VITAMIN C AND P-CARO’IENE IN FRESH AND FROZEN GREEN BEANS AND BROCCOLI IN A SIMULATED SYSTEM

MNG WU, AIKO K. PERRY, and BARBARA P. KLEINl

Division of Foods and Nutrition University of Illinois at Urbana-Champaign

905 South Goodwin Ave. Urbana, Illinois 61 801

Accepted for Publication October 23, 1991

ABSTRACT

lhe reduced ascorbic acid and Pcarotene contents of “retail mark&-fresh” (RMF) and “fi.esh-frozen” (FF) green beans and broccoli in a simulated handl- ing system were determined. changes in ascorbic acid content in the two vegetables during a retail market period and frozen storage jbllowed different patterns. In green beans, ascorbic acid content decreased during refngerated storage f i r up to 7 days, buz in broccoli there was a significant increase. Blanching resulted in a loss of approximately 40% of the ascorbic acid in broccoli. Ascorbic acid content of FF green beans stored at - 20C for 16 weeks was approximately twice that in RMF green beans, but in FF broccoli stored under the same conditions it was only about half that of RMF broccoli. &carotene content of green beans and broccoli did not change during either the retail market simulation of frozen storage and did not differ from that of fresh.

INTRODUCTION

Consumption of fresh and frozen vegetables has increased by over 20% and lo%, respectively, in the past 20 years (Shewfelt 1990; Zind 1989). This increase reflects consumer response to dietary guidelines issued by a number of health- related organizations, as well as the suggestion that certain nutrients (vitamins A and C) may have a protective effect against cancer (HHS 1988; NRC 1982).

‘Correspondence should be addressed to: Dr. B.P. Klein, Division of Foods and Nutrition, Universi- ty of Illinois, 905 South Goodwin, Urbana, Illinois 61801; Phone: 217/333-1325.

Journal of Food Quality 15 (1992) 87-96. All Rights Reserved. 0 Copyright 1992 by Food & Nutrition Press, hc . , IfumbulI, Connecticut. 87

88 Y. WU, A.K. PERRY and B.P. KLEIN

Many consumers regard the nutritional value of fresh vegetables to be superior to processed (Zind 1989), but postharvest handliig and transportation can seriously affect nutrient content (Erdman and Klein 1982; Klein 1987; Salunkhe and Desai 1988). Freezing preservation results in losses of water soluble nutrients, particu- larly during the blanching step (Fennema 1988).

Numerous studies of the effects of freezing preservation on nutrient retention in vegetables were summarized by Fennema (1988). Investigations of fresh or “market-fresh” vegetables can also be found in the literature (Bushway 1986; Hudson et d. 1986; Perrin and Gaye 1986; Barth et ul. 1990; Vanderslice et ul. 1990). However, few of these studies made direct comparisons between fresh, stored and frozen products. Thus, differences due to maturity, planting location, fertilization, season, cultivar, variety, handling procedures and storage period were not eliminated.

In the research presented here, the frozen and fresh vegetables were from the same harvest lot, minimizing differences due to the factors mentioned above. A simulated “retail market-fresh” system was used to control the conditions of postharvest handling and storage. Green beans and broccoli, which contain both @-carotene and ascorbic acid and are two of the most commonly consumed vegetables according to FDA (Food Institute Report 1991), were chosen for this study.

The objective of this study was to compare the reduced ascorbic acid and @- carotene content of “ fresh-frozen” (FF) and “retail market-fresh” (RMF) green beans and broccoli in a simulated handling system.

MATERIALS AND METHODS

Vegetables

Green beans (cv. Bush Blue Lake 94) were grown and harvested by the Depart- ment of Horticulture of the University of Illinois at Urbana-Champaign. Broc- coli (cvs. Packman and Premium Crop) were grown in Tuscola, Illinois. Both commodities were hand-harvested. Green beans were harvested in July and September and broccoli was harvested in July, September, and October. Upon harvest, vegetables were transported to the Food Research Laboratory at the University of Illinois. One bushel was used per replication.

Sample Preparation

Green beans and broccoli were immediately cleaned with tap water after delivery to the Food Research Laboratory. Both ends of green beans were cut off and the edible parts were cut into 4 cm pieces. Broccoli was cut 10 cm long and the

VITAMINS IN FRESH AND FROZEN VEGETABLES 89

diameter of the florets was about 4 cm. Initial samples of sorted and cleaned vegetables were taken approximately 1 h postharvest. The remaining vegetables were divided into two groups for storage or processing.

Retail Market Fresh 0 Simulation. The vegetables were held for an additional 4 h postharvest at ambient temperature in the range of 20-25C (68-77F), refrigerated at 4C (40F) for 3 days to simulate transport time and temperature. Then the green beans were transferred to a produce display case at 10-16C (50-60F) under fluorescent light (Corrigan Misting System, Northbrook, IL. Misting apparatus was not used.), and broccoli was held at 1OC (50F) in a lighted walk-in cooler for 4 days to simulate holding conditions in supermarkets. A small portion of the vegetables was removed after 3 days of storage in display case or cooler and placed in a home refrigerator at 4C (40F) for 3 days to simulate consumers’ storage of the products at home, after purchase from the supermarket. The RMF vegetables were analyzed at the Food Research Laboratory immediately after arriving (1 h postharvest), after 4 h at ambient temperature (5 h postharvest), after 3 days of refrigeration, after 2, 3 and 4 days of display, and after 3 days of display plus another 3 days of refrigeration.

Fresh Frozen (FF) Vegetables. After sorting and cleaning, vegetable samples were prepared for freezing. A modification of the blanching procedure of Van Duyne et d. (1945) was used. Five weighed cheesecloth wrapped bundles of green beans (240 g each) or broccoli (180 g each) were blanched for 3 min in 8 L (green beans) or 12 L (broccoli) of boiling water. The vegetables were then cooled in cold running water for another 3 min and drained. As much excess water as possi- ble was removed from the surface of the blanched vegetables by blotting with absorbent cloth. The vegetables were placed in plastic bags and frozen immediately at -2OC for 2, 4, 8 or 16 weeks.

Analyses

Moisture content of raw RMF green beans and broccoli, vitamin C content of raw RMF and FF green beans and broccoli, and @-carotene contents of raw and blanched RMF and FF green beans and broccoli were determined. Three replications of green beans were done for the determination of vitamin C and 6-carotene, four replications of broccoli for vitamin C, and three replications of broccoli for determination of @-carotene.

Ascorbic Acid Determination. Reduced ascorbic acid (RAA) was determined by titration using the 2 ,ddichlorophenol indophenol method (Pelletier 1985). Ascorbic acid was extracted by homogenizing 50 g sample with 150 mL 3% metaphosphoric acid-acetic acid; the mixture was then diluted to 500 mL with the same solution.

90 Y. WU, A.K. PERRY and B.P. KLEIN

P-carotene Determination. The &carotene concentration of the samples was determined by a modification of the method developed by Dietz ef ul. (1988). The pigments from 20 g sample were extracted by homogenizing with an acetone: hexane mixture (4 : 6). The acetone in the extract was removed by washing with distilled water and the hexane was then evaporated with a Rotavapor. The pigments were diluted to 10 mL with methylene chloride. 0-carotene was determined by reversed phase high-performance liquid chromatography (rp-HPLC) using a mobile phase of methanol : acetonitrile : water mixture (880 : 90 : 30) and an ODs (CI8) column (15 cm) of 5 p particle size. Flow rate was 2 mL/min and the wavelength for detection was 460 rim. 0-carotene was identified by retention time and quantified by peak area. @-carotene was determined in raw RMF samples and also in samples taken at the same time that were blanched before extraction, to verify that carotenes were not being destroyed by enzymatic activity during the analysis. No significant differences in 0-carotene between raw and blanched samples were found (data not shown), so values given are for raw unblanched RMF vegetables.

Moisture Determination. Moisture was determined by drying a weighed ali- quot of ground sample in a holding oven overnight at 55C (131F) and in a vac- cum oven at 6OC (140F) to constant weight.

Statistical Analysis

Means, standard deviations, and variance were calculated. After the F test for treatments was shown to be significant, the means were separated by Fisher’s least significant difference (LSD) using a 5 % level of significance.

RESULTS AND DISCUSSION

Moisture Content

Table 1 shows the moisture contents of RMF and FF green beans and broc- coli. Minor changes in the moisture content of the two vegetables were observed during the retail market storage period. The moisture content of FF beans and broccoli was slightly higher than that of RMF beans and broccoli, respectively, probably due to some water that remained on the surfaces of the vegetables despite attempts to remove it by absorption.

Reduced Ascorbic Acid Content

The RAA content on the wet weight basis (WWB) of RMF and FF green beans and broccoli is shown in Table 1. The vitamin C content calculated on the dry

TAB

LE 1

. M

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TUR

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ASC

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BIC

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ID A

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CA

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AR

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ESH

(R

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A

ND

FR

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atm

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1 ho

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Reta

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5 ho

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3 da

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2 da

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spla

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day

disp

lay

4 da

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1 hou

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t har

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, bla

nche

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4

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8

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Fres

h Fr

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I 16

wee

ks fr

ozen

I M

ean

of 3

repl

icat

ions

Gre

en B

eans

' B

rocc

oliZ

91

.16

bcd

89.3

6 b

90.6

2 cd

89

.42

b 91

.40

bc

89.1

3 bc

90

.81

cd

89.0

5 bc

90

.09

d 88

.83

bc

90.3

9 cd

88

.61

c 90

.79

bc

89.2

1 bc

91.4

8 bc

91

.68

a 92

.29

ab

91.9

5 a

92.2

6 ab

91

.88

a 93

.08

a 91

.90

a 92

.13

ab

91.8

1 a

Gre

en B

eans

' B

rocc

oliz

16

.97

a 10

7.18

bc

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5 ab

11

6.34

a

7.14

cd

118.

29 a

8.

95 c

11

3.78

ab

8.70

c

105.

53 b

c 8.

40 c

d 98

.54

c

6.16

d

104.

47 c

14.1

1 b

62.8

8 d

13.0

8 b

59.6

2 d

12.8

6 b

58.9

9 d

13.0

5 b

57.7

6 d

11.8

3 b

51.9

0 d

2 M

ean

of 4

repl

icat

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M

eans

in a

colu

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with

sam

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ent a

t the

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leve

l.

Sree

n B

eans

l B

rocc

oli1

0.

29 a

0.

41 b

0.26

a

0.45

b

0.33

a

0.48

b

0.33

a

0.43

b

0.27

a

0.48

b

0.32

a

0.41

b

0.30

a

0.36

b

0.36

a

0.44

b

0.30

a

0.40

b

0.25

a

0.40

b

0.28

a

0.45

b

0.27

a

0.38

b

92 Y. WU, A.K. PERRY and B.P. KLEIN

weight basis (DWB) followed the same patterns (data not shown). Ascorbic acid content of green beans decreased during the simulated period of transportation, refrigeration, displaying in market and home refrigeration. Of the initial ascor- bic acid in the green beans on WWB or DWB, about 58% was lost during the first 3 days of refrigeration. The ascorbic acid content then remained constant for the next 3 days of refrigeration or 4 days of displaying. Ascorbic acid content (WWB) did not decrease during frozen storage at -2OC for 16 weeks. About 14% (DWB) or 17% (WWB) of the ascorbic acid in EF green beans was lost during the blanching process before freezing. The ascorbic acid content of FF beans was approximately twice as much as that in the RMF beans, presumably because of inactivation of enzymes by blanching. The RAA content of 1 h postharvest beans and 5 h postharvest beans in this study is in agreement with the value reported in Handbook No. 8-1 1 (USDA, 1984). The decrease in RAA in green beans between 5 h postharvest and 3 day refrigerated beans can be at- tributed to accelerated enzymatic and nonenzymatic degradation, due to time- temperature conditions and rate of wilting (Ezell and Wilcox 1959; Eheart and Odland 1972; Kramer 1979). More recently Albrecht et ul. (1991) observed major losses of ascorbic acid in green beans stored 2 or 3 weeks at 2C. In contrast, RAA in FF green beans did not decrease during frozen storage

at a constant temperature (-20C) for at least 16 weeks. This conclusion is in accord with results reported by Guadagni and Kelly (1958) who found that negligi- ble ascorbic acid was lost in frozen vegetables unless enzymes were not inac- tivated by blanching. According to these investigators, total and biologically ac- tive ascorbic acid remained at essentially the same level for a year or longer, if the vegetables were stored below - 18C.

The changes in ascorbic acid content of broccoli during retail marketing storage differed from green beans. Ascorbic acid content of broccoli increased about 4% (DWB) or 9% (WWB) between 1 and 5 h postharvest, remained constant for the next 3 days of refrigeration and 2 days of display and then decreased for the rest of the 4 day display period. Refrigeration for 3 days beyond the 3 day display time did not result in a significant decline. However, because of the initial postharvest increase in RAA, there was no significant difference between ascor- bic acid levels at harvest and at the end of the retail marketing period. This is in agreement with Albrecht d ul. (1991) who found that vitamin C in broccoli was remarkably stable over a period of several weeks of storage.

About 21 %I (DWB) or 40% (WWB) of ascorbic aicd was lost during the pro- cess of blanching before freezing. This was probably due to the large surface area of broccoli in contact with hot water during blanching, as well as the thin tissue of the florets. Steam blanching might have resulted in less loss (Fennema 1988). The ascorbic acid did not change for 16 weeks at -2OC, but the ascorbic acid content of frozen broccoli was only about 28% (DWB) or 55% (WWB) of

VITAMINS IN FRESH AND FROZEN VEGETABLES 93

that of 1 h postharvest broccoli because of losses during blanching. The RAA content of 1 h postharvest broccoli in this study is in the range of the values ob- tained by other researchers (Eheart and Odland 1972; Vanderslice et al. 1990).

That RAA content of broccoli increases for several days after harvest and then gradually decreases was also found by other investigators. Eheart and Odland (1972) reported that RAA in broccoli (cv. Waltham-29) increased about 36% be- tween 0 and 7 days at 2C. They suggested that sulfhydryl groups in broccoli might have prevented ascorbic acid loss during storage. Sulfhydryl groups were con- sidered to be a factor promoting the conversion of D-glucose and D-galactose to L-ascorbic acid after enzymatic oxidation to glycuronic acid (Isherwood and Mapson 1962). On the other hand, Albrecht et al. (1991) were not able to associate sulfhydryl content with vitamin C retention during storage of broccoli. Payne (1967) noted that the increase of ascorbic acid in corn frozen on the cob may be caused by either a physicochemical change in which ascorbic acid-protein bonds are ruptured or by an unidentified enzyme that regenerates ascorbic acid.

P-carotene Content

Table 1 shows the /3-carotene content (WWB) of RMF and FF green beans and broccoli. 0-carotene content of green beans and broccoli did not change significantly during the retail marketing period or during frozen storage at - 20C for 16 weeks on either WWB or DWB.

The 0-carotene values for both green beans and broccoli obtained in this study are lower than those given in Handbook No. 8-11 (USDA 1984), i.e., 668 IU of vitamin A or 0.400 mg of &carotene per 100 g for green beans and 1542 IU of vitamin A or 0.925 mg of 0-carotene per 100 g for broccoli. The values in this study are similar to those reported by Bushway (1986) and Bushway et al. (1986) who used HPLC to detect the &carotene content of a number of vegetables. The difference between the values of this study and those reported in Handbook No. 8-1 1 (USDA 1984) could be due to overestimation by the AOAC method. The AOAC method may be accurate for samples that only contain 0-carotene but gives high values for samples that contain a complex mixture because the AOAC method does not separate individual carotenoids (Simpson et al. 1985).

CONCLUSIONS

From the results described above, it can be concluded that the patterns of change of ascorbic acid content in different vegetables during simulated retail market and frozen storage conditions varies while @-carotene content is stable. Ascorbic acid content of green beans stored at - 20C for 16 weeks is approximately twice

w Y. WU, A.K. PERRY and B.P. KLEIN

that in retail market fresh green beans. But the ascorbic acid content of broccoli stored at -2OC for 16 weeks is only about half of that of retail market fresh broccoli. The @-carotene content of green beans and broccoli did not change dur- ing either a simulated retail market period or frozen storage and did not differ from that of fresh.

ACKNOWLEDGMENT

The authors gratefully acknowledge the financial support from The Frozen Vegetable Council.

REFERENCES

ALBRECHT, J.A., SCHAFER, H.W. and ZOTTOLA, E.A. 1991. Sulfhydryl and ascorbic acid relationships in selected vegetables and fruits. J. Food Sci. 56, 429.

BARTH, M.M., PERRY, A.K., SCHMIDT, S. and KLEIN, B.P. 1990. Misting effects on ascorbic acid retention in broccoli during cabinet display. J. Food Sci. 55, 1187.

BUSHWAY, R.J. 1986. Determination of @and &Carotene in some raw fruits and vegetables by high performance liquid chromatography. J. Agric. Food Chem. 34, 409.

BUSHWAY, R.J., YANG, A. and YAMANI, A.M. 1986. Comparison of Alpha- and Beta-carotene content of supermarket versus roadside stand produce. J. Food Qual. 9, 437.

DIETZ, J.M., KANTHA, S.S. and ERDMAN, J.W. 1988. Reversed phase HPLC analysis of alpha and beta carotene from selected raw and cooked vegetables. Plant Foods Hum. Nut. 33, 341.

EHEART, M.S. and ODLAND, D. 1972. Storage of fresh broccoli and green beans. J. Am. Diet. Assoc. 60, 402.

ERDMAN, JR,, J.W. and KLEIN, B.P. 1982. Harvesting, processing, and cook- ing influences on vitamin C in foods. In Ascorbic Acid: Chemistry, Metabolism, cad UsesAdvances in Chemistry Series 200, (P.A. Seib and B.M. Tolbert, 4 s . ) p. 500. American Chemical Society, Washington, DC.

EZELL, B.D. and WILCOX, M.S. 1959. Loss of vitamin C in fresh vegetables as related to wilting and temperature. J. Agric. Food Chem. 7, 507.

FENNEMA, 0. 1988. Effects of freeze preservation on nutrients. In Nutritional Evaluation of Food Processing, 3rd ed. (E. Karmas and R.S. Harris, eds.) p. 269, Van Nostrand Reinhold/AVI, New York.

VITAMINS IN FRESH AND FROZEN VEGETABLES 95

FOOD INSTITUTE REPORT. 1991. Nutritional Labeling of Produce, Seafood; Voluntary-or Else! Food Institute Report, July 6, 1991. American Institute of Food Distribution, Inc., Fairlawn, NJ.

GUADAGNI, D.C. and KELLY, S.H. 1958. Time-temperature tolerance of frozen foods. XIV. Ascorbic acid and its oxidation products as a measure of temperature history in frozen strawberries. Food Technol. 22, 645.

HHS. 1988. l?ze Surgeon General’s Report on Nutrition and Health. U.S. Dept. of Health and Human Services. U.D. Govt. Print. Office, Washington, DC.

HUDSON, D.E., CAPPELLINI, M. and LACHANCE, P.A. 1986. Ascorbic acid content of broccoli during marketing. J. Food Qual. 9, 31.

HURT, H.D. 1979. Effect of canning on the nutritive value of vegetables. Food Technol. 33(2), 62.

ISHERWOOD, F.A. and MAPSON, L.W. 1962. Ascorbic acid metabolism in plants. Part 2: Biosynthesis. Ann. Rev. Plant Physiol. 23, 329.

KLEIN, B.P. 1987. Nutritional consequences of minimal processing of fruits and vegetables. J. Food Qual. 20, 179.

KRAMER, A. 1979. Effects of freezing and frozen storage on Nutrient reten- tion of fruit and vegetables. Food Technol. 33(2), 58.

NRC. 1982. Diet, Nutrition and Cancer. The Committee on Diet, Nutrition and Cancer. National Academy of Sciences. The American Institute for Cancer Research, Falls Church, VA.

PAYNE, I.R. 1967. Ascorbic acid retention in frozen corn. J. Am. Diet. Assoc. 52, 344.

PELLETIER, 0. 1985. Vitamin C (L-ascorbic and dehydro-L-ascorbic acids). In Methods of Eftamin Assay, 4th ed., (J. Augustin, B.P. Klein, D. Becker and P.B. Venugopal, 4 s . ) John Wiley & Sons, New York.

PERRIN, P.W. and GAYE, M.M. 1986. Effects of simulated retail display and overnight storage treatment on quality maintenance in fresh broccoli. J. Food Sci. 51, 146.

SALUNKHE, D.K. and DESAI, B.B. 1988. Effects of agricultural practices, handling, processing and storage on vegetables. In Nutritional Evaluation of Food Processing, 3rd ed., (E. Karmas and R.S. Harris, eds.) p. 23, Van Nostrand Reinhold/AVI, New York.

SHEWFELT, R.L. 1990. Quality of fruits and vegetables. Food Technol. 44(6), 99.

SIMPSON, K.L., TSOU, S.C.S. and CHICHESTER, C.O. 1985. Carotenes. In Methods of vitamin Assay,, 4th ed., (J. Augustin, B.P. Klein, D. Becker and P.B. Venugopal, 4 s . ) p. 185, John Wiley & Sons, New York.

USDA. 1984. Agriculture Handbook Number 8-11: Composition of Foods: Vegetables and Vegetable Products, Raw, Processed, Prepared, p. 31, United State Department of Agriculture, Washington, DC.

96 Y. WU, A.K. PERRY and B.P. KLEIN

VANDERSLICE, J.T., HIGGS, D.J., HAYES, J.M. and BLOCK, G. 1990. Ascorbic acid and dehydroascorbic acid content of foods-as-eaten. J . Food Comp. Anal. 3, 105.

VAN DUYNE, F.O., CHASE, J.T., RETZER, J.L. and SIMPSON, J.I. 1945. Retention of ascorbic acid in vegetables preserved by quick freezing, Proc. Inst. Food Technologists, 13.

ZIND, T. 1989. Fresh trends '90-A profile of fresh produce consumers. Packer Focus 96, 37.