Proc. Fla. State Hort. Soc. 91:112-114. 1978.
SECTIONING GRAPEFRUIT BY ENZYME DIGESTION
Joseph H. Bruemmer, Alicia W. Griffin
U. S. Citrus and Subtropical Products Laboratory1
P. O. Box 1909, Winter Haven, Florida 33880
Oladipo Onayemi
University oflFE,
Department of Food Science & Technology
ILE-IFE, Nigeria
Additional index words, pectinase.
Abstract. Mature 'Duncan' grapefruit were vacuum in
fused with a solution of commercial pectinase complex to
evaluate enzyme digestion as a method for sectionizing.
Treatment with 100 ppm enzyme effectively loosened the
peel and segment membranes and freed the intact juice
sections, which could then be easily removed. A 16-member
panel of tasters scored the sections "like moderately" for
flavor and texture. Six commercial pectinase complexes were
evaluated for effectiveness in preparation of intact sections
at standardized treatment conditions (122°F for 30 min). The
pectinases contained cellulase, pectinesterase, polygalacturonase and polymethylgalacturonase. Their effectiveness in
peeling correlated best with the activities of polygalacturonase and polymethylgalacturonase. The major advantage
of enzymic digestion is high recovery of high quality sections.
Because the sections are not cut from the membrane, loss of
juice is minimal and dry packaging could be used for market
ing the product.
The small portion (2 million boxes) of grapefruit pro
duction that is processed as sections has remained relatively
constant over the past 5 years. Current processing technology
is a major deterrent to expanded markets for sections. Proc
essing costs are high because sections must be cut from the
membranes and the yield is only about 60% of the total
juice segments. Quality is low because steam peeling, lye
finishing, and heat pasteurization change the fresh grapefruit
flavor and texture. Finally, treating and disposing the
alkaline waste water from the lye finisher create problems
for the processor. We describe and evaluate the use of en
zyme digestion for peeling and sectionizing grapefruit.
Materials and Methods
Grapefruit (Citrus paradisi Macfad. cv. Duncan) were
obtained from a local packer. Pectinases were provided by
Grinsted Products, Inc., Overland Park, Kansas; G. B.
Fermentation Industries, Inc., Des Plains, Illinois; Novo
Laboratories Inc., Wilton, Connecticut; Miles Laboratories,
Inc., Elkhart, Indiana; Enzyme Development Corporation,
New York, New York, and Ciba-Geigy Corporation, Ardsley,
New York. Pectin, N.F. #3442 and polygalacturonic acid
#3491
(PGA) were products of Sunkist Groves, Inc.,
Corona, California. Carboxymethylcellulose (CMC) was
obtained from Dow Chemical Co., Midland, Michigan.
Gelatin was obtained from a local food supplier.
^Science and Education Administration, U. S. Department of Agri
culture, Agricultural Research. Mention of a brand name or trademark
is for identification only and does not constitute endorsement of the
product by the U. S. Department of Agriculture over others which may
also Y>
tofijte
112
Preliminary tests
In our first attempt to peel grapefruit by vacuum in
fusion with solutions of commercial pectinase, we handscored the peel of a grapefruit in quadrants and placed it in
a solution of 1.0% Irgazyme (one of the commercial
pectinases used later in the study) in a vacuum infusion
apparatus, a heavy-walled glass desiccator connected to a
vacuum pump. We held a vacuum of 28 in. Hg in the ap
paratus for about 5 min, then released the vacuum and re
placed the gas in the fruit with about 100 ml of pectinase
solution. The grapefruit and solution were maintained at
room temperature. Within several minutes after infusion,
the peel began to slowly disintegrate. The peel was easily
removed, and the slimy coating covering the fruit washed
away under the faucet with a gentle stream of water. The
peeled, washed grapefruit was then easily separated into
hemispheres, which were then fanned out on a flat surface.
The segment membranes were easily separated from the
vesicular tissue, so that intact sections could be removed.
None of the juice sections were broken.
In preliminary tests, we standardized the pectinase treat
ment and determined the minimum effective concentration
for each brand of commercial pectinase used. Ease of peel
ing, lack of adhering albedo, ease of removing sections from
segment walls, and appearance of sections were evaluated
and given scores of 1 = good, 2 = fair, or 3 = poor. Con
centrations for each brand were selected so that 45 min of
treatment at 50°C (122°F) was adequate for effective treat
ment (Table 1).
Table 1. Minimum concentration of 6 brands of pectinases required for
effective peeling and sectionizing.2
Pectinase brands
Pectinase
cone; ppm
Treatment
A
B
C
D
E
F
100
100
(1)
(1)
500
(5)
1000
(10)
1000
(10)
3000
(30)
9
11
7
7
13
7
7
scores
15 min
30 min
45 min
10
6
5
8
6
4
4
4
8
4
4
^Criteria of effective peeling and sectionizing: ease of peeling, lack of
adhering albedo; ease in removing sections from segment walls and
appearance of sections. Scores were determined for each treatment by
assigning the values 1 = good, 2, = fair and 3 = poor for each criterion.
Values are the average sums of the four scores for six fruit.
Peeling and sectionizing procedure
The effectiveness of pectinase depends on temperature as
well as time. The manufacturers of the various pectinases
recommended 122°F2 as the optimum temperature for
pectinase infusion. To do this without cooking the fruit, we
submerged the grapefruit, which had initial temperatures of
68°-77°F, for 30 min in a water bath at 140°F. At the end of
the soak, the temperature of the albedo was 122°F, that of
the core, 95°F. The fruit were then removed from the bath,
and their peels were hand-scored from stem-end to blossomend, dividing the sphere into six equal parts. The fruit were
then submerged in the enzyme solution (which was main
tained at 86°F) contained in the vacuum infusion apparatus.
2For metric conversions see table at front of this Yolume. Ed.
Proc. Fla. State Hort. Soc, 91: 1978.
We pulled a vacuum of 28 in. Hg and held it for 5 min be
fore readmitting air into the vessel. We repeated the vacuum
infusion and then removed the fruit from the solution and
placed them into individual plastic food bags. After incubat
ing the fruit for 30 min in an oven at 122°F, we removed the
fruit from the bags, peeled them, and split the fruit in half,
then removed the juice sections from the segment walls.
Quality of enyzme prepared sections
Grapefruit sections prepared by the enzyme digestion
method were evaluated for quality by comparing them to
sections cut from grapefruit peeled by the conventional
method. Grapefruit for the comparison were obtained from
a local canning plant. Steam-peeled, lye-finished grapefruit,
taken directly from the sectionizing table at the plant were
transported to the laboratory in ice chests where sections
were cut from the fruit segments. Unpeeled grapefruit, also
from the plant, were peeled and sectionized by the pectinase
procedure described above. A panel of 20 tasters were pre
sented grapefruit sections prepared by the two methods and
asked to evaluate them and indicate a preference on the
basis of flavor and texture. The sections were also compared
for juice retention. A weighed portion of the two kinds of
sections were stored in plastic bags for three days and then
the sections were drained of juice and reweighed.
Storage stability
Sections were coated with gelatin by dipping chilled
sections into 10% gelatin solution maintained at 77°F and
then setting them on waxed paper at 40°F. Coated and uncoated sections were each packed in polyethylene bags for
storage experiments.
A panel of 12 tasters was selected from a group of 19
panelists who were surveyed for preference for grapefruit
sections. The panel members were presented with pectinaseprepared grapefruit sections stored dry or in freshly pas
teurized grapefruit juice in plastic bags at 40°F. They were
asked to score the fruit for flavor and texture on a scale of
1 to 9; 1 = like extremely, 9 = dislike extremely (1). For
another storage stability test, each member of a second
panel of 16 to 24 tasters was presented with 2 gelatin-coated
sections stored at 40°F, one in air and the other in N2 and
asked to compare the sections for flavor and texture. The
results of this paired comparison test were examined sta
tistically for significance (1).
Enzyme activity
We also wished to evaluate the effectiveness of com
mercial pectinase preparations; to do this we measured the
activity of the constitutent enzymes. We used two methods
reported by Vas et al. (2).
The activities of polygalacturonase, polymethylgalacturonase, and cellulase were measured by decreases in the
specific viscosities of the enzymes' substrates, 1.5% polygalacturonic acid, 0.45% pectin, and 2% carboxymethylcellulose, respectively. Solutions of substrate were incubated
with one of the pectinases at 122°F in Ostwald-type viscometers (Fisher Scientific Instruments, #150) in a thermo
statically controlled water bath. The decrease in specific
viscosity of each substrate was expressed as the percentage
of the original and plotted against the log of the incuba
tion time. From each plot the time necessary for 25% re
duction of specific viscosity was interpolated. Because the
log reciprocal of this time is proportional to the log of the
enzyme concentration, we were able to calculate the amount
of each brand of pectinase needed to reduce viscosity by
25% in 17.5 min.
Proc. Fla. State Hort. Soc. 91: 1978.
The activity of pectinesterase was measured by increase
in H+ liberated by the enzyme's substrate, pectin, during
hydrolysis. Fifty ml of 1% pectin solution was heated to
122°F, and 15 ml of dilute pectinase solution was added.
The temperature was controlled at 122°F in a water-jacketed
vessel by a thermostatically controlled circulating bath. At
zero time the pH of the solution was adjusted to 4.0 with
0.5 N NaOH. As the reaction proceeded, the pH was mon
itored and kept at 4.0 by measured addition of 0.1 N NaOH
from a burette. The volume of alkali added to maintain pH
4.0 was recorded every 2 min for 10 min. The volumes of
0.1 N NaOH consumed were plotted against time and the
slope of the proportionality line was calculated by the
method of least squares. The pectinesterase activity of each
brand of pectinase was expressed as the amount that hydrolyzed pectin at the rate of 20 /Ainoles of ester bonds per
min at 122°F.
Results and Discussion
Grapefruit sections prepared by pectinase treatment ap
peared dry and intact and tasted fresh and clean. The fruit
segments were completely recovered as intact sections. In
contrast, sections prepared by the conventional method of
cutting appeared wet and smaller because 30 to 40% of the
juice vesicles remained attached to the segment membranes.
The cut sections also sustained a drip loss of over 10% after
only 3 days at 40°F, whereas, the new type sections retain all
their juices. In flavor and texture the sections prepared with
pectinase were superior to cut sections. When asked to
evaluate and indicate a preference, all 20 members of a
laboratory taste panel preferred the new type sections.
Storage of pectinase-peeled sections
A panel of tasters judged unpasteurized pectinaseprepared sections stored dry or in grapefruit juice to be
similar in flavor and texture during 14 days of storage
(Table 2). These data suggest that flavor and texture of the
sections are stable during short-time cold storage.
Table 2. Comparison between sections packed dry and in juice.
Average hedonic score z
Storage
days
1
4
8
11
14
Flavor
Texture
Dry pack
Juice pack
Dry pack
Juice pack
3.5
3.3
4.0
3.7
3.3
3.5
3.5
4.1
3.8
4.8
3.1
3.1
2.9
2.9
2.5
3.2
3.3
3.0
2.9
3.2
*Twelve panelists scored on «i scale of 1 to 9; 1= like extremely, 9 =
dislike extremely. Values are the mean of 2 samples per panelist.
Unpasteurized grapefruit sections coated with gelatin
and stored in N2 were preferred by panelists to coated sec
tions stored in air (Table 3). During the 28-day storage test
preference was for N2-stored fruit, but only on one trial
(Exp. 2, 21 days) was this preference statistically significant.
Pectinase-prepared sections can be heat-pasteurized and
canned in juice-syrup, but dry packs of unpasteurized sec
tions have several advantages over conventional packs.
Besides saving the weight and space of the liquid in the pack,
the sections could be displayed in attractive see-through con
tainers. Gel-coating of sections would add structural stability
and could serve as a matrix for flavor-stabilizing substrates
for the metabolically active unpasteurized fruit. For low
113
Table 3. Paired comparison scores between sections stored in air and in
nitrogen.*
activity of constituent enzymes.2
Storage
days
7
14
21
28
Table 4. Concentration of 6 brands of pectinase required for equivalent
Exp. 1
Exp. 2
N2
Air
13
11
9
4
8
11
12
12
Enzyme and
N2
Air
12
5
11
6
15y
5*
11
7
zScores are the number of tasters who preferred one section over the
other.
ySignificantly different at P <0.05.
calorie diet packs, lack of syrups and juice would reduce
sugar and calorie contents, as well.
Degradative activity of the pectinases
According to the manufacturers the pectinases contain
polymethylgalacturonase, pectinesterase, polygalacturonase,
and cellulase, enzymes that degrade pectin, PGA and CMC.
One of these enzymes is more important than the others in
peel degradation. Comparison of the enzyme activities with
effectiveness of peeling of the various brands should indicate
which enzyme is the most important. Pectinase concentra
tions necessary for equivalent activity were compared in
Table 4. Brands A and B, which were the most effective in
peeling, also had the highest activity in degrading pectin
and PGA. The concentration ratios for polymethylgalac
turonase activity compared favorably with the concentration
ratios for effective peeling and sectionizing. The concentrate
ratios for cellulase activity showed no relationship to effec
tive peeling ratio but the ratios for pectinesterase and poly
galacturonase activities showed some similarity. The data in
Table 4 indicate that pectin degradation could be used as
an objective measurement of the peeling effectiveness of a
pectinase. In addition, cellulase activity was not involved in
peeling effectiveness.
substrate
Polymethylgalacturonase
Pectin
Polygalacturonase
PGA
Cellulase
CMC
Pectinesterase
Pectin
Pectinase concentration (mg/g substrate)
~~A
37
(1.0)
22
(1.0)
75
(1.0)
200
(1.0)
B
37
(1.0)
44
(2.0)
83
(1.1)
200
(1.0)
C
111
(3.0)
56
(2.5)
58
(0.8)
1300
(6.5)
D
296
(8.0)
356
(16.0)
66
(0.9)
500
(2.5)
E~
22
(6.0)
256
(11.6)
16
(0.2)
3300
(16.5)
F~
1110
(30.0)
589
(26.7)
833
(11.1)
5800
(29.0)
"Pectin, PGA, and CMC were degraded at the same rate, 25% reduction
in viscosity in 17.5 min at 50°C. Pectin was demethylated by each brand
at the rate of 20 /tmoles of ester bonds per min at 50° C. Ratios of cone
of all brands to brand A shown in parentheses.
Conclusion
Infusion of commercial pectinases into grapefruit effec
tively loosened the peel and segment membranes and freed
intact juice sections which could then be easily removed.
These sections were superior in size, flavor and texture to
sections cut from steam-peeled, lye-finished fruit. Com
mercial pectinases differ in their peeling effectiveness and in
activities of polymethylgalacturonase, polygalacturonase,
pectinesterase and cellulase. Pectinase pelling has potential
as a commercial method of preparing grapefruit sections
because segments are completely recovered as high quality
intact sections by this method.
Literature Cited
1. Larmond, E. 1967. Methods for sensory evaluation of food. Publ.
#1284, Canada Dept. of Agric, Ottawa, Canada.
2. Vas, K., M. Nedbalek, H. Scheffer and G. Kovacs-Proszt. 1967.
Methodological investigations on the determination of some pectic
enzymes. Fruchtsaft-Ind. 12(5): 164-184.
Proc. Fla. State Hort. Soc. 91:114-117. 1978.
MICROBIAL ASSESSMENT OF PASTEURIZED-REFRIGERATED
PEACH HALVES IN TRANSPARENT POUCHES1
R. P. Bates, L. Alcantara, J. Gomez and J. A. Koburger
Food Science and Human Nutrition Department, IFAS
University of Florida, Gainesville, FL 32611
Abstract. The microflora of fresh 'Flordagold' peaches
and microbial changes induced by lye peeling, hot packing in
boil-in-bag pouches, pouch steaming (7 min @ 86°C), rapid
cooling and storage at 2 and 20°C were evaluated. Prepara
tory steps resulted in a progressive decrease in plate counts
to <50 organisms/g immediately after processing. Die-off
proceeded to <l/g during storage except in pouches con
taining unpeeled halves stored at 20C where mold growth,
undetected for the first 2 weeks, proceeded rapidly there
after. Apparently, the combination of high levels of initial
contamination, low pH (3.5), non-refrigerated storage temiFlorida Agricultural Experiment Stations Journal Series No. 1479.
We are grateful to Cryovac Division, W. R. Grace & Co. for a reliable
supply of poudi maieiial and to D. W. Buchanan and E. P. Miller,
Fruit Crops Dept., IFAS, for generous access to experimental peach
plantings.
114
perature (20°C) and pouch oxygen permeability selectively
favored mold development. However, at 2°C, or in the case
of peeled halves, no microbial outgrowth occurred. The
pasteurized-refrigerated process was effective from the stand
point of enzymatic and microbial stability, although the boilin-bag pouch was a marginal package for ambient tempera
ture storage.
Florida is currently developing a modest peach industry
based on varieties developed over the last decade. During
this period, acreage in the state has expanded from about
2,800 to 6,500 (2, 12) and Florida ranks about 10th in U.S.
peach production (5). Home-grown peach plantings have
also increased by a substantial (but undetermined) amount
over the same period. As well as high yield and improved
cultivation characteristics for North Florida, these new
varieties ripen early and are well suited for home-grown,
pick-your-own and local markets.
There is now a need to consider expanded markets and
utilization forms for Florida-grown peaches. Several peach
Proc. Fla. State Hori. Soc. 91: 1978.