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Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

Market Quality and Rate of tespiration of

HEAD LETTUCE
Held in Low-Oxygen Atmospheres

Marketing Research Report No. 777 Be:
Rie

JUL 3

Agricultural Research Service ~

UNITED STATES DEPARTMENT OF AGRICULTURE

ACKNOWLEDGMENTS

The cooperation of Bruce Church, Inc., Harden Farms of California,
Inc., Bud Antle, Inc., and the Grower-Shipper Vegetable Association of
Central California, all of Salinas, Calif., in furnishing the lettuce for these
tests is gratefully acknowledged.

I appreciate the assistance of M. Uota with the carbon dioxide deter-
minations and that of H. M. Couey with the statistical analyses.

CONTENTS
Page

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Methods andamaterials.. 2425. so oes hoe wad fe eee sae eee Zi
DOUNCES OL TESt Leltmees <a 8 es4 wo 6 eee os ae he eee en ee Ps
Atmospheres and temperatures during holding.................. 2
Evaliation ofomarket Gia tty: .... teks ovo atnc ta concatenate 2
Determination..of réspiration rates...) <2 ve: sisb.2s oe. soot ee 3
PREGUIGS? «ai 5 Ssh ehecstenc, saudi Raha tear at'ac olsen aan Sam ranean <tc tee ee 3
Effect of low-oxygen atmospheres on market quality.............. 3
Injurious eects of low oxywen. Sia. es ok Sauken eee ee 3
General eirects: on qualitys.<s.0. fees wcrod oes ete oe eee eee 3
Effect of low-oxygen atmospheres on respiration................ ff
Discussion and. -conclusions...2-% «.s «ieeeee wee oc white Se ees oe Sees 7
Literature cited, <:54045.4%s he sateces « 0 ee a ae con «eee ere eee 9

Washington, D.C. Issued June 1967

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Market Quality and Rate of Respiration of Head Lettuce Held in
Low-Oxygen Atmospheres

By W. J. LIPTON, plant physiologist, Market Quality Research Division,
Agricultural Research Service, U.S. Department of Agriculture

SUMMARY

The market quality of head lettuce was evalu-
ated after 7 days at 36°, 41°, or 50° F. in at-
mospheres containing 0, 14, 1%, 1, 2, 5, or 8
percent O, or in air and after an additional 3 or
A days in air at 50° F.

Concentrations of 0 and 14, percent O, injured
the lettuce. Gray dead patches on the wrapper
and cap leaves and dead young heart leaves were
the principal symptoms of injury. The injuries
became more severe as the temperature was
increased.

Russet spotting was reduced substantially in
all low-oxygen (O.) atmospheres tested, regard-
less of temperature. The incidence of pink rib
was not affected by low-O, atmospheres. The
severity, however, was reduced in 14 percent O,
when all test temperatures are considered, but
increased in 2 percent O, at 50° F.

Decay was reduced, but not significantly, in
1 percent O,. Low temperature was more ef-
fective in reducing decay than low O..

Butt discoloration was generally less intense
in 14 and 1 percent O, than in air, but not after
3 or 4 additional days in air.

The general appearance of nontrimmed let-
tuce was not affected significantly by holding it
in 14 to 8 percent O, at 36° or 41° F.; although

lettuce held at 50° in 1, 2, 5, or 8 percent O, for
7 days plus 3 or 4 days in air appeared signifi-
cantly greener and fresher than that held in air
throughout. However, the quality of lettuce held
in air at 36° or 41° was equal to and often su-
perior to that held under low O, at 50°. After
the lettuce was trimmed, the only benefit from
low O. was a significant reduction in russet
spotting.

The rate of respiration (carbon dioxide
[CO.] production) was reduced by about a third
to a half in 14, 1, or 5 percent O, compared with
that in air, depending upon O, concentration
and temperature. At 41°, a 50-percent decrease
in the rate of CO, production would reduce vital
heat production from 40,000 to 20,000 B.t.u. per
day per 20-ton load of lettuce. This reduction
of 20,000 B.t.u. is equivalent to the refrigeration
capacity of about 140 pounds of ice or 115
pounds of liquid nitrogen (N.).

When N. is used as the refrigerant or as a
supplement to mechanical refrigeration, the O.
concentration should be at least 1 percent, never
less than 14 percent. [or maximum quality
retention, low temperatures must accompany
low-O, atmospheres.

BACKGROUND

Liquid N. has been used recently as a refrig-
erant or as a supplement to mechanical refrig-
eration in some shipments of western lettuce.
The liquid N. expands into a gas as it enters the
load compartment, displaces the air, and there-
by lowers the O., content. Parsons and oth-

Fresno, Calif.

ers (6)? found that very low O. concentrations
reduced russet spotting of lettuce, but Watada
and others (12) reported that O. concentrations
from 1 to 15 percent had little effect on the rate
of deterioration of lettuce. Further, Stewart

“Italic numbers in parentheses refer to Literature
Cited, p. 9.

1

2 MARKETING RESEARCH REPORT NO. 777, U.S. DEPT. OF AGRICULTURE

(10) noted undesirably high transit tempera-
tures in shipments of lettuce refrigerated by N,.
Harvey (3) cited evidence that atmospheres
low in O, injured some crops, although Parsons
and others observed no injury on lettuce held in
very low O, concentrations for 10 days at 33° F.

These divergent results needed to be clarified
so that sound recommendations could be made
to the lettuce industry on the safety or useful-

ness of low-O, atmospheres. Consequently,
tests were conducted to evaluate the response
of lettuce to such atmospheres at various tem-
peratures.

Since a reduction in the heat of respiration is
claimed as one advantage of shipping lettuce
under low O, (1), respiration was determined
under various conditions.

METHODS AND MATERIALS

Sources of test lettuce: —Lettuce of the Great
Lakes type was obtained in 1964 and 1965 from
the Salinas Valley or near Mendota in the San
Joaquin Valley of California. The lettuce was
cut and packed by field crews, but, to reduce
injury, only 18 heads were placed in each car-
ton, instead of the customary 24. Lettuce from
the Salinas Valley was vacuum cooled less than
1 hour after harvest and transported under
refrigeration to the U.S. Department of Agri-
culture laboratory at Fresno, Calif.; that from
the San Joaquin Valley was not vacuum cooled
prior to storage because of the short transit
time involved.

Atmospheres and temperatures during hold-
ing: —The lettuce was placed in stainless steel
chambers and exposed to the appropriate at-
mospheres the same day it was harvested, or
within 24 hours. The lots for each treatment
contained an equal number of heads from each
original carton for a total of 16 or 18. Wet
towels maintained a high relative humidity in
the chambers. Adjacent heads were from dif-
ferent cartons to avoid introduction of potential
nests of decay.

After closure, some chambers were flushed
with N. until the desired O. concentration was
nearly reached. The atmospheres were main-
tained by a continuous flow of the appropriate
mixtures of O, and N, dispensed from high-
pressure cylinders. Atmospheres in chambers
used as controls contained 21 percent O, (air)
and were maintained by a continuous flow of
air.

The desired O, concentration was generally
reached within 4 to 6 hours after closing the
chambers. The lettuce was held in O, con-
centrations of zero, 14, 14, 1, 2, 5, 8, and 21

percent and at temperatures of 36°, 41°, or
50° F. Zero and 14, percent O, were not tested
at 50° because these atmospheres seriously in-
jured the lettuce at 36° and 41°. The N. used
to obtain zero percent O, may have contained
undetected traces of O., which, however, were
not sufficient to prevent low-O, injury. The
rate of flow was sufficient to keep CO, levels
below 14 percent. The effluent gas had nearly
the same O, concentration as that entering the
chambers, as monitored with a Burrell® gas
analyzer.

Evaluation of market quality: —The effect of
each O,-temperature treatment on lettuce was
evaluated in at least three tests. Half the let-
tuce was examined after 7 days in the atmos-
pheres evaluated (simulated transit period), and
the remainder after an additional 3 or 4 days in
air at 50° F. (simulated wholesale-retail pe-
riod). Each head was rated for russet spotting,
pink rib, and decay on a scale from 1 to 9
(1—defects absent, 3—trace, 5—slight, 7—
moderate, 9—severe). Observations on butt
discoloration and injury from low-O, concen-
tration were made when appropriate. The gen-
eral appearance of the heads with and without
wrapper leaves was rated on a scale from 9 to 1
(9—field fresh, no defects; 7—good, minor de-
fects; 5—fair, objectionable defects that can
be removed before sale; 83—poor, generally un-
salable; 1—inedible).

The large number of treatments precluded
their incorporation in any one test. Thirteen

* Trade names and the names of commercial companies
are used in this publication solely to provide specific in-
formation. Mention of a trade name or manufacturer
does not constitute a guarantee or warranty of the
product by the U.S. Department of Agriculture or an

endorsement by the Department over other products
not mentioned.

MARKET QUALITY OF LETTUCE HELD IN LOW-OXYGEN ATMOSPHERES 3

tests were conducted during the two seasons.
Each test included eight treatments, three low-
O. concentrations and one air control at each of
two temperatures. A given treatment was
tested at intervals during a growing season to
avoid seasonal bias. The combined data from
all tests were evaluated on an equal basis by
applying the analysis of variance (Duncan’s
Multiple Range Test) for unequal sample size
to the data which were calculated as a percent-
age of averages obtained for the lots held in air
for a given temperature. (For example, aver-
age severity rating for pink rib at 50° F. in air,
4.0; in 2 percent O., 6.0; relative value for air,
100; for 2 percent O., 150.) The results of tests
that differed substantially from the trend were
also analyzed separately to gain information
that would be lost in the averages of all tests.

Determination of respiration rates; —CO,

production was measured on lettuce grown in
the Imperial Valley of California (January to
March 1966) and obtained from a local whole-
saler 24 to 36 hours after harvest. The lettuce
was held in glass jars at 41° or 50° F. in 14, 1,
5, or 21 percent O. (air). Six heads were used
for the 41° treatments, five heads for the 50°
treatments. Each treatment was tested twice.
The graphs in figure 1 represent the average
values of the replicates at each temperature.

CO, was determined with a positive nondis-
persive-type infrared analyzer (Liston-Becker,
model 15A) connected to a Varian strip chart
recorder set at zero to 0.250 percent CO, full
scale. A given gas sample passed through the
analyzer for about 4 minutes at about 114-hour
intervals, as regulated by an automatic mani-
fold. CO.-free air and O.-N. mixtures were
used in the respiration studies.

RESULTS

Effect of Low-Oxygen Atmospheres
on Market Quality

Injurious effects of low oxygen: —Extensive
gray, dead patches developed on the wrapper
leaves of lettuce held 7 days in zero percent
O, at 36° or 41° F. These injuries were pres-
ent to a lesser degree on wrapper leaves held
in 14, percent O, (pl. 1), but were more wide-
spread and severe after the additional 3 or 4
days in air at 50° than at the first examination.
Cap leaves of heads held previously in zero
percent O. also developed gray, dead patches.
Injury to heart leaves from low-O, concentra-
tions appeared as reddish-brown spots (pl. 2)
on the midribs of leaves near the center of the
head. Zero percent O, caused injury to wrap-
per, cap, and heart leaves, but '4. percent O,
caused the death of only the heart leaves.

Both the incidence and severity of the exter-
nal and internal injuries increased as the tem-
peratures increased. In one test at 50° F., the
heart leaves died even in 0.3 to 0.4 percent O..
The injuries developed further after removal of
the lettuce to air at 50°, and decay often affected
the injured wrapper leaves during this simulat-
ed wholesale-retail period.

Off-flavors—sweet, flat, not typical of lettuce

—were detected in heads held in zero and 14.
percent O, and were sometimes more prominent
in samples held at the higher concentration.
These off-flavors generally disappeared after the
lettuce was held 3 or 4 days in air at 50°.

General effects on quality: —Substantial inhi-
bition of russet spotting (4) was the principal
benefit of low-O, atmospheres in tests where
this disorder occurred. The incidence was re-
duced about equally in zero to 8 percent O, and
at all three temperatures (table 1).

Not only the incidence, but also the severity
of russet spotting was reduced in the low-O.,
atmospheres. The severity, as a percentage of
that in air, ranged from “9 to 66 percent, both
values being significantly lower (99-percent
probability level) than “hat in air. Differences
among individual treatments were not tested
statistically because the sporadic occurrence of
russet spotting resulted in too few data fcr
reliable analysis.

The incidence of pink rib, which was ev’ | ont
in all tests and in all treatments, we not
affected significantly by low-O, atmos} «eres.
The incidence, as a percentage of th:.' in air,
ranged from 74 percent in 14 perce it O, to
‘C3 percent in 8 percent O,.

4 MARKETING RESEARCH REPORT NO. 777, U.S. DEPT. OF AGRICULTURE

3,300

Return to air

4 2,200

1,100

AIR THROUGHOUT
ae ie ee Be

0 O, CONCENTRATION

ahs. 5.0%. |

—*— 1.0%

=O") 0.5%

B.T.U. PER TON-DAY

CO, PRODUCTION mg. per kg.-hour

20 4,400
1S
10
S)
Oo —
0 2 4 6 8 10°

DAYS AFTER TRANSFER TO INDICATED ATMOSPHERES

FrGuRE 1.—CO, production by lettuce held in various Oz concentrations at 41° or 50° F. and after the return to air.

MARKET QUALITY OF LETTUCE HELD IN LOW-OXYGEN ATMOSPHERES

PLATE 1

Wrapper leaves from lettuce held
7% days at 41° F. in % (left)

or 0 percent O» (right). Injured
areas are dead and gray.

PLATE 2

Cross section through lettuce
heads held 7% days in 0 percent
O, at 86° (left) or 41° (right)
plus 4 days in air at 50°.

PLATE 3

Color of butts of lettuce held

7 days at 36° in 21, 2,1, or %
percent O». Note slightly lesser
degree of discoloration after
holding lettuce in % or 1 percent
O.; also, great variation among
butts held in same atmosphere.

OT

6

MARKETING RESEARCH REPORT NO. 777, U.S. DEPT. OF AGRICULTURE

TABLE 1.—I/ncidence of russet spotting, severity of pink rib, and overall quality of lettuce held in
various low-oxygen atmospheres

[All values expressed are relative to their percentages in air (21 percent O.). Figures on a given line without a
common letter differ at the 95-percent probability level. Averages are for all samples at any one QO» concentration;
they are not equal to the averages of each column because of the unequal numbers of samples in each treatment.

At least 48 heads were evaluated for each treatment]

Values for lettuce held 7 days in the following percentages of oxygen 1}

Storage temperature
°

plus 8 or 4 days in air at 50° F.

Y% 1 | 2 5 8 21
Relative incidence’ of russet
spotting at — Pct. Pct. Pct Pct Pct Pct.
OO sce & coue sae ew lanoress a Guatens tousuecstere) 6 38a 2la Ta 13a 9a 100 b
A 2 wsctv ca Danae dec Cah aust eo eae teen: Oa ee 19 a Oa 1a 100 b
DO? sath ch PCa ahh aks eos wee G2) Oa da Qa Va 100 b
AVGTAPE 6 scocicaarieeaecadale aes 2a 17 a 9a 6a 12a 100 b
Relative severity’ of pink rib at —
364i cima sted cis aate ed ecb eee 84a 83 a 89 a 99 a 98 a 100 a
AN 552 argcir a vatbnava asked Gare cemslele ee 8la 85 a 82a 98 a 98 a 100 a
BOS Seve awa dtaraca evar erdiieias eyes Seeteueree 80 a 127 ed 133 d 117 bed 121 ed 100 abe
ANVOLPAG@O .cctia ng, aoa we scdhe eaters 82a 96 ab 100 b 104 b 105 b 100 b
Relative rating’ of overall
quality at —
BO a racevee die reesyerRians cdr aoe e Auestene 108 a 104 a 103 a 107 a 106 a 100 a
AN, 2 Sis elena bisa e Wndr@ cs8-0 tea eyes ee 108 a 105 a 104 a 106 a 10la 100 a
DO! srscargperenwtdie, 61 aie a,recienaterdrie/e.eoernehaye 120 b 138 b 129 b 123 b 127 b 100 a
ANGLAISE va acedy debe wie ale ost 107 a 114 a llla 112 a llla 100 a
1 Data for zero percent and 14 percent Oz not included because these atmospheres seriously injured the lettuce.
2 Actual incidence in air averaged 33 percent at 36° F., 36 percent at 41°, and 14 percent at 50° ; average for all temperatures was 31 percent.

3 No russet spotting occurred in tests that included this treatment.
4 Actual severity rating in air averaged 2.5 at 36° F.,
used: 1 = none, 3 = trace, 5 = slight, 7 = moderate, 9 = severe.

5 Actual quality rating of nontrimmed lettuce in air averaged 6.4 at 36° F., 6.2 at 41°, and 4.6 at 50°;

3.3 at 41°, and 4.3 at 50°;

was 5.8. Rating scale used: 9 = excellent, 7 = good, 5 = fair, 3 = poor.

The severity of pink rib was reduced sig-
nificantly in 14 percent O, (table 1). In con-
trast, the severity was intensified by 1, 2, 5, or
8 percent O, at 50° F. in almost every test; the
increase was greatest in 2 percent O, (table 1).
Pink rib was sufficiently severe to detract from
the market value of much of the lettuce (see
table 1, footnote 4). At 36° or 41°, none of the
atmospheres affected the severity of pink rib
significantly.

The effect of low-O, atmospheres on the in-
cidence of decay was inconsistent. Although
lettuce held in 1 percent O, had slightly less
decay after the simulated transit period plus
3 or 4 days in air (the incidence was 66 percent
of that in air) than that from the other atmos-
pheres, none of the differences were statistically
significant. Temperature exerted a greater in-

average rating for all temperatures was 3.3. Rating scale

average rating for all temperatures

fluence on decay than O, concentration; the in-
cidence of decay increased as temperature in-
creased from 36° to 50° F. Heads held under
low O, at 50° had consistently more decay than
those held in air at 36° or 41°. No trend ex-
isted in respect to severity of decay. The decay
that did occur was usually confined to the wrap-
per leaves and was not serious.

Butt discoloration varied widely within a
given treatment but was generally less severe
in 14 percent or 1 percent O, than in air (pl. 3).
Holding the lettuce 3 or 4 days in air at 50° F.
completely negated any differences that existed
immediately after the lettuce was removed from
14 or 1 percent O,. The cut surface remained
white only at zero or 14. percent O., concentra-
tions that injured the lettuce severely.

MARKET QUALITY OF LETTUCE HELD IN LOW-OXYGEN ATMOSPHERES 7

The appearance of nontrimmed lettuce was
affected very little by storage in 14 to 8 percent
O. at 36° or 41° F., temperatures near and
slightly above desirable levels. At 41°, lettuce
held in low-O, (1 percent) concentrations was
significantly superior to that held in air in only
one test. However, atmospheres in the range of
1 to 8 percent O, improved the appearance of
lettuce held at 50° F. (table 1). Lettuce in
these treatments remained greener and had a
fresher appearance than that held in air. This
difference was not evident immediately after
removal from low O., but only after the simu-
lated wholesale-retail period in air at 50°. The
appearance of lettuce held in air at low tem-
peratures was commonly superior to that of
lettuce held in low-O, concentrations at the next
higher temperature. Therefore, low O, cannot
be considered an improvement over low tem-
perature in maintaining the market quality of
lettuce when russet spotting is not a problem.

Trimming off the wrapper leaves removed
all evidence of the favorable effect of low O, on
general appearance. The amount of O, had
little effect on trimmed lettuce either “on ar-
rival” or after the simulated wholesale-retail
period in air; only those benefits related to
reduction in russet spotting remained. The
effect of low O, on pink rib was variable, as
previously noted.

Effect of Low-Oxygen Atmospheres
on Respiration

Atmospheres with 14, 1, or 5 percent O, re-
duced the rate of CO, production of lettuce held
at 41° or 50° F. (fig. 1), and the percentage
reductions were similar at both temperatures.
In 14 and 1 percent O, the rate of CO. produc-
tion was reduced by about half of that in air
and in 5 percent O. by about a third. These
results parallel those for spinach (7) and fora
variety of other plants (2, 11). As would be
expected, the rates were higher at 50° than at
41°; in fact, at 50°, CO. production in 1% per-
cent or 1 percent was about the same as at 41°
in air.

The rate of CO. production adjusted to the
lowered O. concentrations about 3 days after
the change from air at both temperatures. In
contrast, the return to the normal rate of CO,
production after the lettuce was again placed in
air was more rapid at 50° F. than at 41°.
Further, at 50°, the rates for the lettuce held
previously in 14 or 1 percent O. exceeded the
rates of lettuce held in air throughout by 25
and 11 percent, respectively, before they leveled
off—2 days after the return to air. Bourne
and Ranson (2) observed a similar increase in
CO. production after removal of rhododendron
leaves from an N, atmosphere into air. This
increase in CO, production above that existing
in the air control, was almost absent at 41°.

DISCUSSION AND CONCLUSIONS

The foregoing results indicate that some
definite, but limited, benefits may be derived
from the use of certain low-O, atmospheres
under conditions simulating the transcontinent-
al shipment of head lettuce. The substantial
reduction in russet spotting, even at O. concen-
trations as high as 8 percent, is clearly the ma-
jor benefit. This response, previously observed
by Parsons and others (6) in lettuce held at very
low O. concentrations, is surprising at the high-
er O. concentrations because the rate of CO,
production, a measure of metabolic activity,
was substantially higher in 5 percent O, than
in 14 or 1 percent O, (fig. 1). It appears, there-
fore, that russet spotting is inhibited at O,
concentrations substantially higher than those

that reduce CO, production appreciably. In
contrast, the browning reaction associated with
butt discoloration was prevented only in near
anaerobic conditions, both in these tests and in
those of Parsons and others. Pink rib, also the
result of an oxidative discoloration, was actu-
ally enhanced in 1 or 2 percent O., but reduced
in 14 percent O, at 50° F. These apparently
paradoxical results cannot be clarified on the
basis of the present tests, but they suggest that
the circumstances that lead to russet spotting
differ substantially from those that cause pink
rib.

The small young heart leaves were most sen-
sitive to low-O, injury and, therefore, would be
a good indicator of the adequacy of the O. sup-

8 MARKETING RESEARCH REPORT NO. 777, U.S. DEPT. OF AGRICULTURE

ply for lettuce during transit. The grayish-
brown discoloration, especially at the leaf mar-
gins, was quite distinct from the reddish-brown
discoloration caused by excess CO, (12). How-
ever, the reddish spots on the midrib of the
central head leaves found in zero percent O,
did resemble those described for CO, injury.
Some of the external and internal injuries
observed in lettuce held in zero percent and 14
percent O. resembled those attributed to “sub-
oxidation” by Nelson (5). Parsons and others
(6) did not observe these injuries, possibly
because metabolic activity is lower at 33° than
at 36° F. or above, or because not all entrapped
O. was removed by flushing the chambers with
N. for 10 to 15 minutes twice daily.

The lack of substantial effect of low O. on the
general appearance of lettuce, especially after
trimming, agrees closely with the results of
Watada and others (12), who found that O,
concentrations from 1 to 15 percent neither im-
proved nor harmed lettuce held at 36° or 41° F.
Watada and co-workers did not include 50° in
their tests; this temperature, while undesir-
ably high, does occur in lettuce shipped under
liquid N, refrigeration (10). The retardation
in quality loss resulting from 1 to 8 percent O,
at 50°, but not at 41° or 36°, is related to the
rate of deterioration at these temperatures.
At 41° or below, sound lettuce remains in fairly
good condition for nearly 2 weeks (8), so that
color retention by wrapper leaves is not affected
by low O, concentrations during shorter inter-
vals. In contrast, at 50° the senescence retard-
ing effect of low O. becomes noticeable because
of the substantially higher rate of deterioration
at this temperature. However, during long
periods of storage, such as may occur in the
supply of navy ships, the benefits of low O, also
may be apparent near 40° F. Similarly, any
beneficial effects of low O., on lettuce quality
would be progressively less noticeable as the
transit period is shortened.

The low incidence and severity of decay found
in these laboratory tests, especially at 50° F.,
may be explained by the careful handling the
lettuce received. In commercially packed and
shipped lettuce, decay was substantial in O.
concentrations of 2 percent or lower when
transit temperatures were about 50° F. (10).

The data on CO, production allow deter-
mination of possible savings in the cost of
refrigeration if lettuce is shipped in low-O,
atmospheres. With a rate of heat production of
about 2,000 B.t.u. per ton-day in air at 41° F.,
a 50-percent reduction lowers the heat of respi-
ration from 40,000 to 20,000 B.t.u. per day in a
20-ton load of lettuce. This reduction of 20,000
B.t.u. is equivalent to the refrigeration capacity
of about 140 pounds of ice or 115 pounds of
liquid N.. This relatively small saving lasts
only as long as the O, level remains near 1 per-
cent; if it rises, as has occurred when the sup-
ply of liquid N. was exhausted before arrival
(10), normal rates of CO, production are ap-
proached rapidly or even exceeded. The sav-
ines would be slightly greater with lettuce re-
spiring at the somewhat higher rates reported
for lettuce from the Salinas Valley (8) or the
Rio Grande Valley (9).

It follows from the above discussion that low-
O. atmospheres do not benefit lettuce substan-
tially except in respect to russet spotting, which
was reduced significantly and about equally in
14 to 8 percent O.. Further, for maximum qual-
ity retention, low temperatures (not above 41°)
must accompany low-O, concentrations. When
N, is used as a refrigerant or as a supplement
to mechanical refrigeration, the O, level should
remain near 1 percent or higher, never below
14 percent because of the danger of serious
injury at lower concentrations.

These deterioration and respiration studies
clearly show that low-O, atmospheres neither
stop deterioration of lettuce nor ‘immediately
put [lettuce] to sleep’ as has been claimed (1).

| (1)

(2)

(3)

| (4)

(5)

(6)

(7)

LITERATURE CITED

ANONYMOUS.
1966. USE OF OXYGEN CONTROL TO STOP
DETERIORATION. The Packer
73(4): 16, illus.
BOURNE, D. T., and RANSON, S. L.

1965. RESPIRATORY METABOLISM IN DE-
TACHED RHODODENDRON LEAVES.
Plant Physiol. 40: 1178-1190,
illus.
HARVEY, J. M.
1965. NITROGEN—ITS STRATEGIC ROLE
IN PRODUCE FRESHNESS. Prod.
Market. 8(7) : 17-18.
LIPTON, W. J.
1961. ANATOMICAL OBSERVATIONS ON
RUSSET SPOTTING AND PINK RIB
OF LETTUCE. Amer. Soc. Hort.
Sei. Proc. 78: 367-374, illus.
NELSON, R.
1926. STORAGE AND TRANSPORTATION-

AL DISEASES OF VEGETABLES DUE
TO SUBOXIDATION. Mich. Agr.
Expt. Sta. Tech. Bul. 81, 38 pp.,
illus.

PARSONS, C. S., GATES, J. E., and SPALD-

ENG, D. A.

1964. QUALITY OF SOME FRUITS AND
VEGETABLES AFTER HOLDING IN
NITROGEN ATMOSPHERES. Amer.
Soe. Hort. Sci. Proc. 84: 549-
556.

PLATENIUS, H.

1943, EFFECT OF OXYGEN CONCENTRA-
TION ON THE RESPIRATION OF
SOME VEGETABLES. Plant Physiol.
18: 671-684, illus.

(8) PRATT, H. K., Morris, L. L., and TUCKER,

C. L.
1954.

TEMPERATURE AND LETTUCE DE-
TERIORATION. Conf. Transport.
Perishables Proc., Univ. Calif.,
Davis: 77-83, illus.

(9) ScHoLz, E. W., JOHNSON, H. B., and Bu-
FORD, W. R.

1963.

HEAT EVOLUTION RATES OF SOME
TEXAS-GROWN FRUITS AND VEGE-
TABLES. Rio Grande Valley Hort.
Soc. Jour. 17: 170-175.

(10) STEWART, J. K., and others.

1966.

NITROGEN — ITS EFFECT ON
TRANSIT TEMPERATURES AND
MARKET QUALITY OF WESTERN
LETTUCE SHIPPED IN PIGGYBACK
TRAILERS. U.S. Dept. Agr. Mar-
ket. Res. Rpt. 759, 14 pp., illus.

(11) STILEs, W.

1960.

THE COMPOSITION OF THE AT-
MOSPHERE (OXYGEN CONTENT OF
AIR, WATER, SOIL, INTERCELLULAR
SPACES, DIFFUSION, CARBON DI-
OXIDE, AND OXYGEN TENSION).

Encyclop. Plant Physiol. [W.
Ruhland, -ed.|] 12 (2): d14.
118, illus.
(12) WATADA, A. E., Morris, L. L., and RAPPA-
PORT, L.
1964. MODIFIED ATMOSPHERE EFFECTS
ON LETTUCE. Fruit and Vege-

table Perishables Handling Conf.
Proc., Univ. Calif., Davis: 82-
85, illus.

SYU.S. GOVERNMENT PRINTING OFFICE: 1967—238-598

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