Transcript Document

Assessment of Microwave Blanching as a Preparatory Tool for
Home Freezing of Yellow Squash (Cucurbita pepo)
J. ROBERTS, L. T. Walker and J.C. Anderson
Department of Food & Animal Sciences
Alabama A & M University, P.O. Box 1628, Normal, AL 35762
ABSTRACT
Microwave blanching of fruits and vegetables has been identified as a process that retains nutrients better than conventional blanching methods (boiling water and steam). Only low energy level (500-700 Watt) microwaves using
selected vegetables have been investigated in the past. Further studies are necessary to determine the effect of today’s higher energy level microwaves on the blanching of vegetables. Yellow squash was blanched in covered containers
for 3 min using: boiling water (BW), steam (ST), and 3 microwaves (1000 watt - MW1, 1200 watt - MW2, and 1300 watt - MW3). Samples were ice-cooled, placed in freezer bags, and stored at –18oC for 6 months. Enzyme activity,
physico/chemical, nutritional and sensory parameters were assessed during and following 6 months of frozen storage. Peroxidase (POD) activity decreased from 2.77-4.03 units (unblanched - UB) to 0.005-0.138 units after initial
blanching. The MW3 treatment retained 96% Fe (170 mg/kg) and 93% K (2133 mg/kg), which was significantly higher than the other microwave blanch treatments. Total ascorbic acid (TAA) retention was highest (14.4 mg/100g) for
the ST blanched treatment (97%). There were no significant TAA retention differences among the MW3 and ST treatments. Texture values were least firm for the BW treatment (66 Newtons) due to an increased cooking effect. Sensory
preference scores indicated a level of acceptance that was no different from the commercial (control) product. The study indicated that the overall quality of MW (all 3 energy levels) blanched yellow squash was as good as or superior
to BW and ST blanching methods. The availability of this and other such information to home preservers of fruits and vegetables could lead to a higher quality of products for consumption.
Texture. Unblanched yellow squash shear force was 128 N at initial
Microwave ovens are now being employed for meal preparation and food
POD enzyme activity in fresh, unblanched yellow squash varied from 2.77-4.03 measurement. Texture was firmest for the ST blanched treatment (106
preservation instead of conventional stove top and conventional oven
units to 0.005-0.138 units after initial blanching. POD activity was retarded the N) and least firm for the BW blanched treatment (66 N) after initial
approaches.
most for MW1, MW2, MW3 and BW treatments at initial blanching and after 4 blanching (Figure 3). There were no significant textural differences
(p>0.05) among the three MW and ST treatments at initial blanch and
Most vegetables require a short heat treatment called blanching. It is the primary and 6 months frozen storage. Essentially, no POD regeneration occurred for all
after 4 months frozen storage.
means of inactivating oxidative enzymes present in vegetables and fruits in order blanching treatments (Figure 1). The ST treatment was significantly higher
to preserve quality prior to and during freezing and for reducing the surface
200
(p<0.05) than the other blanching treatments for POD activity.
INTRODUCTION
RESULTS and DISCUSSION
microbial load. Blanching also aids in removing tissue gases, shrinking the
product, peeling, cleaning and stabilizing color.
Conventional blanching processes utilize rapidly boiling water or steam as a
heating medium and result in leaching of solids which reduces nutritional quality
(Brewer, 2002).
Twenty-first century microwave blanching has proven to be 1) the most
economically efficient, 2) better at retaining the nutrient content of treated fruits
and vegetables, and 3) a better time saving method for the home preparation of
vegetables for freezing (Barlow, 1998). Since over 93% of United States
households own a microwave (IMPI, 2003), it is imperative that microwave
blanching as a form of food preparation for home freezing be researched and
usage guidelines communicated to the public.
OBJECTIVE
The goal of this study was to compare the effects of microwave blanching of
yellow squash utilizing three levels of microwave energy (MW1, MW2 and
MW3) with those of conventional blanching (BW and ST). The objective of this
study was to to assess enzyme activity, chemical, physical, nutritional and
sensory parameters following 6 months of frozen storage of microwave blanched
yellow squash.
0.14
Shear Force (Newtons/15g )
Enzyme Activity (units POD/mL filtrate)
0.16
Initial
4 mo
6 mo
0.12
0.1
0.08
0.06
180
Initial
160
4 mo
140
6 mo
120
100
80
60
40
20
0
UB
0.04
MW1
MW2
MW3
BW
ST
Treatments
0.02
Figure 3. Shear Force Resistance of Unblanched and Blanched Yellow
Squash at all Storage Phases
0
MW1
MW2
MATERIALS and METHODS
BW
ST
Treatments
Sample Preparation
Yellow squash was harvested fresh in mid July. The vegetables were rinsed with
tap water three times (to remove dirt and debris), blanched, and assayed for
peroxidase activity within 4 hr of harvest. Yellow squash (200 g samples) was
blanched for 3 min by three methods in covered containers using the required
amounts of water: BW (1900 mL), ST (300 mL), and MW1, MW2, & MW3 (60
mL). Blanching time and proportion of vegetable/water were based on average
times for BW and ST recommendations. MW blanch time was established in a
previous study. This was the microwave blanch time required to inactivate POD
activity. The vegetables were then ice-cooled for 5 min and drained. Samples
were removed and packed in 1-L plastic freezer bags until further analyses.
MW3
Figure 1. POD Enzyme Activity of Yellow Squash at Initial Blanching
and after 4 and 6 Months Frozen Storage
Minerals. MW3 blanch treatment retained 94% Ca (197 mg/kg), 96%
Fe (170 mg/kg), 99% K (2133mg/kg) and 69% Na (82 mg/kg) for
yellow squash. The MW3 blanch treatment was significantly greater for
potassium retention compared to the other microwave blanch treatments.
There were no differences in the blanch treatments for calcium retention,
but the MW3 blanch treatment was significantly greater than the BW and
ST blanch treatments for iron and sodium retention (Table 1).
Analyses
Peroxidase activity, minerals (calcium, iron, potassium and sodium), total
ascorbic acid and texture of unblanched and blanched yellow squash were
determined. Sensory evaluation was determined after 6 months on frozen and
cooked yellow squash.
Table 1. Mineral Retention in Yellow Squash Originally and after 6 Months
Frozen Storage
Treatment
Peroxidase (POD) Activity
POD activity was determined spectrophotometrically as described by
Chance and Maehly (1955) & revised by Sigma-Aldrich (1994).
Absorbance (420 nm) was read at 20 sec intervals for 3 min. The
unblanched vegetable was used as the control. Enzyme activity was
expressed as units POD/mL vegetable filtrate.
Minerals (Ca, Fe, K and Na)
A microwave-assisted acid digestion procedure for preparing samples
(based on US EPA Method 3051 for soil analysis and modified for
appropriate foods) was used to prepare the vegetable samples for analyses
(Pingitore, 1996). The digestate was analyzed using Inductively Coupled
Plasma (ICP) Spectrometry and concentrations expressed in mg/kg (SW846, 1994).
Ca
Fe
K
Na
(mg/kg)
(mg/kg)
(mg/kg)
(mg/kg)
UB
209a
178a
2150a
119a
MW1
177a
124bc
1227b
51c
MW2
190a
150abc
1398b
62bc
MW3
197a
170ab
2133a
82b
BW
194a
117c
750c
47c
ST
99a
109c
1182bc
50c
Sensory Evaluation. The results revealed that after 6 months frozen
storage, there were no significant differences in preference among the
treatments (MW1, MW2, MW3, BW, ST and Control).
CONCLUSION
The three MW blanch and BW blanch treatments decreased
POD activity better than the ST blanch treatment. Mineral
losses may be attributed to interactions with other chemical
compounds and large volumes of water used in the BW blanch
treatment. TAA losses during the blanching process occurred
mostly by leaching of soluble solids or aqueous extraction
rather than by chemical degradation. Also, Ball (1997) stated
that ascorbic acid oxidase and lipoxidase present in squash can
oxidize TAA by generating free radicals from the oxidation of
polyunsaturated fatty acids, which in turn can react with and
damage TAA. This could also explain some TAA loss in
yellow squash. Texture shear force values were least firm for
the BW blanch treatment which was probably due to a
greater cooking effect caused by greater amounts of heat
produced by boiling water. The preference scores for this
experiment indicated that a level of acceptance that was not
different from the commercial (control) vegetable.
The study indicated that the overall quality of MW blanched
yellow squash for all three energy levels was as good as or
Total Ascorbic Acid retention was highest for the ST blanch treatment (97%,
Total Ascorbic Acid (TAA)
superior to BW or ST blanched methods. These microwaves
TAA was determined by HPLC using a UV detector set at 272 nm. The
14.4 mg/100g) followed by the MW3 blanch treatment (87%, 12.9 mg/100g).
when used as a preparatory blanching step for freezing of
analytical column was a 250 x 4.6 mm x ¼ in Valco Microsorb (MV100-5) MW1 and MW2 blanch treatments were significantly less in TAA retention than
yellow squash could yield a higher quality product for the home
column. The method described by Russell (1986) was utilized for this
the MW3 blanch treatment but equivalent to the BW blanch treatment
experiment. The mobile phase consisted of 9.5% acetonitrile in DD water,
consumer.
abcMeans
0.4 L/L ammonium hydroxide, 0.95 g/L hexane sulfonic acid (pH to 2.8
with phosphoric acid). Concentration of TAA was expressed as mg/100g.
(Figure 2).
Texture
A TMS-Texturepress (model FTA-300 Force Transducer) was used for
texture evaluation. Homogeneous samples of chopped, unblanched &
blanched vegetables (15 g) were used to fill the Allo-Kramer ten blade test
cell (Model CS-2 Thin Blade Shear-Compression). A one-bite mode test
was performed on each sample (Ponne, 1994; Bourne, 2002). The
transducer cal number was 780 and the transducer speed was set to 1.
Texture was determined as maximum force and expressed in Newtons (N).
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SELECTED REFERENCES
Bourne, M.C. (ed). 2002. Food texture and viscosity: concept and
measurement. 2nd ed. Academic Press, San Diego, CA.
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TAA (mg/100g)
Sensory
A 30 or more member consumer sensory panel (Alabama A&M University
faculty, staff and students) used the Multiple-Paired Comparison Test to
evaluate the single attribute, preference. Commercial frozen yellow squash
was used as the control versus the other 5 blanch treatments for this
characteristic. Sensory evaluation was conducted only on cooked yellow
squash after 6 months of frozen storage (Meilgaard et al.,1999).
with the same superscript letter in a column are not significantly different (p>0.05).
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Chance, B. & Maehly, A.C. 1955. Methods in Enzymology. 2:773-775.
8
EPA Method 3051. 1994. From SW-846 Online.
http://www.epa.gov/epaoswer/hazwaste/test/3series.htm.
6
Meilgaard, M. et al. 1999. Sensory Evaluation Techniques, 3rd Ed., CRC
Press, Inc., Boca Raton, FL.
4
2
Pingitore, N.E. et al. 1996. The Border Basket: Analysis of Toxic Metals in
Retail Foods, El Paso-Juarez. SCERP Project # EHPP961VI-2. The Univ.
of Texas at El Paso.
0
Statistical Analyses
Data were analyzed by ANOVA (analysis of variance) and significant
different (p<0.05) means were determined using Tukey’s HSD test (SAS,
2001). Values were reported as the mean of the four replicates.
Statistical analysis for sensory evaluation used Friedman’s analysis
T test
(Meilgaard et al, 1999). Significant differences (p<0.05) were
determined
using Tukey’s HSD test.
UB
MW1
MW2
MW3
BW
ST
Russell, L.F. 1986. J. Food Science, 51(6):1567-68.
Treatments
Figure 2. Total Ascorbic Acid Retention in Unblanched and Blanched Yellow
Squash after 6 Months Frozen Storage
Sigma-Aldrich, Inc. 1994. Enzymatic assay of Peroxidase (EC 1.11.1.7).
Sigma quality control test procedure for Sigma product P-1432.
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.