 1958; Hughes & Swain, 1962a, b: Heisler et al., 1963, 1964, 1969: Hunter et al., 1957; Smith, 1959; Wurster & Smith, 1963, 1965; Vertregt, 1968). Sawyer (1956) had indicated that irradiation could enhance after-cooking darkening of potatoes. In our studies on

sprout inhibition of potatoes grown in India also it was observed that irradiated (10 krad) tubers on prolonged storage, developed a tendency to darken after cooking. The present work was therefore initiated to examine the nature of this darkening and to evolve suitable methods to prevent or reduce this phenomenon which could otherwise adversely affect consumer acceptability.

77

Materials and methods

P A U L THOMAS AND M. R. J O S H I

Potatoes of the variety 'Kufri Chandramukhi" grown in Babugarh, Meerut District, were irradiated within a month of harvest to 10 krad in a Cobalt-60 package irra- diator in air at 25~ The dose rate was approximately 0.5 krad per rain. Irradiated and control tubers, 100 kg each, were stored at ambient temperature (28 to 32~ R.H. 55 to 75'~o) and at 15~ R.H. 80 to 85"~o in wide-meshed jute bags in 20-kg lots. In addition 20 kg of control tubers were stored at 2~ the conventional cold storage temperature for comparative purposes. These studies were performed in two harvest seasons during 1973 and 1974. Since similar trends were observed in both the years, only the results for 1974 are reported.

Cooking tests and evaluation of darkenhlg Whole or prepeeled tubers (5 in each treatment), were cooked by placing them in boiling water for 30 to 35 min either in the presence or absence of added chemicals as shown in Table 2. After cooking, the tubers were exposed to air either as such or after cutting into two longitudinal halves. The intensity of darkening in each tuber soon after cooking and as a function of time of exposure to air was assessed subjec- tively taking into account the overall flesh colour of the potatoes and recorded on a 0 to 10" scale, from 0 (no darkening) to 10 (extreme darkening; dark grey to black). The final rating was obtained by averaging out the marks of individual tubers.

To tal phenolics Skin tissues from 10 tubers selected at random were removed with a potato peeler. The skin and pulp tissues were separately cut into small pieces and mixed thoroughly. Triplicate samples of 10 g each from skin and 20 g each from pulp were frozen in liquid nitrogen followed by blending with 100 ml of 85~ ethyl alcohol in an Omni- mixer for 10 rain. Total phenolics content in the supernatant were determined by Folin-Denis reagent according to Swain & Hillis (1959). The values reported are averages of six independent estimations.

Total carotenoids Five tubers selected at random were cut into small pieces and mixed thoroughly. Carotenoids were extracted from 25 g tissues in duplicate with cold acetone: n- hexane mixture (75 : 60 v/v) till the filtrates were colourless. The filtrate was freed of acetone by washing with distilled water, the hexane layer containing carotenoids was dried over anhydrous sodium sulphate and the absorbance at 436 nm was measured in a spectrophotometer. The amounts were calculated with reference to a standard graph made for [3-carotene. The values reported are averages of at least two separate estimations.

In separate studies it was observed that the heavy bacterial rotting occurring in potatoes, either irradiated or unirradiated during storage under tropical ambient temperature (28-32 ~ could be significantly reduced by holding at 15 ~ hence this temperature was selected for long-term storage of irradiated potatoes.

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Results and discussion

Table 1 shows that storage temperature and duration of storage only have a slight affect on after-cooking darkening in control tubers but a very marked affect on dark- ening in irradiated tubers. In the latter case the intensity and distribution of darkening increased during storage at 15~ but not at 28-32~ At 3 months the darkening was limited to the stem end and at 4 to 5 months the darkening appeared along the peripheral areas of the whole tuber with maximum intensity at periphery at stem end. In tubers stored for 6 months and above, the intensity and distribution of darkening showed a further increase, the entire flesh turning grey to dark grey, the intensity being more in the peripheral region with maximum at periphery at stem end. The darkening was found to increase as a function of time when the cooked potatoes were exposed to air. The observation that darkening was more intense in the pe- ripheral areas of whole potatoes indicated that either the level of substrates are higher in the zone adjacent to the skin than elsewhere in the tuber and/or that sub- strates diffuse into this zone from the skin during cooking. These hypotheses were tested by cooking pre-peeled irradiated tubers with or without added skin tissues. It was found that tubers without added skin tissue darkened very little after cooking whilst tubers with added skin darkened to a certain extent but not as much as whole tubers (Table 2). These results indicate that, although a reduction in discoloration may be partly due to substrates being lost more readily into the cooking liquor in pre-peeled tubers than whole tubers, substrates which influence after cooking dark-

Table 1. Intensity of after-cooking darkening in potatoes as a function of storage temperature and time.

Treatment Storage Storage period Mean value for ~ temperature ( ~ (months) discoloration

Control ambient. 2 3 0 28-32 4-5 1

15 2 3 0 5-6 I

2 23 0 56 1 89 1

Irradiated ambient 2-3 0 28-32 4-5 1

15 2 0 3 1 5 3 6 5 8-9 I0

Discoloration was ewlluated subjectively as described in 'Materials and methods'.

Potato Res. 20 (1977) 79

PAUL THOMAS AND M. R. JOSHI

Table 2. Effect of pre-peeling, added phenolics, FeSO4, EDTA and citric acid on after-cooking darkening of irradiated potatoes.

Treatment Additions in Type and intensity of flesh discoloration cooking water soon after after 4 h exposure

boiling to air

type score type score

Unirradiated 2 nil straw yellow. 0 straw yellow, 0 Whole tuber normal normal

Irradiated j Whole tuber Whole tuber Whole tuber. reconditioned 3 Pre-peeled tuber Pre-peeled tuber Pre-peeled tuber Pre-peeled tuber

Pre-peeled tuber Pre-peeled tuber

Pre-peeled tuber Pre-peeled tuber

nil grey 5 dark grey 10 EDTA pale grey 1 medium grey 3 citric acid white 0 pale grey 1 nil pale yellow 0 pale grey 1

nil white 0 pale grey 1 skin tissues pale grey 2 grey 5 caffeic acid medium grey 3 grey 5 caffeic medium grey 4 grey 6 acid + FeSO4 chlorogenic acid pale grey 2 medium grey 3 chlorogenic medium grey 3 medium grey 4 acid + FeSO4 P-coumaric acid pale grey 2 medium grey 3 P-coumaric pale grey 2 medium grey 3 acid + FeSO,, FeSO,~ pale russened a pale grey 2 Pre-peeled tuber

1 Irradiated (I0 krad) tubers stored for 8 months at 15~ were used. Pre-peeled tubers were soaked for 16 h in solution containing phenolic compounds (0.01",i,), FeSO4 (0.01~ EDTA (1",,) or citric acid (1 oo) followed by cooking in the same solutions. 2 Tubers of the same variety stored for 8 months at 2~ 3 At 34-35 ~ for 4 days.

ening are re leased f rom the skin du r ing cooking . I f in whole tube r subs t ra tes in the skin di f fused more inwards and not o u t w a r d s du r ing c o o k i n g they cou ld be a con- t r ibu t ing fac tor in pe r iphe ra l da rken ing . This hypo thes i s was inves t iga ted fur ther by ana lys ing the skin and flesh tissue and by add ing to the c o o k i n g so lu t ion subs t ra tes though t to inf luence a f t e r - cook ing da rken ing . Several r epor t s have ind ica ted that the a f t e r - cook ing d a r k e n i n g o f po t a toes is inf luenced by the con ten t o f i ron and phenol ic subs tances present in the t issue (Hughes & Swain, 1962a; Heis le r et at., 1963; K i e r m e i r & Ricker l , 1955a, b; Smith , 1959). A c o m p a r a t i v e s tudy o f the dis- t r ibu t ion o f phenol ics af ter s to rage revealed that skin tissues con ta ined 3 to 4 t imes the quan t i t y o f tha t present in pu lp tissues (Table 3). It is a p p a r e n t tha t the relat ive con ten t s o f phenol ic c o m p o u n d s in pu lp and skin tissues var ied in i r r ad ia t ed and con t ro l tubers s to red for 7 to 7�89 months . Con t ro l tubers s to red an 2~ showed

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P R E V E N T I O N OF A F T E R - C O O K I N G D A R K E N I N G OI: I R R A D I A T E D POTATOES

Table 3. Total phenolics content of skin and pulp tissues of irradiated and control potatoes stored under varying conditions.

Treatment Storage Storage period Phenolics (mg/100 g temperature (days) fresh weight) ~ (~

Initial

Control

Irradiated

2 215 2 220 2 230

15 215 15 220 15 230

pulp skin

53 I27

39 167 39 173 44 192

61 149 53 140 63 167

' Values are averages of six separate estimations.

lower levels of phenolics in pulp and higher levels in skin tissues in comparison to irradiated tubers stored at 15~ for similar periods (Table 3). With a view to find out the possible role of phenolics and Fe + + ions in darkening, irradiated tubers after removal of skin were soaked in solutions containing different phenolic substrates and FeSO4 either alone or together. Irradiated pre-peeled tubers when cooked in the presence of peel tissues or caffeic acid and FeSO4 developed a similar discolora- tion as that observed in cooked whole tubers though the intensity of discoloration was comparat ively less than that of whole tubers (Table 2). Chlorogenic and para- coumaric acids were less effective in inducing darkening. In all cases tubers cooked in the presence of both phenolics and FeSO.~ showed more darkening than either of them added alone. However, when freshly harvested potatoes of the same variety either irradiated or unirradiared were cooked in the presence of phenolics and FeSO,~ the darkening was negligible. Preliminary chromatographic studies showed that skin and pulp tissues of irradiated tubers stored for 8 months at 15~ contained more caffeic than chlorogenic acid while unirradiated tubers stored for a similar period at 2~ had more chlorogenic acid. Apart from the above two, a number of other phenolic compounds could also be detected in both irradiated and control potatoes some of which were present in larger amounts in irradiated tubers. These findings would suggest that even though the total content of phenolics present in control and irradiated potatoes did not vary appreciably, qualitative changes in phenolics had occurred in irradiated potatoes during storage. The chromatographic data and the observation that pre-peeled irradiated tubers darken more in presence of added caffeic acid would point to a preferential accumulation ofcaffeic acid and/or some other as yet unidentified phenolic compound in irradiated tubers which may be contributing to the darkening phenomena.

The after-cooking darkening of irradiated potatoes could be reduced by soaking

Potato Res. 20 (1977) 81

P A U L THOMAS AND M. R. JOSH1

EDTA or citric acid solutions followed by cooking in the same solutions (Table 2). The dark complex formed in irradiated cooked tubers was found to disappear com- pletely when the tubers, after removing the skin or cutting into two longitudinal halves, were placed in a 0.2!',,i solution of citric acid. This complete disappearance of the dark complex can be attributed to better penetration of citric acid into cooked tubers.

The use of chelating or sequestering agents as a foliar spray or as a dip were re- ported to reduce the after-cooking darkening (Greig & Smith, 1955; Hughes & Swain, 1962b; Smith, 1958; Smith & Muneta, 1954).

Table 4 shows the changes in carotenoids during storage under varying tempera- tures. Increased levels of carotenoids were noticed during the first three months in both irradiated and control tubers stored at ambient temperature or 2~ followed by a gradual decrease thereafter. After 6 months of storage the carotenoids content was almost similar to initial level. In contrast, irradiated tubers stored at 15 ~ showed a continued disappearance of carotenoids which resulted in whitish flesh colour after cooking as compared to straw yellow colour of tubers stored at ambient tem- perature or 2~ It is likely that this whitish background colour of irradiated tubers

Table 4. Effect of storage temperature on carotenoids content of potatoes.

Storage period (days) Temperature (~ Treatment

Initial

30 28-32 control (ambient) 28-32 irradiated 15 control 15 irradiated 2 control

100 28-32 control 28-32 irradiated 15 control 15 irradiated 2 control

195 28-32 control 28-32 irradiated 15 irradiated 2 control

Irradiated. stored for 210 days at 15~ and reconditioned at 34-35~ for 4 days Irradiated, stored for 210 days at 15~ and reconditioned at 34-35~ for 20 days

Carotenoids content (~g/100 g fresh weight)

140

116

112 140 72 84

212 181 160 55

195

110 112 14 93

42

62

1 Values are averages of two independent estimations.

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P R E V E N T I O N OF A F T E R - C O O K I N G D A R K E N I N G O|: I R R A D I A T E D POTATOES

stored at 15~ would have accentuated the after-cooking discoloration. Recondi- tioning of such tubers at 34 to 35~ for 4 to 6 days was found to result in renewed synthesis of carotenoids (Table 4) and also significantly reduced the intensity of after-cooking darkening (Table 2). The reconditioned tubers had pale yellow flesh colour on cooking.

The observation that only irradiated tubers stored at 15 ~ developed the tendency to darken after cooking may suggest the interaction of various factors in the manifes- tation of this phenomenon. Bate-Smith et al. (1958) and Hughes & Swain (1962a) have reported that of the various factors known to influence the after-cooking darkening, the level of citric acid was the most important, the best correlation being between blackening and ratio of chlorogenic to citric acid. It is likely that apart from the qualitative changes in phenolics, changes in the content and distribution of citric acid have occurred in irradiated potatoes during prolonged storage. Our preliminary findings indicate higher pH values in irradiated tubers stored at 15~ for over 6 months in comparison to control stored at 2~ which would implicate a possible reduction in the level of citric acid or other organic acids in irradiated potatoes. Such suggestion, however, needs confirmation. Low temperatures towards the final stages of tuber maturation in the field was found to increase the darkening in certain potato varieties which could be reduced by storage at higher temperatures (Smith et al., 1942). Our own limited studies have shown that the intensity of darkening in irradiated potatoes could be reduced by holding the tubers at higher temperatures (34 to 35~ for a few days (Table 2). The results of the present study suggest that the after-cooking darkening of irradiated potatoes is of a similar nature to that reported in certain potato varieties which have a natural tendency to darken.

In India potatoes are normally cooked as whole and peeled before they are used for culinary and table purposes. With irradiated potatoes stored at 15 ~ it would be necessary to remove the skin tissues prior to cooking or recondition the tubers at higher temperatures to overcome the darkening of the flesh. Further studies are underway to determine the possible role of temperature as well as individual phenolic compounds and other factors in the darkening phenomena.

Acknowledgment

The authors thank Dr G. B. Nadkarni for his helpful suggestions and Dr S. C. Verma of the Central Potato Research Institute, Simla for supplying the potatoes.

References

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Heisler, E. G., J. Siciliano, R. H. Treadway & C. F. Woodward, 1963. After-cooking discolo,'ation of potatoes. Iron content in relation to blackening tendency of tissues. J. Fd Sci. 28 : 453-459.

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