Plant Foods for Human Nutrition 37:47-57 (1987) 47 © Martinus Nijhoff/Dr W. Junk Publishers, Dordrecht - - Printed in the Netherlands

Effect of waxing and cellophane lining on chemical quality indices of citrus fruit

M A Q B O O L A H M A D & I S M A I L K H A N Nuclear Institute for Food & Agriculture, P.O. Box No. 446, Peshawar, Pakistan

Received 30 April 1986; accepted in revised form 26 September 1986

Key words: waxing, film lining, internal quality indices, ethanol, acetaldehyde

Abstract. Effects of waxing of the fruit and of the use of cellophane for lining storage boxes on chemical quality indices (including ethanol and acetaldehyde) and flavour scores of Feutrell's Early mandarin during storage at room conditions were determined. Maximum changes during storage of waxed mandarins in film-lined boxes occured in ethanol contents which were followed by acetaldehyde, total soluble solids (TSS)/acid ratio, TSS and acid contents. There were negative and highly significant linear relationships between flavour scores, and ethanol content and acetaldehyde content. The r 2 values indicated that over 90% of the variation in flavour scores could be explained by changes in ethanol and acetaldehyde contents whereas only 50% of the variation in flavour scores could be predicted by changes in total acid and TSS/acid ratios. There were comparatively greater increases in ethanol and acetaldehyde content, lesser increases in TSS and greater decreases in total solids of waxed mandarins during storage than those of unwaxed fruits stored in film lined boxes; the ratio of ethanol and acetaldehyde (E/A) of waxed mandarins was also higher. Decreases in citric acid content during storage correlated linearly to increases in the E/A ratio at the 1% level of significance indicating that citric acid metabolism was affected during storage of waxed mandarins at ordinary room conditions.

Introduction

Studies o f chemical changes o f c o m p o u n d s re la ted to the f lavour o f ci trus fruits have been l imited main ly to TSS, t i t ra table acid and p H [15, 26]. H a r d i n g [18] r epor ted tha t bo th TSS and t i t ra tab le acid o f M a r s h grapefrui ts decreased slightly dur ing s torage bu t were no t affected by the s torage tempera ture , Resul ts o f pos tharves t s torage exper iments on Pakes tan i manda r in s revealed tha t deter- m ina t ion o f rou t ine in terna l qual i ty indices (acidi ty, sugars, ascorbic acid) cou ld no t account for the effect o f different t rea tments like waxing or use o f film for l ining s torage boxes. [1, 16]. Sealing with high densi ty po lye thylene film had no effect on the r ipening indices o f M a r s h grapefru i t s tored for 8 weeks at 20°C, bu t sealed Shamout i oranges lost to ta l acid at a sl ightly bu t no t significantly s lower ra te than the non-sea led fruits [3]. Ha le et al. [17] could no t find any differences in solids, acid, p H or juice percentage a t t r ibu tab le to ei ther waxing, film l ining or c o m b i n a t i o n o f the two t rea tments . W a x i n g or sea l -packaging grapefrui t s wi th low densi ty po lye thy lene film had no effect on ascorbic acid and TSS/ac id ra t io bu t to ta l acid levels t ended to be lower in the juice o f sea l -packaged fruits s tored for 82 days at 21°C [29]. Volat i les o f ci trus juice especial ly e thanol also vary dur ing m a t u r a t i o n / s t o r a g e and to a greater extent than solids, ac id or p H

48

[8, 9, 12, 13]. The present work was instituted with the following objectives: i) To study the effect of waxing and the use of cellophane lining on internal

chemical quality indices including ethanol and acetaldehyde of Pakistani mandarins stored under room conditions.

ii) To study the relationship between chemical and sensory characters during postharvest storage of fruit.

Materials and methods

Procurement, processing and storage of fruit

Feutrell's Early mandarins were procured from gardens around Faisatabad in the month of January and transported in wooden boxes lined with newspaper sheets to the laboratory by road. The stems were cut close to the shoulders and bruised and injured fruits were discarded. Mandarins were washed in running tap-water and air-dried on wire trays. All the fruits were dipped for two rain. in an aqueous suspension of 1000ppm of thiabendazole (TBZ). The fruits were divided into four equal lots. Two lots were coated separately with diluted commercial waxes i.e., Britex-561 and Seal Britex-65, procured from Florida, USA, (diluted to 2:1, wax:water ratio) and air-dried. Newspaper (0.093 mm) was used as a lining material for waxed and one lot of unwaxed (control) fruit whereas cellophane film (0.030 ram) was employed as a container liner for the remaining unwaxed lot. All the lots of the fruit were packed separately in perforated wooden boxes (39 x 30 x 18 cm size with 3 slits of 1 cm) lined with the different lining materials and stored under room conditions (16-22°C, 59 83.5 RH).

Chemical analysis

TSS and acidity were determined by standard AOAC methods [2]. TSS and acid ratio were calculated by dividing the former by the latter. Ethyl alcohol and acetaldehyde contents of juice of control as well as treated mandarins during storage were determined by gas chromatographic analysis of headspace as reported by Davis and Chace [10]. The juice of five mandarins was combined and filtered before analysis. Samples of 5-ml juice were incubated in 60-ml glass capped tubes for two hours in a constant temperature water bath at 40°C. Headspace samples of 2 ml were taken by syringe (after first injecting 2 ml of air and flushing the syringe three times) for injection into the gas chromatograph. A Shimadzu GC-7AG Gas Chromatograph equipped with flame ionization detector was employed, nitrogen was used as a carrier gas with a flow rate of 3ml/min. The column was stainless steel (1.5m x 0.002m) and packed with 10% carbowax 20 M. Oven and detector temperatures were 105°C and 180°C, respectively. Standards in water were prepard by volume from 95% ethanol and

Tab

le 1

. E

ffec

t o

f w

axin

g an

d c

ello

phan

e li

ning

on

the

che

mic

al c

hara

cter

s o

f F

eutr

ell'

s ea

rly

man

dari

ns s

tore

d at

roo

m c

ondi

tion

s(16

-22~

'C,5

9-83

.5%

R

H).

Tre

atm

ents

T

otal

sol

uble

A

cidi

ty

TS

S/a

cid

Eth

ano

l (E

) A

ceta

ldeh

yde

(A)

E/A

so

lids

(T

SS)

gm

cit

ric

* R

atio

*

mg/

100

* m

g/1

00

ml

* R

atio

*

acid

/100

m

l.

ml.

%

*

Fre

sh f

ruit

7.

0 0.

61

11.4

8 75

0.

42

178

1st

wee

k C

ontr

ol

7.5

1.07

0,

49

0.80

15

.31

1.33

10

0 1.

33

0.45

1.

07

222

Bri

tex-

561

7.5

1.07

0.

52

0.85

14

.42

1.26

13

5 1.

80

0.49

1.

17

275

Seal

Bri

tex-

65

7.5

1.07

0.

55

0.90

13

.64

1.19

13

0 1.

73

0.47

1.

12

276

Cel

loph

ane

7.5

1.07

0.

50

0.82

15

,00

1.31

11

0 1.

47

0.44

1.

05

250

3rd

wee

k C

ontr

ol

7.9

1.13

0.

52

0.85

15

.19

1.32

16

0 2.

13

0.60

1.

42

266

Bri

tex-

561

7.5

1.07

0.

50

0.80

15

.00

1.31

32

0 4.

27

0.90

2.

14

355

Seal

Bri

tex-

65

7.5

1.07

0,

50

0.80

15

.00

1.31

35

0 4.

67

1.05

2.

50

333

Cel

loph

ane

7.5

1.07

0.

54

0.89

13

.89

1.21

15

0 2.

00

0.70

1.

67

214

5th

wee

k C

ontr

ol

8.6

1.23

0.

49

0,80

17

.55

1,53

21

0 2.

80

0.85

2.

02

247

Bri

tex-

561

8.0

1.14

0.

43

0.71

18

.60

1.62

49

0 6.

53

1,27

3.

02

385

Seal

Bri

tex-

65

8.0

1.14

0.

45

0,74

17

.77

1.55

50

0 6.

67

1.22

2.

90

409

Cel

loph

ane

8.7

1.24

0.

50

0.80

17

.40

1.52

21

5 2.

87

0.83

1.

98

259

1.24

1.

54

1.55

| .

40

1.49

1.

99

1.87

1.

20

1.38

2.

16

2.39

1.

45

* R

atio

co

mp

ared

to

fres

h fr

uit.

50

99.5% acetaldehyde. Each volatile was processed as reported for juice and the concentration was calculated on a weight basis. The concentration of alcohol and acetaldehyde in juice was calculated as:

Peak area under juice mg/100ml = concentration in standard x

Peak area under standard

In order to compare the extent of change in different chemical characters during storage, a ratio of observed values with that of fresh fruit was calculated by dividing the former by the latter.

Organoleptic evaluation

The sensory evaluation of treated as well as control fruits for flavour was carried out during storage by a panel of trained judges using a scoring method [2]. A scale of 0 to 10 was used where 0 was extremely disliked and 10 was extremely liked.

Stat&tical analysis

Data were analysed statistically by the analysis of variance with least significant differences (LSD) between treatment and storage interval means determined. Regression equations to establish relationship between internal chemical characters and flavour scores were also calculated [34].

Results and discussion

Chemical characters

Fresh Feutrell's Early mandarins contained 75 mg ethanol and 0.42 mg acetal- dehyde per 100 ml juice (Table 1). There were wide variations in ethanol and acetaldehyde contents between varieties, stage of maturity and between in- dividual fruits of the same variety and harvest. Values of ethanol and acetal- dehyde reported in the literature for fresh oranges range from 150 to 800 ppm, respectively, and for fresh grapefruit from 70 to 520ppm and 0.3 to 190ppm, respectively (8, 10, 12, 19, 20, 24, 30). It is clear from the results in Table 1 that during storage of mandarins, maximum changes ocured in ethanol contents which were followed by acetaldehyde, TSS/acid ratio, TSS and acidity. This confirms the findings of other workers [8, 9, 12, 13] on different citrus cultivars.

The treatments had significant effects on the ethanol contents of Feutrell's Early mandarins; ethanol contents were significantly (P < 0.05) higher in waxed fruits than those of control and cellophane lined fruits throughout the storage period of 5 weeks. However, there was no significant difference between the two waxes as far as increases in ethanol content during storage. Although

51

Table 2. Mean scores (0-10) showing effect of waxing and cellophane lining on flavour of Feutrell's early mandarins stored at room conditions (16-22°C, 59-83.5%RH).

Storage periods Control Britex Seal Bri tex Cellophane L.S.D. (weeks) 561 65 (1%)

1 8.0 7.4 7.4 8.2 -

2 6.5 4,9 4.4 6.7 1.79 3 6.7 4.5 3.9 6.8 1.95 4 6.0 3.9 2.9 6.3 1.16 5 5.2 3.0 2,5 5.4 1.63

waxed mandar ins contained comparat ively more acetaldehyde during storage than the controls and the fruits stored in cellophane lined boxes, this difference was statistically insignificant. Waxing as well as film lining had no significant effect on TSS, acidity and TSS/acid, however, these physicochemical charac- teristics (except acidity) as well as ethanol and acetaldehyde increased signifi- cantly (P < 0.01) dur ing a storage period o f 5 weeks.

Flavour scores

The results in Table 2 show that use o f bo th o f the diluted commercial waxes resulted in the development o f off-flavour after one week storage. The flavour o f fruits stored in newspaper and cellophane lined boxes was, therefore, superior to that o f waxed fruits; the difference was significant at the 1% level.

10

o

I

o

t.n W r~ 0 q) b~

oz ._.1 11

2

r- 0.7471 ".0.0893 n:12 Flavour---1.~9198.26.6431

Acidity

•11

• tP

• Ib

a ,. I ......... ! S, |

0 0.2 0.4 0.6 0.13 1.0 ACIDITY (RATIO THAT OF FRESH FRUI T)

Fig. 1. Linear regression between titratable acidity and flavour scores of waxed and film-lined Feutrell's early mandarins stored for I, 3 and 5 weeks at room conditions (16-22°C, 59-83.5% RH).

10

FLavour:18.7346- 9.4663 Tot(a[ s o [ u b [ e sot idS/Acid

' ~ . r : - 0 . 6 9 7 0 z 0.1039

: 2

o

1

o

L,q Ld C~ 0

r'r"

0

...J LL

8

6

4

2

52

I I . . . . . . . . i[ I I

0-5 1.0 1.5 2.0 2.5 TOTAL SOW BLE SOLIDS/ACID

( RATIO THAT OF FRESH FRUIT)

Fig. 2. Linear regression between total soluble solids/acid ratios and flavour scores of Feutrell's early mandarins stored for 1, 3 and 5 weeks at room conditions (I 1-22°C, 59-83.5% RH).

Relationship between flavour scores and other internal characters

TSS, total acid contents and their ratio are the main parameters which usually have been related with internal organoleptic attributes of citrus fruits [t5]. Regression analyses were therefore done to establish the relationship between the chemical characters studied and flavour scores (0-10) of waxed mandarins and unwaxed stored in film lined boxes for 1, 3 and 5 weeks at room conditions. The correlation between TSS and flavour scores was non-significant (r = -0 .373 , N = 12). The relationship between total acids and flavour scores is shown in the linear regression plotted in Fig, 1. There was a positive linear relationship (r = 0.74) and the regression equation was:

Y = - 1 5 . 9 2 + 26.64X.

The correlation between TSS/acid ratios and flavour scores was linear but negative (r = -0 .697) (Fig 2) and the regression of flavour scores on TSS/acid ratios was:

Y = 18.73 - 9.47X.

The decrease in flavour scores during storage of mandarins, therefore, correlates

53

10

r :-0.9671 +. 0.0131

o8~ % n-12 ' Ftavour = 8.9165 - 0.99 2 9 Ethar:io[

c )

~ 6 klJ or" 0

r r 4

0

I ,I I . . . . . . . . . I I I

0 2 4 6 8 10 12

ETHANOL (RATIO THAT OF FRESH FRUIT )

Fig. 3. Linear regression between ethanol contents and flavour scores of waxed and film-lined Feutrell 's early mandar ins stored for 1, 3 and 5 weeks at room conditions (16-22°C, 59-83.5% RH).

to decreases in total acid contents at the 1% level of significance, and to increases in TSS/acid ratios at the 5% level of significance.

There was a negative and highly significant linear relationship between etha- nol contents and flavour scores (r = - 0.967) (Fig 3) and acetaldehyde contents and flavour scores (r = - 9 8 6 ) (Fig 4). The regression equations for these relationships were:

Flavour versus ethanol Y = 8.92 - 0.99X

Flavour versus acetaldehyde Y = 10.77 - 2.73X.

The decrease in flavour scores during storage of this cultivar of mandarins, therefore, correlates to increases in ethanol and acetaldehyde contents at the i % level of significance.

The coefficients of determination (r 2) in case of ftavour scores versus ethanol contents and flavour scores versus acetaldehyde contents correlations were 0.94 and 0.97, respectively. This indicates that 94% and 97% of variations in flavour scores can be explained by changes in ethanol and acetaldehyde contents, respectively. The coefficients of determinations (r z) of flavour scores versus total acid contents (0.55) and flavour scores versus TSS/acid ratios (0.48) relation- ships, however, indicate that only 55% and 48% of variations in flavour scores can be predicted by changes in total acid contents and TSS/acid ratios, respec-

10

r : -0.9861 *.

n - 1 2

O

I

O

b~ 1~1 I3Z 0 C) bO

0

Ix.

54

Flav0ur=10.77- 2-7307 Acetoldehyde

I I I I I I

0.5 1.0 1.5 2.0 2.5 3.0 A C E T A L D E H Y D E ( R A T I O THAT OF FRESH FRUIT

Fig. 4. Linear regression between acetaldehyde contents and flavour scores of waxed and film-lined Feutrell's early mandarins stored for I, 3 and 5 weeks at room conditions (16-22°C, 59-83.5% RH).

tively. Hence, the correlation between ethanol or acetaldehyde contents and flavour scores is stronger than that between flavour scores and TSS, acid or their ratio during storage of this cultivar of mandarins. Earlier reports also indicated that TSS and acid contents of citrus fruit could not be taken as infallible guides for consumer acceptance [31]. Stronger correlation was observed between co- lour measurements and the results of taste tests than between taste and TSS, citric acid or their ratio [22, 23]. Other workers hae reported a build up of ethanol and a distinct abnormal flavour during storage of excessively waxed citrus fruits [11, 25, 27].

Decreases in 02 contents and increases in CO2 contents of the internal atmosphere during storage of waxed oranges [14], tangerines [36], grapefruits [35] and Temples [11] have been reported. Waxing of grapefurits tended to reduce respiratory gas exchange in comparison to unwaxed fruits [29]. Com- paratively greater increases in ethanol and acetaldehyde, lesser increases in TSS and greater decreases in total acid (Table 1) of waxed mandarins in the present studys were most probably due to anaerobic respiration during storage at room conditions. Ethanol is a product of sugar and acid metabolism under anaerobic conditions in stored fruits [32]; Citrus fruits are capable of metabolizing glucose [28], and malic acid [7] to pyruvate and hence to acetaldehyde and ethanol.

The ratio of ethanol and acetaldehyde (E/A) of waxed mandarins was also higher during storage than those of the control or the fruits stored in cellophane

40

55

E

Cb C.) <[ £.)

iz l - -

C )

30

20

10

r= - 0 . 7 5 n= 13

Y= 33.52 - 0.02 5 X

I A I I I

0 100 200 300 400 500

ETHANOL/ACETALDEHYDE

Fig. 5. Linear regression between titratable acidity and the ratio of ethanol and acetaldehyde of waxed and film-lined FeutrelI's early mandarins stored for 0, 1, 3 and 5 weeks at room con- ditions(16-22°C, 59-83.5% RH).

lined boxes (Table 1). The E/A ratio gives an indication of oxidation-reduction state of coenzyme nicotinamide adenine dinucleotide (NAD) in plant tissues in the following equilibrium [5]:

E/A = 0.3NADHz/NAD.

During fruit maturation and senescence both E/A [9] and NADHz/NAD in- creased [4]. A number of NAD-dependent oxidation reactions in orange juice vesicles were depressed when the redox ratio was increased [6]. Similarly flavour changes in waxed mandarins with high E/A ratios during storage reported here might have been due to lowered activity of NAD-dependent oxidative reactions. In citric acid metabolism through the Krebs cycle, the formation of oxaloacetic acid, the precursor of citric acid, requires the NAD-dependent oxidation of malic acid. If citric acid metabolism is affected by storage of waxed citrus fruits at ordinary room conditions, and if citric acid formation is dependent upon the availability of oxaloacetic acid from malate, decreases in acidity of mandarins during storage should correlate with increases in E/A ratio. The relationship between titratable acidity, expressed as # moles of citric acid and E/A ratios of Feutrell's Early mandarins is shown in the linear regression plotted in Fig 5. The regression of citric acid on E/A was as follows:

Y = 33.52 - 0.025X'.

56

The decrease in citric acid dur ing storage of Feutrel l ' s Early manda r in s cor- relates to increases in the E/A ratio at the 1% level of significance (r = - 0 . 7 5 ) . This s tronger l inear correlat ion between these two variables indicates a mutua l relat ionship and strengthens the hypothesis that the N A D ratio is impor t an t in control l ing metabol ic pathways in citrus fruit. Oxygenated c ompounds account for mos t citrus f lavour [33, 37], a shift in equi l ibr ium towards reduced forms might have dropped their concent ra t ion to lower flavour levels and increased the level of other reduced components .

Conclusion

The present results clearly demonst ra te that storing of waxed citrus fruits at r oom condi t ions can lead to anaerobic respirat ion with higher levels of e thanol and acetaldehyde. Changes in e thanol contents and their s t rong correlat ion with flavour scores dur ing storage prove its superiority over other chemical qual i ty indicators. In ternal e thanol content can, therefore, facilitate qual i ty evaluat ion, the development of proper storage techniques, and the prevent ion of condi t ions

deterimental to fruit.

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