Discusses the various tests currently used to measure quality of fruits and vegetables.Agribusiness,

Watada, Alley E.

English

Research Papers in Economics

MEASURING QUALITY OBJECTIVELY AND NONDESTRUCTIVELY
by
Alley E, Watada
Agricultural Marketing Research Institute
Beltsville, Maryland
Discusses the various tests cur-
rently used to measure quality of
fruits and vegetables.
Quality can be defined clearly in com-
munication only when the parameters consti-
tuting quality are identified and measured
objectively. Most subjective measurements
of quality are relative and the base used
for measurement differs among people and
changes unknowingly within an individual.
These differences and changes cause uncer-
tainty in the description of quality. There-
fore, to develop a scale for different grades
of quality, the factors constituting the
quality need to be defined and measured
objectively.
Factors considered under quality of
fresh produce are observed ina sequence and
can be separated into 3 groups. Factors
that are noted initially are those which are
judged by sight. These include composition,
color, defects, and size. The second group,
which includes hardness and firmness, is
judged when the item is touched by hand.
These attributes are vulnerable at the re-
tail level in that repeated handling and
measurement during distribution hastens
softening or loss of firmness. The third
group of factors is judged when the com-
modity is consumed. These factors, which
include texture, flavor, aroma, and taste,
have received the least attention because
satisfactory objective methods of measure-
ment are not available.
Current objective methods of measuring
quality do not necessarily measure the
attributes directly. Firmness, hardness,
external defects, and external color are
measured directly. On the other hand,
flavor, taste, aroma, internal defects, and
state of maturity commonly are determined
by measuring associated changes within the
commodity. For example, flavor, aroma, and
taste often improve with advanced physio-
logical age or maturity and concomitantly
the composition of the colored pigments and
density undergo correlated changes. Thus ,
color and density are used as criteria for
measuring some of the sensory qualities,
including those which can be measured di-
rectly. The color and density of a tissue
are altered by pathological infection or
physiological disorders; thus, these defects
also canbemeasured indirectly by measuring
the internal color.
Firmness or Hardness
The easiest and quickest method to
measure firmness is by feeling the product
by hand. This indicates firmness to the
individual but the measurement cannot be
described concisely. In addition, each
time a commodity is tested for firmness, the
stress caused by the pressure of the hand
weakens the tissue, hastens the softening
process, and shortens the shelf life.
The first mechanical device for meas-
uring firmness was a puncture device devel-
oped in 1919 (10). Magness and Taylor (12)
modified the device and engineered a porta-
ble spring-load pressure tester for measur-
ing the puncture resistance of apples . This
instrument measures firmness by recording
the amount of force required to depress a
plunger 7/16 of one inchin diameter into an
apple for a distance of 5/16 of one inch.
The Magness-Taylor pressure tester has been
modified and is presently used as a standard
firmness measuring device by the United
States Department of Agriculture for esti-
mating the picking quality, texture, and
potential storage life of fruits and vege-
tables .
Journal of Food Distribution Research February 73/page 23Texture or firmness needs to be mea-
sured nondestructively in many situations.
The earliest method involved a measurement
of pressure requiredto compress a commodity
for a given distance bya flat-surface plate
(14) , This technique has been modified con-
siderably since 1930 and current manually
controlled instruments measure firmness
either by a single-point (9) or a multi-
point compression (11). Electronically con-
trolled instruments minimize human error
and also record the deformation curve of a
sample. The deformation curve is informa-
tive in interpreting the texture and firm-
ness of a sample. The speed of such in-
struments is too slow for industrial use.
Other techniques and principles are
being explored for use in high-speed non-
destructive measurement of firmness. Per-
haps the most promising technique involves
the useof random vibration. The vibration
test was first introduced in 1942 (6) and
the technique was pioneered by Abbott et al
(1) and Finney and Norris (8). Finney (7)
indicated that on the basis of frequency
response curves, a l/3-octane measurement
of the accelerometer signal at 2000 Hz would
be sensitive to differences in peach firm-
ness. He found that the random vibration
signal at 2000 Hz ranged from 45.3 to 73.3
dB for peaches that ranged from 0,9 to 22.4
pounds, which was measured by a 5/16-inch
diameter Magness-Taylor probe mounted in an
Instron testing machine. On the basis of
his data, the vibration signal might beused
to separate peaches as follows: fruits with
a vibration signal exceeding 64 dB would be
eliminated due to immaturity; those exhib-
iting 60 to 64 dB would be ideal for long-
distance shipment; and those with a signal
of 57 to 60 dB would be considered too soft
for shipment but could be used locally.
Fruits having a vibration signal of 40 to
50 dB would be optimum for consumption.
These values should not be considered as
the final separation point of the different
groups until all the variables involved in
fruit softening are studied completely;
nevertheless, the technique has potent:al
for use on mechanical harvesters, production-
area packing lines, and perhaps in terminal
market prepackaging plants.
Color
External color is a good indicator of
eating quality for some commodities and
commonly is measured by the reflectance
technique. Most laboratory model instruments
are capable of measuring the lightness, red-
ness or greenness, and yellowness or blueness
of a color. Commercial model instruments
generally measure only a specific color and
the measurement indicated on the instrument
is derived differently with each instrument.
The reading of an instrument used to deter-
mine raw tomato juice color is a product of
an equation, which utilized all three mea-
surements described for the laboratory model.
Instruments used to determine the color of
citrus, apples, potatoes, and tomatoes give
the reflectance measurement at a specific
light wavelength or the difference of mea-
surements at two wavelengths. The potential
of the reflectance instrument differs with
each commodity.
Quality of all items cannot be cate-
gorized satisfactorily on the basis of ex-
ternal color. The use of external color as
a criterion of quality is difficult when it
is not uniform or is mixed with streaking
or speckling of other colors. In some com-
modities, pigments in the epidermis mask
internal color, and consequently, the exter-
nal color gives a deceiving appearance of
the fruit color. In most commodities inter-
nal color is a better index of quality than
external color.
The internal color of a commodity can
remeasured nondestructively by light trans-
mittance. This technique depends on the
absorption of the light by chlorophyll
(green), carotenoid (red and yellow),
anthocyanin (red and blue) and other pig-
ments in the tissue. Commodities differ
in pigment composition; thus the wavelength
that is used for light-transmittance mea-
surement differs with the commodity. Several
reports have described this technique to
measure internal quality or maturity.
A current laboratory model instrument,
termed the “Light-Transmittance Difference
Meter” (LTDM), measures the light trans-
mittance or optical density at two wave-
lengths and indicates the difference be-
tween the two measurements . Maturity,
quality, and defects were found to
February 73/page 24 Journal of Food Distribution Researchbe correlated more closely to
the difference in optical density of two
wavelengths than transmittance measurement
at a single wavelength.
Maturity of tomatoes can be determined
bytheuse of the LTDM. With the instrument,
the degree of ripeness of fruit which are
green externally as well as of those that
are red can be determined, Ripeness of a
green tomato is determined by the AOD of
510-600 nanometers (rim) and ripeness of a
red tomato is determined by the AOD of
600-690 nm. Currently measurement of inter-
nal tomato color is limited to laboratory
studies; however, this principle could be
applied and used commercially for separating
tomatoes according tomaturity. This method
of sorting might be used on mechanical
harvesters to eliminate immature fruit that
will not ripen properly; in the packing shed
to separate the green fruits into various
maturity categories so as tohave fruit that
will ripen at a desired date; and in termi-
nal markets to categorize the red tomatoes.
If all fruits had uniform external color at
the wholesale point, tomatoes could be
resorted to estimate the shelf life of the
fruit or to determine the temperature at
which the fruit should be maintained to
hasten or retard ripening.
The LTDM has been used in the labora-
tory to measure the quality of peaches. The
eating quality of a peach, like many other
fruits, improves with the decreasing green
color (chlorophyll content) and the increas-
ing ripe color. On the basis of this rela-
tionship, Sidwell et al (13) measured the
chlorophyll content of the peach with the
LTDM to determine eating quality. They
found that the instrument reading @OD of
695-725) ranged from 80 to -30 for fruit
that had an eating quality score ranging
from 4 to 50, with increasing score indi-
cating greater preference. On the basis of
these scores, peaches can be separatedto
about four quality groups with the LTDM.
Before such quality categories can be for-
mulated for commerical use, all the conuner-
cially grown varieties would need to be
studied with the LTDM and the relationship
between LTDM values and firmness determined.
The latter is particularly important in that
maturity is currently based on firmness.
The LTDM has been used for sorting Red
Delicious, Golden Delicious, Stayman, Rome
and Winesap apples into various categories
of quality (2, 15). In these studies, fruit
were categorized into4 or 5 classifications
with theLTDM and submitted to a taste panel
for preference score, The tests showed a
very close correlation in the ranking of
eating quality as indicated by the machine
and by the taste panel. Other laboratory
studies also have been encouraging, thus
USDA is coordinating a program to evaluate
a commercial model for sorting apples.
The LTDM can beused to detect internal
defects such as water core of apples. Water
core is a physiological disorder in which
watery tissue develop around the internal
vascular bundles often near the core and
during storage the water-soaked appearance
may spread and eventually become visible
externally. Birth and Olsen (5) found that
optical densityof water-core tissue differed
from sound tissue. On this basis, the LTDM
was used to measure the optical density of
apple fruit suspected to have water core.
They found that fruit free of water core had
a AOD (750-810 nm) value exceeding 50, those
with a slight amount of water core had a
AOD value ranging from 20 to 40, and those
with moderate to severe water core had a
AOD value below 20. Since water core at
harvest frequently isat an incipient stage,
a sorter could be used to segregate water-
core apples inxnediately after harvest.
Hollow heart of potatoes, an internal
defect, can be measured with the LTDM by
determining the AOD of 710-800 nm. Effec-
tiveness of the instrument is dependent upon
the discoloration of hollow heart. Birth
(4) measured theAOD of Katahdin and Cobbler
potatoes suspected to have hollow heart and
cut the tubers to determine the degree of
hollow heart. With Irish Cobbler potatoes,
those without any hollow heart had a AOD
reading ranging from -.09 to -.33 with the
largest number of readings centered between
-,20 to -.28. Those with a small cavity
and slight discoloration had a reading
ranging from -.33 to -.42. Potatoes with a
medium degree of hollow heart had a reading
ranging from -.36 to -.65. The largest
proportion of these potatoes had a reading
ranging from -.35to -.48. The potatoes with
a large degree ofhollowheart had a reading
Journal of Food Distribution Research February 73/page 25of - .40 to- .80 with the largest group having fat content in hamburger and other meat
a reading of -.50 to -,55. on the basis of products.
this research, it is conceivable that the
LTDM technique canbe used commercially to Other objective tests, such as X-rays,
separate potatoes with hollow heart from can be used to measure quality; however,
sound potatoes or to sort hollow-heart those listed here probably have the greatest
potatoes into different categories. potential application for commercial use.
Although the current commercial use of these
The LTDM also is used to measure oil, techniques is minimal, with the increasing
protein, and water content of soybeans (3). need for objective measurements of quality
The potential for the LTDM to measure these and exploratory challenges initiated by the
factors in other material is excellent, industry, advancement in the development of
Studies are being conducted to evaluate the commercial instruments is anticipated.
applicability of the instrument to measure
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Literature Cited
Abbott, J. A., G. S. Bachman, N, F.
Childers, J. V. Fitzgerald, and F. J.
Mahisek. 1968. Sonic techniques for
measuring texture of fruits and vege-
tables . Food Technology 22(5): 1o1-
112.
Aulenbach, B. B., J. N. Yeatman and
J. T. Worthington. 1972. Quality
sorting of Red Delicious apples by
1ight transmission. USDA , ARS ,
Mktg. Res. Rpt. No. 936, 21 pp.
Ben-Gara, I. and K. H. Norris. 1968.
Determination of moisture content in
soybeans by direct spectcophotomety.
The Israel J. of Agri. Res. 18(3) :
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Birth, G. S, 1960. A nondestructive
technique for detecting internal dis-
coloration in potatoes. Amer. Potato
J. 37: 53-60.
Birth G. S. and K. L. Olsen. 1964.
Nondestructive detection of water core
in Delicious apples. Proc. Amer. Sot.
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Clark, H, L. and W. Mikelson. 1942,
Fruit ripeness tester. U. S. Patent
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Finney, E. E., Jr. 1971. Random
vibration technique for nondestructive
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Finney, E. E. and K. H. Norris. 1968.
Instrumentation for investigation dy-
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(9)
(lo)
(11)
(12)
(13)
(14)
(15)
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USDA, Circular 350.
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Ernest, and C. Golumbic. 1961. The
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February 73/page 26 Journal of Food Distribution Research

MEASURING QUALITY OBJECTIVELY AND NONDESTRUCTIVELY

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