Effects of storage conditions on colour change of selected perishables

Effects of storage conditions on colour change of selected perishables T. K. Pai and S. K. Sastry** Department of Food Science, The Pennsylvania State University, University Park, PA 16802, USA *Department of Agricultural Engineering, 590 Woody Hayes Drive, The Ohio State University, Columbus, O H 43210, USA Received August 1988; revised 25 J u l y 1989

The effects of refrigerated storage temperature and relative humidity (RH) on the colour change of mature green tomato (Dombito), mushroom (Sylvan hybrid white) and apple (Red Delicious) were investigated at three temperatures (5, 10 and 15°C) and four different relative humidity levels (91, 94, 97 and 100%). Colour change rates of the tomatoes (a value) showed an apparent zero-order reaction with an activation energy (Ea) of 45.15 kcal mol-l. The rates of loss of whiteness of mushrooms was not significantly affected (p >0.0l) by RH ranges at the same temperature. The loss of whiteness of mushrooms showed an apparent first-order reaction, with an activation energy of 20.48 kcal mol- t. The colour change rates of apples showed decreasing slopes, but the results were generally not clear, due to the relatively short duration of storage.

(Keywords:storageconditions;colourchange;perishables)

Effets des conditions d'entreposage sur le changement de couleur de quelques denrres prrissables On a effectuO des recherches sur les effets de 3 temp&atures (5, 10 et 15 de9 C) et de 4 humiditOs relatives (HR) (91, 94, 97 et 100%) sur le changement de couleur des tomates vertes (Dombito), des champignons (Sylvan hybrid white) et des pommes (Red Delicious). Le taux de changement de couleur des tomates (valeur a) a prOsentk une rbaction apparente d'ordre zbro avec une ~nergie d'activation (Ea) de 45,15 kcal.mole -~. A la mOme tempbrature, les diffbrents niveaux d'humiditb relative n'ont pas considbrablement affectb la couleur blanche des champignons (p > 0,01). La perte de blancheur des champignons a prbsentO une rkaction apparente du premier ordre avec une bnergie d'activation de 20,48 kcal. mole- 1. Le taux de changement de couleur des pommes avait une pente dbcroissante, ranis, de fa¢on gbnbrale, les rbsultats n'~taient pas trks clairs btant donnOe la durke d'entreposage relativement courte.

(Mots clrs: conditions d'entreposage; changement de couleur; prrissables) On being removed from the plant, fruits and vegetables are deprived of their normal source of water, minerals and simple organic molecules which normally would be translocated to them from other parts of the plant. However, the harvested fruits and vegetables are still living as they continue to perform metabolic reactions and maintain the physiological systems as though they were still attached to the plant, while being solely dependent on their own reserves and moisture content. Therefore, losses of respirable substances and water are not replaced and deterioration begins. The length of time for which fruits and vegetables can be stored is a function of composition, resistance to attack by microorganisms and external factors such as temperature, relative humidity and various gases in the environment 1. Of the environmental factors affecting post-harvest changes of fruits and vegetables, temperature and relative humidity may be the most important factors. The quality of many horticultural products is judged by appearance, specifically colour. As the product changes (ripens or deteriorates) in storage, the colour undergoes a continuous change. Many physical, chemical and quality t To whom correspondenceshould be addressed PennsylvaniaAgricultural ExperimentStation Journal SeriesNo. 7750 0140-7007/90/030197-06 © 1990 Butterworth & Co (Publishers)Ltd and IIR

changes in foods have been found to follow the laws of reaction rate kinetics 2. Consequently, mathematical models for colour change that are based on laws of reaction kinetics would be most useful in the design and optimization of refrigerated storage. Such models have been developed for processed foods, however information is lacking for refrigerated storage conditions. The objective of this study was to determine the effect of storage temperature and relative humidity on colour change rates of tomatoes, mushrooms and apples, and to characterize these changes, whenever possible, by reaction rate models.

Materials and methods The colour changes of mature green tomato, mushroom and apple in cold storage were investigated at three temperatures (5, 10 and 15°C), and at four different relative humidity levels (91, 94, 97 and 100%) at each temperature. Experimental

Air-tight containers (0.28 x 0.42 x 0.25 m) were specially designed and fabricated in the laboratory using 0.006 m thick Plexiglass. Two litres of distilled water or solutions

Bey. Int. Froid 1990 Vol 13 Mai

1 97

Effects of storage conditions on co~our change. T. K. Pal and S. K. Sastry Data Loggger

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Figure 1 Schematic diagram of experimental set-up Figure 1 Schkma de l'installation exp&imentale Table 1 Actual relative humidity and temperature values obtained during experimentation Tableau 1 Valeurs r~elles de l'humiditb relative et de la tempkrature au cours de l'exp&ience

Relative humidity (%) Temperature Chamber (°C) 1

Chamber 2

Chamber 3

Chamber 4

Tomato

5.4 10.0 14.8

98.9 99.0 98.7

97.4 96.2 96.0

93.7 92.3 93.1

91.2 89.6 90.5

Mushroom

5.8 9.5 14.6

98.5 98.1 98.1

97.6 96.8 97.3

94.4 92.7 92.9

92.1 91.8 91.6

Apple

5.6 10.0 14.8

99.8 99.4 99.3

97.7 96.4 96.5

94.6 93.5 93.7

91.3 91.4 91.3

Sample

of certified sodium chloride for biological work (Fisher Scientific, Fairlawn, N J, USA) were placed at the bottom of each container to assist in maintaining a desired relative humidity. A one-way valve was used for ventilation to prevent a counter stream of air entering from the outside of the container. As illustrated schematically in Figure I, four storage chambers were placed in an environmental chamber (Model C10632H6M, Lunaire Environmental Inc., Williamsport, PA, USA) by which the temperature was controlled at test levels. To achieve the desired relative humidity (RH) in the storage chambers, distilled water and sodium chloride solutions of various concentrations were used (Table I). For 100% RH, distilled water was used; the sodium chloride concentrations were 5% for 97% RH, 9.5% for 94% RH and 14% for 91% RH. For the 5°C tests, sodium chloride solutions of higher concentrations were used. These concentrations of sodium chloride solutions were recommended for use in the temperature range 15-50°C 3.

198 Int. J. Refrig. 1990 Vol 13 May

Dry compressed air was first humidified by passing through distilled water and this pre-humidified air was then separated into four streams. These separated air streams were bubbled through three containers with "different salt solutions and one container with distilled water (for 100% RH). The actual relative humidities and temperatures deviated slightly from the intended values. The actual values are shown in Table I. The rate of air flow through each chamber was controlled by separate air flow meters (Model FMl l00-VI, Matheson Instruments Inc., Horsham, PA, USA). The flow rate was 0.00071 m 3 s- 1 of standard air through each chamber. For studies involving mushrooms, stem elongation was slowed by maintaining the carbon dioxide concentration within the chambers at 1% (accomplished by the introduction of carbon dioxide into the air stream). The temperature of each chamber was recorded every 15 min using a copper-constantan thermocouple (diameter 0.127ram; gauge 30; Omega Engineering Inc., Stamford, CT, USA) and a KAYE Digistrip III data logger (Kaye Instruments Inc., Bedford, PA, USA). Relative humidity of each chamber was recorded using a dew point hygrometer (Model 1200APS, General Eastern Instruments Co., Watertown, MA, USA) and a KAYE Digistrip III data logger.

Sample preparation Mushroom. A white strain of cultivated mushrooms (Agaricus bisporus), obtained from the Mushroom Test and Demonstration Facility, The Pennsylvania State University, was used in this study. Mushrooms were brought to the laboratory at the day of harvest and carefully brushed to remove the excess dirt. For each temperature, 24 mushrooms were used for colour change studies. Mushrooms were divided into four subgroups and each subgroup (six mushrooms each for colour studies) was placed in each storage chamber.

Effects of storage conditions on colour change: T. K. Pai and S. K. Sastry Plastic weighing boats were used for handling the mushrooms to avoid damage. Each experiment was performed for 8 days. The colour measurement was performed every other day.

Tomato. Fresh mature green tomatoes of variety Dombito were hand-picked from the Pepperidge Farm of the Campbell Institute for Research and Technology, Washingtonville, PA, USA. These tomatoes were brought to the laboratory within 2 h of harvest. For each temperature experiment, 24 tomatoes were used for the colour test. The tomatoes were divided into four subgroups and placed in each storage chamber. The colour measurement was performed every 4 days, with a total test duration (at each temperature) of 20 days. Apple.

Red Delicious apples were purchased from a local wholesaler (J. E. Nelson and Sons, Inc., Altoona, PA, USA). Forty-two apples were used for each test. Forty apples were divided into four subgroups and placed in the storage chamber. The tests were performed for 10 days each. Colour tests were performed at 5-day intervals.

Colour measurement A Gardner digital colorimeter (Model XL-10, Gardner Laboratory, Inc., Bethesda, MD, USA) was used for measuring the colour values of tomatoes and mushrooms. For tomatoes, the a value of the blossom end was measured. Since slow colour changes may be expected in cold storage, all tomato colour measurements were performed at the blossom end (where the colour changes begin) to ensure measurable changes during the experiment. For mushrooms, the L value of the cap was measured. A Hunter tristimulus colorimeter (Model D25L-9, Hunter Associates Laboratory, Inc., Reston, VA, USA) was used for apple colour measurements. For each apple, the a value of the side was measured at three points and averaged to assure the consistency of the measurement.

Data analysis For the colour change reaction kinetics, regression procedures were performed using programs developed by Minitab (University Park, PA, USA). Colour change rates at different temperature and relative humidity values were performed by (1) linearly regressing colour against time for each treatment, and (2) testing the independent regressions for homogeneity as described by Steel and Torrie 4.

Kinetics. To analyse the kinetics of colour change for each commodity, the following procedure was used. Unless otherwise stated, the change in colour for all commodities was initially modelled as a first-order reaction 2 which can be written as: ln( Co/C)= kt

(1)

where C O= the colour value of the sample at time zero; C = the colour value of the sample at time t when t = 0, C=Co; k=reaction rate constant (s -1 or d a y - l ) ; and t = time (s or day) By regressing the natural logarithm of colour ratio versus time, the values of reaction constant (k) were determined at various temperatures. Other reaction orders were used if the first-order fit was poor. For tomatoes, a values increased with time, changing

from negative to positive during the storage period. To prevent mathematical difficulties arising from negative colour ratios, the colour values of tomatoes were transformed to positive values by adding 10 (the minimum a value was -9.17). By substituting these values for a o and a:

(2)

ln[(C o + lO)/(C + 10)] = - k t

The relationship between the rate constant of a reaction and the temperature is described by the Arrhenius equation 5: k = Ae- e./Rr

(3)

where A =frequency factor (s-1 or day-1); Ea = activation energy (kcalkg -1 mol-1); R=universal gas constant (kcal kg-1 mol-1 K-1); and T = absolute temperature (K). A regression analysis of In k and 1/T was performed to determine Ea and A. Lag correction was not necessary because the storage period was long compared to the time required to cool the samples to the experimental temperatures. Results and discussion

Colour Tomato.

The rates of colour change (a value) of tomatoes at each RH during 20 days storage are shown in Table 2, along with the results of t-tests for the homogeneity of the independent regressions. All the colour change rates of the tomatoes at each RH level showed increasing slopes, indicating that the colour was changing from green to red. The rate of colour change of the tomatoes stored at 14.8°C was significantly (p < 0.05) higher than the rate for those stored at 5.4°C and 10.0°C. However, there was no significant (p>0.05) difference between the rates at 10.0°C and 5.4°C. At 5.4°C, the rate of colour change was very slow and there was no significant (p > 0.05) difference at different levels of RH. In the case of tomatoes stored at 10.0°C, relative humidity affected the colour change rate more than the tomatoes at other temperatures. However, the trend was not correlated with RH. The colour change

Table 2 Rate of change of colour for tomato at each RH during storage Tableau 2 T a u x de changement de couleur des tomates pour chaque H R , au cours de l'entreposage

Rate of colour change (a value per day) Relative humidity (%) Temperature (°C) 14.8 10.0 5.4

100~

97

94

91

Total

1.31

1.90

1.76

0.91

1.47

xa b

xb

xb

xc

x

1.09

0.10

0.08

0.34

0.40

xa

yc

yc

yb

y

0.08

0.10

0.09

0.13

0.I0

ya

ya

ya

za

y

a These values represent the expected humidities and may differ from the tested humidities b At each temperature, the rates having the same letter (a-c) are not significantly different (p < 0.05). At each RH, the rates having the same letter ( x - z ) are not significantly different (p<0.05)

Rev. Int. Froid 1990 Vol 13 Mai

199

Effects of storage conditions on co~our change. 7-. K. Pai and S. K. Sastry t4.0 C

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Colour change relationships with time for tomatoes Relations entre le changement de couleur des tomates et le

Reaction constants (k) for colour change Tableau 3 Constantes de rkaction (k) pour le changement de couleur

Table 3

Commodity

Colour parameter

Temperature (K)

k (day 1)

Correlation coefficient (%)

Tomato

a

Mushroom

L

Apple

a

278.4 283.0 287.8 278.8 282.5 287.6 278.6 283.0 287.8

0.101 _0.008 0.400+0.018 1.469_+0.045 0.016_+0.001 0.021 _+0.002 0.048+0.002 0.006 + 0.007 0.002 + 0.002 0.009 + 0.000

97.8 99.2 99.6 98.7 98.0 99.6 42.9 48.6 99.9

chromoplasts. This observation may account for the slow colour change at 5.4°C. The colour change relationships with time are shown in Figure 2. The reaction constants (k), obtained from the first-order reaction model, were changing with time at 14.8 ° C, which indicated that the assumed model did not fit. Therefore the change in colour was modelled as zeroand mixed-order reactions and reaction constants were calculated. Of these, the reaction constants obtained from the zero-order reaction fitted best. The reaction constants were 0.101, 0.400 and 1.469 d a y - 1 at temperatures of 5.4, 10.0 and 14.8°C, respectively (Table 3). These data show that the reaction can be modelled as zero-order for the colour change of tomatoes. The Arrhenius plot for Ink versus the inverse of absolute temperature is shown in Fiyure 3. The activation energy (Ea) for change of colour of tomatoes, calculated from the slope of the plot, was 45.15 kcal mol- 1 (Table 4).

Mushroom. The rate of loss of whiteness for mushrooms at each RH during storage is shown in Table 5, along with results of t-tests for the homogeneity of the independent regressions. The rate of loss of whiteness (the L value) of stored mushrooms showed a highly correlated linear relationship with storage time at each relative humidity. At 14.6°C, the rate of loss of whiteness was significantly (p < 0.05) higher than those at other temperatures. There was no significant (p>0.05) difference between the whiteness loss rates for the mushrooms stored at 9.5 and 5.8°C. At the same RH level, the rate of loss of whiteness became higher as the temperature increased, except the

Table 4 Activation energies (Ea) and frequency factors for colour change Tableau 4 Energies d'activation (E,) et facteurs de fi'Oquence du changement de couleur

2

~c:

1-

Commodity

Colour parameter

O-

Tomato Mushroom Apple

a L a

-1-

Ea (kcalmol 1)

Correlation coefficient (%)

Frequency factor (day- 1)

45.150+0.867 20.475+3.822 N/A ~

100.0 96.6 N/A

3.98 x 10 ~9 1.59 x 10 TM N/A

" N o t available: the value was not meaningful due to the lack of consistency of the data Table 5

Rate of loss of whiteness for m u s h r o o m at each RH during storage Tableau 5 Taux de perte de blancheur des champignons h chaque HR, au cours de l'entreposage

-4-

~5

3.4

3.5

t000 /

3.6

Rate of loss of whiteness (L value per day)

3,'/

T

Figure 3

Arrhenius plot for colour change (a value) for tomatoes Figure 3 Graphique d'Arrhbnius pour le changement de couleur des tomates (valeur a)

Relative humidity level (%) Temperature (°C) 14.6

at 99.0% RH was the highest at 10.0°C followed by 89.6% RH. Chlorophyll breakdown and the synthesis of lycopene are important in the colour change in tomato fruit. Moline 6 reported that mature green tomatoes stored at chilling temperatures failed to ripen normally. He suggested that this observation was partially due to the interruption in the c o n v e r s i o n of chloroplasts to

200

Int. J. Refrig. 1990 Vol 13 May

9.5 5.8

100~

97

94

91

Total

-3.10 xab b - 1.77 ya -1.04 za

-3.90 xa - 1.34 ya -1.17 yab

-3.27 xb - 1.67 ya -1.82 yb

- 3.47 xb - 1.66 ya -1.17 za

-3.34 x - 1.61 y -1.30 y

"These values represent the expected humidities and may differ from the tested humidities b At each temperature, the rates having the same letter (a, b) are not significantly different (p < 0.05). At each RH, the rates having the same letter (x-z) are not significantly different (p<0.05)

Effects of storage conditions on co~our change." T. K. Pai and S. K. Sastry .4

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20.475 kcal m o l - 1 (Table 4), which fell within the range of E a values ( 1 0 - 3 0 k c a l m o l - I ) for colour, flavour and texture changes suggested by Saguy and Karel 2.

•

°~ o

,~

,

Time Iday)

Figure 4 Colour ratio relationships with time for mushrooms

Figure 4 Relations entre le changement de couleur du champignon et le temps -2-

-2.5-

-3-

Apple. The colour changes of Red Delicious apples (a value) during storage are shown in Figure 6. The apples did not show any trend in rate of colour change at the same storage temperature except at 5.6°C. The rate of colour change of the apples showed a decreasing slope at each RH except the rate at 10.0°C with 96.4% RH (Table 6). At the same storage temperature, the apples did not show any significant (p > 0.05) difference in rates of colour change at different levels of RH. Except for the rates between 99.4 and 96.4% RH at 10.0°C and between 99.8 and 97.7% RH at 5.6°C, there was no significant difference in colour change rates at different temperatures at the same RH level. In Red Delicious apples, the colour of the skin changes from green to red, which may be due to the formation of a flavonoid compound idaein 9, although anthocyanin degradation may also play a role. Colour change implies that the a values of the skin of the apples are increasing. The apples used in this experiment were purchased from a

-3.5-

27 -4-

26

2sT

"

24 'a' value

Iooo / T

Figure 5 Arrhenius plot for colour change (L value) for mushrooms Figure 5 Graphique d'Arrh~nius pour le changement de couleur des champignons (valeur L)

rate at 9.5°C with 92.7% RH which was lower than that at 5.8°C with 94.4% RH. The loss of whiteness of mushrooms results for the most part from the activity of o-diphenoloxidase on phenolic compounds. These compounds are hydroxylated and then oxidized to the corresponding quinones by this enzyme. These quinones condense to form brown melanins 7. Minamide et al. 8 observed the loss of whiteness for pileus (cap) and stipe (stem) separately. After 8 days of storage at 6°C with 98% RH, the Hunter L values were about 60 and 40 for pileus and stipe, respectively. The stipe lost whiteness more rapidly than the pileus. Relative humidity ranges at the same temperature in this experiment did not cause a significant effect (p > 0.05) on the rate of loss of whiteness of the mushrooms. The rate of loss of whiteness became higher as the temperature increased. The whiteness ratio relationships with time are shown in Figure 4. The reaction constants were 0.016, 0.021 and 0.048day -1 at temperatures of 5.8, 9.5 and 14.6°C, respectively (Table 3). These data show that the reaction is of apparent first-order in loss of whiteness of mushrooms. The Arrhenius plot for logarithms of k versus the reciprocal of absolute temperature is shown in Figure 5. The activation energy (E,) for loss of whiteness of mushrooms, calculated from the slope of the plot, was

22 o--

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21 20

I

1

I

I

1

;2

3

4 5 6 time(day)

I

I

I

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7

8

9

10

Figure 6 Colour change (a value) of apples during storage (.) 5.6; (B) 14.8°C; (O) 10.0°C Figure 6 Changement de couleur (valeur a) des potatoes au cours de l'entreposage (o) 5.6°C; (l) 14.8°C; (0) 10.0°C Table 6 Rate of change of colour for apple at each RH during storage Tableau 6 Taux de changement de couleur des potatoes h chaque HR, au cours de rentreposage

Rate of change of colour (a value per day) Relative humidity level (%) Temperature (°C)

100a

97

94

91

Total

14.8

--0.07

--0.13

--0.19

--0.14

--0.13

xa

xa

-0.04

-0.16

xya b

xya

0.02

x

10.0

-0.03

-0.05

xa

xa

xa

xa

X

5.6

-0.24 yab

-0.29 ya

-0.13 xb

-0.20 xab

-0.22 x

aThese valuesrepresent the expectedhumidities and may differfrom the tested humidities bAt each temperature, the rates having the same letter (a) are not significantlydifferent (p < 0.05). At each RH, the rates having the same letter (x~y) are not significantly different (p<0.05)

Rev. Int. Froid 1990 Vol 13 Mai

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Effects of storage conditions on colour change: T. K. Pai and S. K. Sastry

local wholesaler a few months after the harvest, so they might be losing colour pigments. The colour data for apples were not consistent and the regression equations for the reaction constants were not highly correlated (Table 3), thus the kinetic data were not meaningful (Table 4). The experimental period of 10 days is apparently too short to indicate the colour change behaviour of apples having relatively long storage time.

References 1

2 3 4

Conclusions The colour change of tomatoes (a value) can be represented by a zero-order reaction with a frequency factor of 3.98 x 1079 day- 1 and an activation energy of 45.15 kcal m o l - 1. The loss of whiteness of mushrooms (L value) can be represented by a first-order reaction with a frequency factor of 1.59 × 1014 day- 1 and an activation energy of 20.48 kcal m o l - 1. The trends in colour change of apples were generally not clear, given the relatively short duration of this experiment.

202

Int. J. Refrig. 1990 Vol 13 May

5 6 7 8 9

10

Biale, J. B. Synthetic and degradative process in fruit ripening, in Postharvest Biology and Handling of Fruits and Vegetables (Eds N. F. Haard and D. K. Salunkhe) The AVI Publishing Co., Westport, CT, USA (1975) 5 Saguy, L and Karel, M. Modeling of quality deterioration during food processing and storage Food Technol (1980) 34 78 Chirife, J. and Resnik, S. L. Unsaturated solutions of sodium chloride as reference sources of water activity at various temperatures J Food Sci (1984) 49 1486 Steel, R. G. D. and Torrie, J. H. Linear regression, in Principles and Procedures of Statistics 2nd Edn, McGraw-Hill Book Co., New York, USA (1980) Ch 10, 239 Laidler, K. J. Reaction kinetcs, in Physical Chemistry with Biological Applications The Benjamin/Cummings Publishing Co., Menlo Park, CA, USA (1978) Moline, H. E. Ultrastructural changes associated with chilling of tomato fruit Phytopathology (1976) 66 617 Mathew, A. G. and Parpia, H. A. Food browning as a polyphenol reaction Adv Food Res (1971) 19 75 Minamide, T., Habn, T. and Ogata, K. Effect of storage temperature on keeping freshness of mushrooms after harvest J Food Sci Tech (Japan) (1980) 27 28l Hnlme, A. C. and Rhode, M. J. C. Pome fruits, in The Biochemistry of Fruits and their Products Vol 2 (Ed A. C. Hulme) Academic Press, London, UK (1971) 333 Spector, P. C., Goodnight, J. H., Sail, J. P. and Sarle, W. S. The GLM Procedure. SAS User's Guide: Statistics SAS Institute Inc., Cary, NC, USA (1985) 433