Effects of microwave blanching vs. boiling water blanching on retention of selected water-soluble vitamins in turnip greens using HPLC

Paper 92A-8. Presented at the Institute of Food Technologists Annual Meeting, Chicago, IL, July 15, 2003.

Abstract

Blanching is an effective way of preserving fruits and vegetables. However, it has been shown that conventional boiling water blanching of vegetables results in the leaching of water-soluble vitamins. This experiment was designed to determine the effectiveness of different blanching methods on the retention of selected water-soluble vitamins in turnip greens. The objective was to employ a HPLC method in the determination of the level of selected water-soluble vitamins in turnip greens that were blanched using conventional and microwave blanching methods. Turnip greens (Brassica rapa) were purchased from a local supermarket. They were thoroughly washed, chopped and separated into three treatment groups including unblanched (UB) which served as the control; boiling water blanched (BWB); and microwave (1300 watts) blanched (MWB). A 100 gm sample from each treatment group was subjected to blanching treatment (according to designation) for 5 minutes. The samples were cooled in iced-water and an extract prepared using a modification of a method previously described by Russell (1986). A 10µl sample (in duplicate) from each treatment extract was separately injected in a Varian ® HPLC with a C18 column and a UV detector set at 272nm. Concentrations of ascorbic acid, folic acid, thiamin and riboflavin were determined using external standards.

The result showed that, compared to control samples, BWB lost 16% ascorbic acid, and 100% folic acid, thiamin and riboflavin while MWB lost 28.8% ascorbic acid, 25.7% folic acid 16.9% thiamin and 7.2% riboflavin.

The results indicate that MWB is more effective in the retaining the selected water-soluble vitamins with the exception of ascorbic acid. This is also in congruence with earlier findings indication that microwave blanching is more effective in retaining nutrients in vegetables.

Introduction

Blanching is the process of exposing vegetables or fruits to high temperature for a short period of time.  It is done not only to prolong the self-life of vegetables by inactivate the enzymes responsible for browning (lipoxygenase and peroxidase) but also improves both color and flavor. Proper blanching is important as under-blanching is ineffective in inactivating the enzymes that reduce the quality while over-blanching can result in overall quality reduction and the leaching of essential vitamins and minerals. Conventionally blanching is done through the use of boiling water or steam, however microwave blanching may be a suitable alternative which could lead to improved overall quality and retention of essential minerals and vitamins. However the high cost of the equipment made the use uncommon in food industries. Boiling water blanching is most used at home. It is easy, simple and inexpensive, but has highest potential of leaching water-soluble vitamins and minerals compared to other methods. Convectional steam blanching is currently the most commonly used method in the food industry today. It is relatively inexpensive and retains minerals and water-soluble vitamins over-boiling water blanching.

Objectives

• The objective was to employ a HPLC method to determine the level of selected water soluble vitamins in turnip greens blanched using boiling water and microwave.

Materials and Methods

• Turnip greens (Brassica rapa) were purchased from a local supermarket.
• They were thoroughly washed, chopped (approximately 0.5 inches) and separated into three treatment groups.
• From each group 100g samples were separated for microwave blanching (MWB), boiling water blanching (BWB) and control-unblanched (UB).
• Microwave blanching was conducted using a domestic Panasonic Genius microwave with 1300 wattage for 5 min in a 2 quart/2 liter Pyrex container containing 60 ml tap water.
• Boiling water blanching was conducted in a 2 quart/2 liter enamel double boiler containing 1900 ml tap water. Boiling was conducted for 5 min.
• The sample was cooled in iced water and extracted.

Preparation of samples

• The extract from each treatment and control were prepared by blending 100g in 100g of 6% metaphosphoric Acid for 60 seconds.
• 50g of the sample was mixed with 35ml methanol and centrifuged at 13,300 X G at 4°C for 15mins.
• A10 ml aliquot of the supernatant was diluted to 100ml with 1.5mM pyrogallol and passed through a 0.45µm filter.
• A10µl of each extract was separately injected into a Varian HPLC with a C18 column with a mobile phase flow rate of 2ml/min and detected using a UV detector set at 272nm.
• 1mg of each vitamin was diluted with1000µl of mobile phase and 10 µ l was injected in the HPLC for analysis.

Preparation of Mobile Phase

• 9.5% acetonitrile in water containing 0.4µl NH4OH (Ammonium Hydroxide) and 0.9 of hexane sulphonic acid pH adjusted to 2.8 with 85% of phosphoric acid.

Statistical Analysis

• Data were analyzed by ANOVA, and differences in means were determined using Tukey's studentized range tests with SAS statistical program, 2001, Version 8. Differences were considered significant at P0.05.

Results and Discussion

• The results showed that boiling water blanching lost 99.9% ascorbic acid, 100% folic acid, thiamine and riboflavin while microwave blanching lost 28.8% ascorbic acid, 25.7% folic acid, 16.9% thiamine and 7.2% riboflavin when compared to control.

Figure 1: A comparison of retention of water soluble vitamins in turnip greens blanched in boiling water vs. microwave blanching

Table 1: Effect of microwave blanching vs. boiling water blanching on water soluble vitamin content on turnip greens (mg/100g)

	Ascorbic Acid	Folic Acid	Thiamine	Riboflavin
Control	20.0	536.0	3.06	14.0
Microwave blanching	14.2	398.0	2.54	13.0
Boiling water blanching	0.17	0.01	0.01	0.01

Conclusion

• The results indicate that Microwave blanching was more effective in retaining water-soluble vitamins in turnip greens. This is also in congruence with earlier findings indicating that microwave blanching is more effective in retaining nutrients in vegetables compared to conventional blanching methods.

Selected References

• Frank L. Vandemark (1981). Analysis of Water-Soluble Vitamins. J. Liquid chromatography. Vol. 4: 1157 Liquid Chromatography Applications
• Laurence L. Saettel (2000) Turnip Brassica rapa.
• (www.dietobio.com/aliments/en/turnip.html)
• Owen R. Fennema. (1996) Food Chemistry 3rd Edition Marcel Dekker, Inc. pp 541, 543, 561, 577
• Russell, L.F.(1986) High Liquid Chromatographic Determination of Vitamin C in Fresh Tomatoes. J. Food Science. Vol.51, No 6: 1567

This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 00-51110-9762.

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Permission is granted to reproduce these materials in whole or in part for educational purposes only (not for profit beyond the cost of reproduction) provided the authors and Alabama A&M University receive acknowledgment and this notice is included:

Reprinted with permission of Alabama A&M University. M. A. OSINBOYEJO, L. T. Walker, S. Ogutu, and M. Verghese. 2003. Effects of microwave blanching vs. boiling water blanching on retention of selected water-soluble vitamins in turnip greens using HPLC. Normal, AL: Alabama A&M University, Food and Animal Sciences Department.

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