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

1

Effects of microwave blanching vs. boiling water
blanching on retention of selected water-soluble
vitamins in turnip greens using HPLC M. A.
OSINBOYEJO, L. T. Walker, S. Ogutu, and M.
Verghese. Department of Food Animal Sciences,
Alabama AM University, PO Box 1628, Normal, AL
35762







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.
• METHOD AND MATERIALS
• 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 4ºC 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

• Statistical Analysis
• Data were analyzed by ANOVA, and differences in
  means were determined using Tukeys studentized
  range tests with SAS statistical program, 2001,
  Version 8. Differences were considered
  significant at P?0.05.

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.

• CONCLUSIONS
• 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.

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

• REFERNCES
• 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

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 20.0a 536.0a 3.06a 14.0a
Microwave blanching 14.2b 398.0b 2.54a 13.0a
Boiling water blanching 0.17c 0.01c 0.01b 0.01b

ACKNOWLEDGEMENTS This material is based upon work
supported by the CSREES/USDA, under agreement No.
00-51110-9762
abcMeans in the same column with the same letter
are not significantly different by Tukeys
Studentized Range (HSD) Test (Plt0.05).