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154

NITROGEN—ITS EFFECT ON TRANSIT
TEMPERATURES and MARKET QUALITY
OF WESTERN LETTUCE SHIPPED
in

PIGGYBACK TRAILERS

Marketing Research Report No. 759

Agricultural Research Service
U. S. DEPARTMENT OF AGRICULTURE

ACKNOWLEDGMENTS

The cooperation and assistance of the following organizations and firms are
gratefully acknowledged:
Shippers providing test loads of lettuce:
Arrow Lettuce Co., Salinas, Calif.
Bud Antle, Inc., Salinas, Calif.
Garin Co., The, Salinas, Calif.
Jackson Produce Co., Inc., El Centro, Calif.
Mutual Vegetable Sales, Salinas, Calif.
Sunrise Packing Co., El Centro, Calif.
West Coast Farms, Watsonville, Calif.
Carriers and suppliers ‘providing equipment or personnel, or both, to assist
in initiating tests:
Atchison, Topeka and Santa Fe Railroad Co., Refrigeration Dept..
Chicago, Til.
Best Express, Div. of The Best Fertilizer Co., Oakland, Calif.
Bud Antle, Inc., Salinas, Calif.
Linde Co., Div. of Union Carbide Corp., Tonawanda, N.Y.
Pacific Fruit Express Co., San Francisco, Calif.
Western Pacific Railroad Co., San Francisco, Calif.

CONTENTS
Page
UAT So io eee ote ee ee seh as em ed
InirodtiiOn. sn wecd contend setaes eee oa ee 1
Methods oie os Ss See ee ee ee 2
MCGINNIS 2255 See ao ee ee eee ee 3
Temperatures in conventional trailers vs. those in traliers refrigerated with
inquidl MiPOgei 2. e ce a tle 3
Temperatures 1 in conventional trailers vs. those in trailers with liquid nitrogen
plus: mechanical Perriperavion) <1 206 eee ed og ee 4
Atmosphere modification. in thé travers 2222. ee ee ee 4
Quality of lettuce shipped in conventional trailers vs. that shipped in trailers
refriverated with. hguid aitrogengl. so oeve es _ eee es 2 ee 6
Quality of lettuce shipped in conventional trailers vs. that shipped in trailers
with liquid nitrogen plus mechanical refrigeration _-----_------------------ 8
IDISCUSSIGN <i es ee ee ee 13
Appendix: Atmospheres obtained by using nitrogen to break the vacuum-cooling
CV ONO oar teed wae keg eee ee ee ee 14

Washington, D.C. Issued December 1966

LLL

NITROGEN —=ITS EFFECT ON TRANSIT TEMPERATURES
AND MARKET QUALITY OF WESTERN LETTUCE
SHIPPED IN PIGGYBACK TRAILERS

By J. K. STEwart, horticulturist, J. M. HaRvey, investigations leader, M. J. CEPONIS, pathologist, and W. R. WRIGHT,
pathologist, Market Quality Research Division, Agricultural Research Service

SUMMARY

Market quality of lettuce and transit tempera-
tures in shipments made in piggyback trailers
refrigerated with liquid nitrogen and in trailers
equipped with both liquid nitrogen and me-
chanical refrigeration units were compared with
those in shipments in conventional, mechanically
refrigerated piggyback trailers. The lettuce was
shipped from California to midwestern or east
coast markets.

Trailers in which liquid nitrogen was the only

source of refrigeration had excessive temperature

variation within the load and average load tem-
peratures were often too high, especially during
the latter part of the trip. Decay was significantly
higher in these trailers than in the conventional,
mechanically refrigerated trailers, because of the

high temperature. Incidence of russet spotting
was significantly lower in these trailers than in
the conventional trailers; this was the result of
the low-oxygen atmospheres produced in the trail-
ers refrigerated with liquid nitrogen.

Trailers equipped with both liquid nitrogen
and mechanical refrigeration units had tempera-
tures similar to those in conventional, mechanical-
ly refrigerated trailers. Decay was essentially the
same in lettuce from both types of trailers. Inci-
dence of russet spotting was significantly less in
lettuce shipped in the mechanically refrigerated
trailers supplemented with liquid nitrogen than
in that shipped in conventional trailers with nor-
mal atmospheres.

INTRODUCTION

Trailers refrigerated with liquid nitrogen and
trailers with both mechanical refrigeration and
liquid nitrogen for atmosphere modification have
been available to lettuce shippers since about 1964.
Few critical studies have been made on transit
temperatures in such equipment or on the effects
of resulting low-oxygen atmospheres on produce
quality. The U.S. Department of Agriculture, in
cooperation with lettuce shippers, carriers, and
suppliers of trailers equipped with liquid nitrogen
systems, made tests in 1963, 1964, and 1965 on
shipments from California to midwestern and
eastern markets.

1Wartapa, A. E., Morris, L. L., and Rappaporr. L.
MODIFIED ATMOSPHERE EFFECTS ON LETTUCE. Fruit and
Veg. Perishables Handling Conf. Proc.. Univ. Calif., Mar.
23-25: pp. 82-85. 1964.

Controlled-atmosphere storage tests with let-
tuce, made by Watada and coworkers,’ showed
that lettuce was neither benefited nor harmed by
oxygen concentrations as low as 1 percent for up
to 8 days at 41° F. High concentrations of carbon
dioxide, however, were harmful to lettuce. Other
laboratory tests by Parsons and coworkers?
showed that, at 33°, lettuce held at 1 or 0 percent
oxygen developed less russet spotting than lettuce
held in air. Lipton’ stated that oxygen levels up
to 8 percent during storage of lettuce reduced

2Parsons, C. S., Gates, J. E., and Spaupinc, D. H.
QUALITY OF SOME FRUITS AND VEGETABLES AFTER HOLDING
IN NITROGEN ATMOSPHERES. Amer. Soc. Hort. Sci. Proc.
84: 549-556. 1964.

3 Personal communication from W. J. Lipton, plant
physiologist, U.S. Department of Agriculture. 1965.

2 MARKET RES. RPT. 759, U.S. DEPT. OF AGRICULTURE

russet spotting. However, he stated that oxygen
levels at 0.5 percent or lower during storage.
within the temperature range studied (36° “to
50°), sometimes injured the heart leaves.

This report gives the results of shipping tests

made under commercial conditions to determine
comparative effects of liquid nitrogen and me-
chanical refrigeration on transit temperatures, on
the quality of lettuce, and on the various disorders
affecting lettuce on the market.

METHODS

Fifteen paired trailer shipments of vacuum-
cooled lettuce were made from California to mid-
western and east coast markets. Most shipments
were made during the summer, from the Salinas-
Watsonville district; but a few were made in the
winter months, from the Imperial Valley of
southern California (table 1). The trailers moved
by rail on flatcars, except for a few short hauls
by truck to or from railheads.

One trailer of each pair in a shipment was a
conventional, mechanically refrigerated (MR)
trailer; the other either was refrigerated by liquid
nitrogen alone (N) or was refrigerated mechan-

ically with liquid nitrogen used as a supplement
(NMR) to modify the atmosphere. The nitrogen
was dispensed in the trailers through a perforated
tube running along the ceiling. The N trailers
had the tube at the centerline; the NMR trailers
had the tube near a wall. Four’ storage tanks, each
holding 375 pounds of liquid nitrogen, were lo-
cated in the nose of the N trailers. One tank,
holding 100 gallons of liquid nitrogen, was at-
tached to the undercarriage of each NMR trailer.
The N trailers had no fans but depended only on
convection and the release of nitrogen for air
movement within the load compartment.

TaB.E 1.—Trailer type, shipping dates and points, and destinations of test
shipments of lettuce from California to midwestern and east coast markets

ehinine Destination
Point
Watsonville _-- | Chicago, Ill.
See Wee ee dQea 5525 Do.
S22) (Salinas (2222222 Do.
cove vill dowoas-= Do.
El Centro —---- Do.
el eee dois === Do.
Salinas ~---___ Waverly, N.J.
ese (eee Se do_----- | Hawthorne, N.J.
ire do__---- | Elmsford, N.Y.
See, |pesvens do------ Do.
Holtville __---~- New York, N.Y.
ee |e eee dowsse Do.
eee do__---- | Chicago, Ill.
2 ap en eee do_----- | Decatur, Mich.
Watsonville __- | Chicago, Ill.
Beene) [pera do__--_- | Decatur, Mich.
af mS do__---- | Philadelphia, Pa.
a Ue tee dow 22252: Do.
Salinas ~-----_ Chicago, Ill.
Shh | peas dost eee Do.
fl See d0iz2 225 Do.
Ti || Set de Ci (ee Saar Do.
oie ee do_____ | Elkhart, Ind.
pee | eset do___-__ | Chicago, Tl.
El Centro ___~-~ New York, N.Y.
=) Holtviller S25. Do.
El Centro ___-- Do.
=") Holtvillee 222" Do.
El Centro ____~- Do.
Seas dostes.= Do.

Trailer Na. per
Date

Ws BS Se N July 9, 1963 __-

Dose eee see MR © (| eee = do_-_-

Sac eve tse Ne? Vitesse i pace

AO Nie eS e MRie \ [ieetee dose

De fa Sock ke N Feb. a * 1964 2

Osetesee sie cees MR. = ji) }2i = do---

ase eee Se N Aug. 25, 1964 __

Siete ote L aes MR. ca do__-

0" ce seccceneoee N Aug. 26, 1964 _-
10 2.52550 22 2e2 MR pe do_--
Wine ase ee eee N Mar. 22, 1965 __
12 Secu oot ese MR “oteese do_--
1332 Seu sees N Mar. 25, 1965 __
14) Joe cee eee IMR: @ jilPeu ese do2==
) 5S eee epee a N Aug. 5, 1965 _--
1G6j22322. 2 Seeu MRS) eee ee do ---
ny epee le NO epi N Aug. 17, 1965 __
if) eae ee MRA sbeb ee doses
Oi 2 el ee ee NMR May 22, 1964 __
20x Ss MRE fe ieee 2s dois
PS Na coated NMR June 4, 1964 __
DO ion ee ee MRE lees does:
933 22) oe Bae NMR July 29, 1964 __
Abi. Mite oat MIR}. | ieee do_.-
Di) Moatte 2 eo aoe NMR Jan. 26, 1965 __
DGjae ar ae MR 2) oe to seers doze—
21 eel So oh TS NMR Mar. 9, 1965 ___
4 3 ee ee RR Lae MR) |e do__-
DO! Fs ayaa sk NMR Mar. 10, 1965 __
BOe 2s oe ee MR eee do__-

1N, liquid nitrogen refrigeration (low-oxygen atmosphere) ; MR, mechanical refrigera-

tion (normal atmosphere) :
oxygen atmosphere).
2 Rail car used for this shipment.

NMR. liquid nitrogen plus mechanical refrigeration (low-

|

NITROGEN’S EFFECT ON MARKET QUALITY OF WESTERN LETTUCE 3

The thermostats in the conventional and NMR
trailers were located in the air return behind the
bulkhead at the front of the trailers. The ther-
mostats in the N trailers were in the load com-
partment at the ceiling, about 6 feet from the
bulkhead and 27 inches to the left of centerline.

The nitrogen-dispensing system in the N trailers
was activated by the thermostat, except for N
trailers 5, 7, and 9. The nitrogen release in these
trailers was controlled by an oxygen sensor. These
three N trailers also had thermostats that could
shut off the supply of nitrogen if the temperature
became too low.

The oxygen concentration in the NMR trailers
was controlled by an oxygen sensor that activated
the flow of nitrogen when the oxygen level in the
trailer rose above the set point.

The oxygen concentration in the N trailers was
lowered initially by the same system used to
refrigerate the trailer in transit. The oxygen con-
centration in the NMR trailers was lowered im-
mediately after loading by purging the trailer
with nitrogen from an auxiliary supply.

Atmosphere samples were drawn from the load
compartment through a %-inch plastic tube that
led from the center of the load (between cartons)
to the outside of the trailer through a water drain.
Samples were taken at shipping point and at
destination. Oxygen and carbon dioxide concen-
trations were determined with an Orsat gas ana-
lyzer. In some tests it was not possible to obtain
samples at shipping point; and, if the trailer doors
had been opened before a sample could be taken,
no reading was obtained at destination.

Transit temperatures were obtained with small
recording thermometers placed in the center of
lettuce cartons at various locations throughout
the load. In most trailers, the thermometers were
located in cartons in the middle and top layers at

the front, in the middle layer at half-length (one
at the wall and one at centerline), and in the top
and bottom layers at the rear of the trailer. All
thermometers were at the centerline of the trailer
except the one at the half-length adjacent to a
wall. The trailers had a thermometer attached to
the ceiling to record inside air temperatures. One
trailer in each pair had a thermometer attached
to the undercarriage to record outside air tem-
perature.

Lettuce temperatures were measured manually
at various positions during unloading.

Lettuce for the test cartons for each trailer of
the paired shipments was selected from the same
field to assure comparable lettuce. The lettuce was
in the “firm” to “very firm” range of maturity.
The six test cartons per trailer were located in the
same positions as the six thermometers placed
in the load.

Quality evaluations were made at arrival and
after a simulated marketing period (4 days at
50° F.). One layer in each carton was used for the
arrival examination and the remaining layer for
the second examination. The arrival examination
was made on the day of unloading or, in cases
where one trailer was unloaded on a different day
from the other trailer, the test cartons from the
first trailer to be unloaded were held at 38° or 40°
until those from other trailers could be unloaded.

External appearance of the heads was rated by
the following scale: 1, unsalable; 2, poor; 3, fair;
4, good; and 5, excellent. Each head was com-
pletely torn apart to detect decay, russet spotting,
pink rib, tipburn, or rib discoloration. These de-
fects were rated by the following scale: 1, none;
2, trace; 3, slight; 4, moderate; and 5, severe.

All quality examinations were made by per-
sonnel of the U.S. Department of Agriculture’s
Market Pathology Laboratories at Chicago, Ill.,
or Belle Mead, N.J.

RESULTS

Temperatures in Conventional Trailers Vs. Those in Trailers
Refrigerated With Liquid Nitrogen

The average load temperature of all the trailers
refrigerated with liquid nitrogen (N) was 41° F.
and that of the companion conventional, mechan-
ically refrigerated (MR) trailers was 36° (table
2). The average temperatures ranged from 2° to
11° higher in the N trailers than in the MR
trailers in individual paired tests. The two pairs
of trailers in which average load temperatures
differed by only 2° (trailers 5 and 6, 13 and 14)
were shipped during February and March when
outside temperatures were low. The average mini-
mum and maximum outside air temperatures dur-

ing shipment of trailers 5 and 6 were 25° and
45° F, respectively; those during shipment of
trailers 13 and 14 were 45° and 70°.

Transit temperatures at the coolest and warm-
est positions and the average temperature of all
positions in each trailer are given in table 2 and
shown graphically in figures 1 and 2 for the N
and companion, conventional MR trailers. The
variation between coolest and warmest positions
was excessive in most N trailers. However, the
variation was not so great in the N trailers
shipped during cool weather (5°-6°) as it was in
those shipped during hot weather (6°-16°). The
variation between the coolest and warmest posi-
tion in the conventional MR trailers was some-

4 MARKET RES. RPT. 759, U.S. DEPT. OF AGRICULTURE

TABLE 2.—Temperatures during shipment of lettuce in trailers refrigerated with liquid nitrogen (N)
vs. during shipment in trailers with mechanical refrigeration (MR)

Average transit temperatures i
. = Maximum
Trailer Trailer | Thermostat Outside air temperature
No. type? setting Inside air Load Coolest Warmest |————_—_——— | at destina-
at ceiling average | position? | position’ |Average | Average tion 4
low high

°F °F. °F. eek °F. ° F. °F. °F
N 45 | ---------- 39 37 43 60 90 54
Die ecetans MR BB it pt ppaseeeeeaee 35 34 36 60 90 37
| N 45 | =--------- 41 37 44 60 90 55
eas MR 5 i 35 34 37 60 90 43
§-stsele NEG /RAQag eS ee 45 39 36 42 25 45 39
Gree sect MR 36 36 37 36 38 25 45 39
(een N 36 53 43 39 50 64 84 60
Sie MR 36 38 38 37 39 64 84 41
LS onlin N 36 46 41 37 46 58 V7 53
102 2E DS MR 36 35 36 35 38 58 TT 42
fn aren ees N 39 43 40 37 43) ) iP eeeseee hen osees 50
12s eos MR 36 36 36 35 CY (OO ad Sa ee ea 39

: N 45 47 38 36 41 45 (O's bape ee

lg tlh MR 36 35 36 35 37 45 Ob cil penetrates
1s ea N 34 54 46 36 52 56 91 66
AG koe fy MR 34 31 35 33 39 56 91 42
Wis kant N 37 51 44 39 52 54 79 61
i: ean oe MR 36 38 39 38 41 54 79 41

Average:

Nisatlobosttenn capes wad beet oe 41 37 46 poise Se eee ele 55
MR eo tooo. ts ete 36 35 883 trahtseds siieokoss— 41

1N, liquid nitrogen refrigeration (low-oxygen atmosphere) ; MR, mechanical refrigeration (normal atmosphere).
2Coolest average transit temperature; not coolest position in load at all times during transit.

3 Warmest average transit temperature; not warmest position in load at all times during transit.

4Manual temperatures obtained by U.S. Department Agriculture personnel during unloading.

times, but not always, greater during hot than
cool weather, and the variation was always less
than in the companion N trailers.

The average difference between coolest and
warmest positions in all the N trailers was 9° and
in all the MR trailers was 3°. The highest average
transit temperature at any one position in a N
trailer was 52° F. and in a MR trailer was 41°
(table 2).

The coolest position in all N trailers was in the
middle layer at the front (adjacent to the liquid
nitrogen storage tanks), and the warmest posi-
tion was in the top layer‘at the rear of the trailer.
The positions of the coolest and warmest tem-
peratures were not consistent in the conventional
MR trailers.

The average of the maximum temperatures of
all the N trailers at destination was 55° F. and of
all the conventional trailers was 41° (table 2).

Temperatures in Conventional Trailers Vs. Those in Trailers
With Liquid Nitrogen Plus Mechanical Refrigeration

The average load temperature during transit
was 38° F. in the nitrogen plus mechanically re-
frigerated yee) trailers and 40° in the conven-
tional, mechanically refrigerated (MR) trailers
(table 3).

Transit temperatures at the coolest and warm-
est positions and the average temperature of all
positions in the trailers are given in table 3 and
shown graphically, in figures 3 and 4. Temperature
variation within the load was generally slightly
less in the NMR trailers (averaged 2°) than in
the companion MR trailers (averaged 3°). The
greater temperature uniformity in the NMR
trailers may have been due to chance, to differ-
ences in the amount of supplemental nitrogen, or
to differences in the design of the trailers, me-
chanical refrigeration units, or the thermostats.

The temperature spread between coolest and
warmest positions was greater in trailers 23 and
29 (4°) than that in any of the other NMR
trailers tested (table 3).

The average of the maximum temperatures
measured during unloading was 39° F. in the
NMR trailers and 41° in the conventional MR
trailers (table 3).

Atmosphere Modification in the Trailers

Most of the atmosphere data are limited to
oxygen and carbon dioxide determinations of
samples taken before shipment and at destination
(table 4). However, oxygen concentration in the

NITROGEN’S EFFECT ON MARKET QUALITY OF WESTERN LETTUCE 5

60

WARMEST POSITION
AVERAGE LOAD TEMPERATURE
COOLEST POSITION

50

AV ERAGE
40 TRAILER 2 (MR)

a
fo}

n
°

TEMPERATURE, °F
a
°o

TRAILER | (N)

ey
°

TRAILER 8 (MR) AVERAGE

THERMOSTAT

60

TRAILER 7 (N)

TEMPERATURE, °F.
a
°o

Db
o

50
AVERAGE
TRIP
TRAILER 16 (MR) TEMP
40 , 39°

THERMOSTAT 34°

w
fe}

a
fo}

TRAILER I5 (N)

TEMPERATURE, °F.
a
°

40

36°

60

50
AVERAGE
TRIP

40 TRAILER 4(MR) TEMP

& 30
e
Ww
“60
=
Ee
-¢
c
a
250
Z TRAILER 3(N) ——_ coganitincnnneee 4ie THERMOSTAT. 45°
40
30
NM N M N M N M N M N M N M N M N M
) \ 2 3 4 5 6 7 a0
DAYS
60
50
AVERAGE
TRIP
TRAILER 10 (MR) TEMP.
40 sas
36°
ae 9
uw THERMOSTAT 36°
° 30
a
£60
Fe
9
ac
w
a
=50
iw TRAILER 9 (N)
40
30
NM N M N M N M N M N M N M N M N M
) I 2 3 4 5 6 7 8
DAYS
AVERAGE
TRIP
TRAILER 18 (MR) TEMP.
4le
pry
.
we
«60
=]
&
a TRAILER 17 (N)
a
£50
Ww
i
40

4
DAYS

FicuRE 1.—Transit temperatures of lettuce shipped during hot weather in piggyback trailers under mechanical refrig-
eration (MR) and liquid nitrogen refrigeration (N). Nitrogen release in trailers 7 and 9 was controlled by an
oxygen sensor. The thermostat was a safety device for shutting off the nitrogen if the temperature became too
low. See table 1, p. 2, for shipping dates and points of departure and destination.

6 MARKET RES. RPT.

- WARMEST POSITION
* AVERAGE LOAD TEMPERATURE
= COOLEST POSITION

TRAILER 6 (MR)

TEMPERATURE, °F.

TRAILER 14 (MR)

THERMOSTAT 36°

TEMPERATURE, ° F.

THERMOSTAT 45°

4
DAYS

load was measured continuously during transit
in N trailer 7 (fig. 5). The oxygen concentration
in this trailer dropped to about 5 percent in 8
hours and only increased above this concentration
briefly two times during the rest of the trip. The
oxygen concentration in this trailer dropped to
below 1 percent during one brief period in transit.
It averaged about 2.6 percent during transit.

Purging the nitrogen plus mechanically refrig-
erated (NMR) trailers with nitrogen reduced the
oxygen concentration to 1 to 2 percent at de-
parture.

Oxygen concentrations were generally lower in
the nitrogen-refrigerated (N) trailers at destina-
tion than in the NMR trailers because the nitrogen
supply in the latter was usually exhausted, or
nearly so. The lowest oxygen concentration found
at destination in the N trailers was about 0.2 per-
cent and in the NMR trailers was 2.0 percent.
Average oxygen concentrations of less than 1

oF

a

TEMPERATURE

759, U.S. DEPT. OF AGRICULTURE

60

50

TRAILER 12 (MR)
40

o
io]

uo
°

TRAILER II (N)

py
°

nN N N WN NN ON ON A A ON SN
5 6 io No i 613 614 15 6 OIF

Figure 2.—Transit temperatures of lettuce,shipped during
cool weather in piggyback trailers under mechanical
refrigeration (MR) and liquid nitrogen refrigeration
(N). Nitrogen release in trailer 5 was controlled by
an oxygen sensor. Trailers 11 and 12 were in transit
for 12 days; the others were in transit 5 to 7 days.
See table 1, p. 2, for shipping dates and points of
departure and destination.

percent during simulated transit conditions have
injured lettuce in laboratory tests.* Oxygen con-
centrations in the conventional trailers ranged
from 17.8 to 20.6 percent at the shipping point and
from 16.1 to 20.4 percent at destination.

The carbon dioxide concentration in the trailers
was generally about 1 to 3 percent at destination.
The highest carbon dioxide concentration found
in any of the trailers at destination was 7.1 per-
cent (N trailer 7). Continuous concentrations of
5 to 10 percent carbon dioxide may be detri-
mental to lettuce.®

Quality of Lettuce Shipped in Conventional Trailers Ys. That
Shipped in Trailers Refrigerated With Liquid Nitrogen

The external appearance of lettuce with wrap-
per leaves attached was slightly better when
shipped in conventional, mechanically refrig-

4 Personal communication from W. J. Lipton. 1965.
5 See footnote 1, p. 1.

Se Sea — a -

TEMPERATURE, - F.

NITROGEN’S EFFECT ON MARKET QUALITY OF WESTERN LETTUCE 7

erated (MR) trailers than when shipped in nitro-
gen-refrigerated (N) trailers (table 5). Wrapper
leaves of lettuce from both types of trailers de-
teriorated in quality at the same rate during 4
days at 50° F.

After the wrapper leaves were removed from
the heads, there was no significant difference in
the appearance of the lettuce shipped in the N or
conventional MR trailers. Butt discoloration was
slightly more severe in lettuce from the N trailers
than in that from the conventional MR trailers.
This was probably due to the relatively high
temperatures in the N trailers.

The average severity ratings for russet spot-
ting, pink rib, tipburn, and rib discoloration were
not significantly different for lettuce from the two
types of trailers (table 6). Decay was slightly
more severe in lettuce from the N trailers than
in that from the conventional MR trailers. Inci-
dence and severity of decay, russet spotting, and

70

60 seem AVERAGE LOAD TEMPERATURE
2524 00LEST POSITION

50 TRAILER 20 (MR) one
TEMP.

46° A506
jae 45

e
ic]

oO
ic]

TRAILER 19 (NMR)

TEMPERATURE, ° F.
ow
°

40

70

60
AVERAGE
TRIP
50
TRAILER 24 (MR)
244°
42°

h
°

THERMOSTAT 38°

TEMPERATURE, °F
2 8

TRAILER 23 (NMR)

pink rib increased significantly during the 4 days
at 50° F. (table 6), but the increase in these dis-
orders was not influenced by trailer type.

The severity ratings for disorders in lettuce did
not reflect differences in the incidence of decay
and russet spotting. Although the average sever-
ity ratings for these disorders were between
“none” and a “trace,” lettuce from the N trailers
had about five times as many heads with decay
as that from the conventional MR trailers on
arrival and about twice as many heads with decay
after 4 days at 50° F. (table 6). Lettuce from the
N trailers had about half as many heads with
russet spotting at the first examination and about
one-third as many at the second examination as
lettuce from the conventional MR trailers. The
higher incidence of decay in the N trailers was
the result of the higher temperatures in those
trailers than in the MR trailers. The lower inci-
dence of russet spotting in the N trailers was due

70

60

AVERAGE
TRIP

50
EMP
TRAILER 22 (MR) s roo

= 41°

39°

°F
+b
°

uw
°

TEMPERATURE,
a
°

TRAILER 21 (NMR)

ey
°

Figure 3.—Transit temperatures of lettuce shipped during
hot weather in piggyback trailers under mechanical
refrigeration (MR) and liquid nitrogen plus mechan-
ical refrigeration (NMR). See table 1, p. 2, for ship-
ping dates and points of departure and destination.

8 MARKET RES. RPT. 759, U.S.

sions WARMEST POSITION
FAVERAGE LOAD TEMPERATURE
“COOLEST POSITION

CAR 26 (MR)

THERMOSTAT 36°

TEMPERATURE, °F.

TRAILER 25 (NMR)

THERMOSTAT 35°

oz
F 4
-z
4
nz
z
uz
E
4
z
z
z
z
=
z
z=
z
xz

TRAILER 28 (MR) AVERAGE

er

THERMOSTAT 34°

TEMPERATURE

TRAILER 27 (NMR)

N M N WM
3 4
DAYS

nN
a

n
°o

OXYGEN CONCENTRATION (PERCENT )

LENGTH OF RUN ( HOURS )

FicurE 5.—Oxygen concentration in load compartment of
liquid nitrogen (N) trailer (No. 7) shipped from
California to Waverly, N.J.

Data from Linde Division, Union Carbide Co.

DEPT. OF AGRICULTURE

70

60

50
TRAILER 30 (MR)

.
°

THERMOSTAT 34°

TEMPERATURE, °F.
a ow
°o fe}

TRAILER 29 (NMR)

e
°o

Ficurp 4—Transit temperatures of lettuce shipped dur-
ing cool weather in piggyback trailers, except as in-
dicated, under mechanical refrigeration (MR) and
liquid nitrogen plus mechanical refrigeration (NMR).
Trailers 29 and 30 were in transit for 11 days; the
others were in transit 8 days. See table 1, p. 2, for
shipping dates and points of departure and destina-
tion.

to the lower oxygen concentrations. The relative
increase in incidence of decay and russet spotting
was about the same in lettuce from both types of
trailers during 4 days at 50°.

Incidence of pink rib, tipburn, and rib dis-
coloration was not significantly different in lettuce
from the two types of trailers either at arrival or
after 4 additional days at 50° F.

Quality of Lettuce Shipped in Conventional Trailers Vs. That
Shipped in Trailers With Nitrogen Plus Mechanical
Refrigeration

Average ratings for external appearance, either
with or without wrapper leaves, were not sig-
nificantly different in lettuce from the two types
of trailers (table 7). Neither did butt discolor-
ation differ in relation to trailer type. The relative
deterioration in appearance was about the same in

NITROGEN’S EFFECT ON MARKET QUALITY OF WESTERN LETTUCE 9

Tas_e 3.—Temperatures during shipment of lettuce in trailers refrigerated with liquid nitrogen plus
mechanical refrigeration (NMR) vs. during shipment in conventional trailers with mechanical

refrigeration (MR)

Average transit temperatures Maximum
Trailer Trailer Thermostat Outside air oe
No. type setting Inside air Load Coolest Warmest: | - at paced
at ceiling average | position? | position? | Average} Average tion 4
low high
°F. °F. °F. °F °F. ae Bae cy °F
192-2. NMR 34 36 42 41 43 58 86 38
20... MR 34 41 45 44 46 58 86 41
7} in NMR 36 40 41 40 42 60 83 42
See een MR 36 40 41 39 43 60 83 42
73 NMR 38 33 37 34 38 63 92 38
2), eee MR 38 37 42 38 44 63 92 40
25 ee A NMR Sb. | eeeseecees 37 36 Oe Mie ee 40
2 MR 36 37 38 37 40) Wicteoses Ieee 41
On a NMR 34 35 38 37 38 37 46 40
Beat Pe MR 34 35 36 35 37 37 46 35
ho i NMR 34 33 35 33 37 33 53 38
BOL aoc sa MR Of ‘|| Seastaceces 40 40 41 33 53 44
Average
INVER) 2b. ectee bh Soca 38 37 BOP | i Wecoeesolleeca eet 39
MRI) cho 2227-22. ||| Pooch sees 40 39 (VR Sera es | ene ne 41

1NMR, liquid nitrogen plus mechanical refrigeration (low-oxygen atmosphere) ; MR, mechanical refrigeration (nor-

mal atmosphere).

2 Coolest average transit temperature; not coolest position in load at all times during transit.
3 Warmest average transit temperature; not warmest position in load at all times during transit.
4Manual temperatures obtained by U.S. Department Agriculture personnel during unloading.

lettuce from both types of trailers during the 4
days at 50° F. Average severity ratings for decay,
russet spotting, pink rib, tipburn, and rib dis-
coloration were not significantly different in let-
tuce from the liquid nitrogen plus mechanically
refrigerated (NMR) trailers than in the conven-
tional, mechanically refrigerated (MR) trailers
(table 8). The relative increase in severity of the
disorders was about the same in lettuce from
both types of trailers during 4 days at 50°.

The incidence of decay, pink rib, tipburn, and
rib discoloration was not significantly different
in test cartons of lettuce shipped in the two types
of trailers (table 8), but incidence of russet
spotting was significantly less in lettuce from the
NMR trailers than in that from the conventional
MR trailers. The relative increase in incidence
of the various disorders was about the same in let-
tuce from both types of trailers during the 4 days
at 50° F.

10 MARKET RES. RPT. 759, U.S. DEPT. OF AGRICULTURE

TaBLE 4.—Oxygen and carbon dioxide concentrations in test trailers of lettuce shipped from California
to midwestern and east coast markets

Atmosphere iti indi
Gp isvelatise mosphere composition at indicated place

Trailer No. Trailer type! purging Oz level control Shipping point Destination
O2 CO2 O2 SO2
Percent Percent Percent Percent Percent Percent
J mamma | ae Le arenes |e Se ee 74 2.0
7 ean BLT E VR IMR Pesce Soe Se a ee eS ee SS | a | ee |
5 ee eee IN; Bios See ee Soe Sa es See a2 |ee ee eee 3.8 1.6
7, ape eee nan eR MR | ban Soeccsocce es] oe ee SE ee eee eee
Bp are ee ING? UG bees See ae TONY he Oe ae ae as oe ee | ee | ee
eee Ane ee MRO 6 pc Sescwetes Sea ee See oe a | ee |
{Soa PE N® oo pee sa sees E 23.0 6.5 1.0 17 71
8 ------------ MR ~~ {---------------|--------------- 20.5 0 217.7 22.5
a eee NG) = plcbat eee ees 23.0 7.0 2.0 27°00) |e ee
1O.22 5222 MR Of ee 2 a eee oe ee 19.0 OG (is Sees | eee
a fi ie eo ee ee 1, en Meee | Pe ee ee 1210) °° |ebo eee secs 3.7 5.0
AD oe oe 10 | | ee 20:6 Wickes 20.0 4
1S See ease too INS eS l saat enc eee eS 13.7 1.6 2 13
14ers MR « Wesel oseoeee ce Sales es See 19.0 15 16.3 13
be See es NS Alipsectece ee. See ee ee eee 1.0 9) (toto aeee ee ee
Ge ee oe 0) ns a ee a 19.2 Gs 2. bes ee | eee
0 Ly (is ae ee IN © ‘(Lessee ca 2k Sess te Sees 15.5 1.9 8.8 2.9
ie ie eae eee eee MR | sesso 2 shes e cds ee 20.5 3 20.4 4
193+ ae ee IND sec ea feet eee eee 4:0!) © )veseeeee 2.0) ~ “| 222-2
20 ee MR io Nseceseeeel es Solace Se a ae kee Sa seeks ee a Las | ee
OF cee NMR Sb: “ol Ces ee eee (bie 2 Hise ees 18.8 2.0
9 | Ea MB) besos ot oh ee Sa ee ae ze 200: etikeeeesusses 20.0 1.0
Es aed ene ee NMR BB GES metas ee Se 47 1.0 13.9 1.0
OA es MRE. oo \> « beeeeseasese esas eae oe eee 17.8 2.5 16.1 2.0
OF oe oats NMR 27 | San | [Peet iS gent) con! pede awee SEE Se! |e eet et 16.4 3.4
DG a IR cs ee as | ca ae ee ee eee 1.2
DG san ee soe NMR DG iw a eR eee ee 6.1 2.3
7: (ee ee See MR fh2secn eed ee eee ee |
| ar NMR DiDiOi = 5 inline eee! ae) phere eee et ee ee 17.4 15
| ee IME ech secu ee Saas Lee eh ee ee 18.3 2.4

1N, liquid nitrogen refrigeration (low-oxygen atmosphere) ; MR, mechanical refrigeration (normal atmosphere) ;
NMR, liquid nitrogen plus mechanical refrigeration (low-oxygen atmosphere).

2 Data obtained by Linde Co. personnel.

3 Vacuum was broken by nitrogen after vacuum cooling of lettuce.

4 Atmosphere measurements obtained the day after loading.

> Data obtained by Best Express Co. personnel.

Tasie 5.—Market-quality ratings of lettuce after shipment in trailers refrigerated with liquid nitrogen |
(N) vs. after shipment in conventional trailers with mechanical refrigeration (MR)

[Each rating represents average of ratings from 7 replicated test trailers]

. ee ; External-appearance rating? Butt-
Time of examination Trailer ———] discoloration
type! With wrapper | Without wrapper rating 3
leaves leaves
Arrival: (222-2222 22322 seSee nse a See N 3.3 3.5 4.0
DO> a2 25255255 -554555---sess-cesSsS MR 3.5 3.7 3.6
4 days after arrival *.__.___.._-.----=..===- N 2.8 3.0 3.9
Do*# ~-.--.---~----=--=--~---=+--==-=: MR 3.1 3.2 3.9
Statistical significance :>
Trailertype-CL). 22 =22222-.22 2222 sn. Se ee eee ~ NS *
Bxamination (BE) <2. es eas ois ve NS
WD XA eet eee aes NS NS NS

1N, liquid nitrogen refrigeration (low-oxygen atmosphere) ;MR, mechanical refrigeration (normal atmosphere).
2 Rating scale: 1, unsalable; 2, poor: 3, fair; 4, good; 5, excellent.

3 Rating scale: 1, none; 2, trace; 3, slight; 4, moderate; 5, severe.

4Held at 50° F.

5 NS, not significant: *, significant at 5-percent level: **, significant at 1-percent level.

NITROGEN’S EFFECT ON MARKET QUALITY OF WESTERN LETTUCE 11

| Taste 6.—Severity ratings and incidence of various disorders of trimmed lettuce heads after shipment
in trailers refrigerated with liquid nitrogen (N) vs. after shipment in conventional trailers with me-
chanical refrigeration (MR)

Severity of indicated disorder? 3

Traiber | | | AA
Time of examination type ! Russet Pink Rib
Decay spotting rib Tipburn discolora-
tion
SRATIIVAl) 2 2c253) see ses ce ese ces. N 1.2 a Bs b 1.0 nD 1.1
WO} 6 ee tS a hh MR 1 13 1.0 1.1 11
4 days after arrival* _________________L. N 1.9 12 1.2 12 11
DOA tes tene een * a os eee! MR 1.5 1.5 1.1 1.2 1.0
Statistical significance :5
Trailer type (CL) -2--3-2 2h es he * NS NS NS NS
Examination (E) ~----_----~___-__-____-__-__-_- +e +e * NS NS
MIA eos co eee eee eet Cocueaweecece NS NS NS NS NS
. Incidence of indicated disorder ® 7
Laver |
Time of examination type ! Russet Pink Rib
Decay spotting rib Tipburn discolora-
tion
Percent Percent Percent Percent Percent
APTIVAN: 22-22 sole ec cece Se eccuce®: N 6.2 3.7 0.6 3.8 LZ
DO) 22 toe abe S oe eS MR 13 6.0 ae 2.1 L7
4 days after arrival* -__-__-__________-. N 25.5 8.5 6.0 3.9 4.0
MOF oo MR 12.1 25.2 4.0 3.8 2.1
Statistical significance :6
Trailer ‘type. (1): 2-2-2-2— 2 we 2 oe +t bid NS NS NS
Mxamination:: (WM) a. +t +f = NS NS
TX Wao es eG NS NS NS NS NS

1N, liquid nitrogen refrigeration (low-oxygen atmosphere) ;MR, mechanical refrigeration (normal atmosphere).

2Hach rating is average of ratings from 7 replicated test trailers, except for tipburn (6 replications).

3 Rating scale: 1, none; 2, trace; 3, slight; 4, moderate: 5, severe.

4 Held at 50° F.

5 NS, not significant; *, significant at 5-percent level; **, significant at 1-percent level.

6 Hach percentage based on 9 replicated test trailers, except tipburn (6 replications) and rib discoloration (7 rep-
lications).

7 Geometric mean of percentage of heads affected.

Taste 7.—Market-quality ratings of lettuce after shipment in trailers with liquid nitrogen plus me-
chanical refrigeration (VMR) vs. after shipment in conventional trailers with mechanical refrig-
eration (MR)

[Each rating represents average of ratings from 5 replicated test trail-
ers, except “external appearance with wrapper leaves” (4 replications) ]

Exte 2 ‘ne 2 =
Time of examination Trailer _., aeeteenab appearance rate * wie ca
type 1 With wrapper | Without wrapper tion rating 3
leaves leaves

ATTA 2222-2 a ae eas NMR 3.4 3.6 3.8
0D i ae pee ees MR 3.5 3.6 4.0

4 days after arrival* __.___-___-__-__-___ NMR 3.0 3.4 4.1
DO* S20 ees ea ete eck MR 3.0 3.2 4.0

Statistical significance :*
Tratlertype.(T). .--- 22-222 es ee Se NS NS NS
Mxamination: (@)) s-222222=2.-5-2-2.25 055 52.3 ee * +* NS
NK Ws NS NS NS

1 NMR, liquid nitrogen plus mechanical refrigeration (low-oxygen atmosphere): MR, mechanical refrigeration
(normal atmosphere).

2.Rating scale: 1, unsalable; 2, poor; 3, fair: 4, good; 5, excellent.

3 Rating scale: 1, none: 2, trace: 3, slight: 4, moderate; 5, severe.

4 Held at 50° F.

5NS. not significant: *, significant at 5-percent level; **, significant at 1-percent level.

12 MARKET RES. RPT. 759, U.S. DEPT. OF AGRICULTURE

TABLE 8.—Severity ratings and incidence of various disorders of trimmed lettuce heads after shipment
in trailers with liquid nitrogen plus mechanical refrigeration (NMR) vs. after shipment in conven-
tional trailers with mechanical refrigeration (MR)

Severity of indicated disorder? 3

Trailer
Time of examination type 1 Russet Pink Rib
Decay spotting rib Tipburn discolora-
tion
Arrival 2c seccetes voce see eee eee! NMR 1.0 lal 11 1.2 1.3
IDO: joe ose oa ee eee MR V1 1.2 lal aa 1.2
4 days-after’arrival’* ou 2222-02 Ses NMR 1.2 1.5 13 1.2 1.4
Dor {222 see ee bo a Fe eee: MR 13 1.6 1.2 1.3 13
Statistical significance :5
Trailer type UL)? 222 22-2422 toe ea NS NS NS NS NS
Pxamination s(h) Seer. eee ** NS =e NS NS
WX Wicsse ese eesokeoen cee ee NS NS NS NS NS
: Incidence of indicated disorder ® 7
Trailer
Time of examination type 1 Russet Pink Rib
Decay spotting rib Tipburn discolora-
tion
Percent Percent Percent Percent Percent
Arrival 2.vsls 0 sa8s-s58525. 225525 SossS cus NMR 0.8 2.1 3.3 9 9.0
DO? 222 sse2ee So a se ee MR 1.6 6.9 5.7 4.3 9.5
4 days after arrival 4 _.________-_---___-. NMR 8.4 8.4 6.3 8.4 10.0
DO? 222222 8 ee as Se MR 10.9 7 a ba 6.4 15.3 9.7
Statistical significance :5
Trailer. type ih) 2252225222022 eee ee NS = NS NS NS
Hxeamination. (YW) \2hsn.- 2222 econ ee *e * NS na? NS
AD XK Weck ee Se Se NS NS NS NS NS

1NMR, liquid nitrogen plus mechanical refrigeration (low-oxygen atmosphere) ; MR, mechanical refrigeration
(normal atmosphere).

2 Hach rating is average of ratings from 5 replicated test trailers.

3 Rating scale: 1, none; 2, trace; 3, slight; 4, moderate; 5, severe.

4 Held at 50° F.

5 NS, not significant; *, significant at 5-percent level; **, significant at 1-percent level.

6 Each percentage based on 5 replicated test trailers.

7 Geometric mean of percentage of heads affected.

NITROGEN’S EFFECT ON MARKET QUALITY OF WESTERN LETTUCE 13

DISCUSSION

Liquid nitrogen serves two purposes in trans-
portation equipment for produce. It provides re-

.) frigeration and it modifies the atmosphere by

lowering the oxygen concentration. The only ben-
efit of low-oxygen atmospheres to lettuce quality
observed in these tests was a statistically signifi-
-cant reduction in incidence of russet spotting at
destination. Since russet spotting is one of the
more serious market disorders of lettuce, the use
of liquid nitrogen in transportation equipment
appears to have some potential. However, when
liquid nitrogen was the only source of refrigera-

‘| tion, temperature variation and maximum tem-

peratures within the load were excessive. Decay
was significantly greater in lettuce from the
trailers in which liquid nitrogen was the only
source of refrigeration than in that from trailers
that were mechanically refrigerated because of
the high temperatures in the former trailers.
Since decay is such a serious factor in determin-
ing the marketability of lettuce, the high incidence
of this disorder in lettuce shipped in nitrogen-
_ refrigerated equipment presents a problem yet to
be solved. Some modifications in the system are
_ being tested, which may improve temperature uni-
| formity and provide lower average load temper-
_ atures,

Lettuce shipped in trailers having both a liquid
nitrogen system and mechanical refrigeration
(NMR) had a significant reduction in russet
spotting with no increase in decay, compared with
that shipped in conventional, mechanically refrig-

erated (MR) trailers with normal atmospheres.
Incidence and severity of decay were essentially
the same in lettuce from both types of trailers be-
cause the temperatures were similar.

Russet spotting of lettuce is not a problem in
all shipments of lettuce; some lots of lettuce de-
velop excessive amounts of russet spotting during
transit and marketing while others have little or
none. Lipton ® found that when air temperatures
in the field exceeded 86° F. for two or more con-
secutive days, 9 to 14 days before harvest, there
was a substantial increase in russet spotting. The
cause of russet spotting and methods of complete
control are not known. However, it can be reduced
by proper precooling, proper transit refrigera-
tion,’ and by providing low-oxygen atmospheres
during transit.

The performance of conventional piggyback
trailers still leaves something to be desired. More
than one-fourth of these trailers had average load
temperatures of 40° F. or higher during transit,
even though all but one thermostat was set at
36° or lower. The ‘average transit temperature of
all conventional MR trailers was 38°.

6 Lipton, W. J. INFLUENCE OF MAXIMUM AIR TEMPERA-
TURES DURING GROWTH ON THE OCCURRENCE OF RUSSET
SPOTTING IN HEAD LETTUCE. Amer. Soc. Hort. Sci. Proc.
83: 590-595. 1963.

7 Lipton, W. J., and BARGER, W. R. MARKET QUALITY OF
HEAD LETTUCE IN RELATION TO DELAYS BETWEEN HARVEST
AND PRECOOLING AND TEMPERATURE AFTER COOLING. U.S.
Dept. Agr. Agr. Res. Serv. ARS 51-5, 14 pp. 1965.

14 MARKET RES. RPT. 759, U.S. DEPT. OF AGRICULTURE

APPENDIX: Atmospheres Obtained by Using Nitrogen To Break the Vacuum-Cooling Cycle

Breaking the vacuum-cooling cycle with nitro-
gen rather than air affords the possibility of
quickly lowering the level of oxygen within the
lettuce carton. Although low-oxygen atmospheres
have some desirable effects on lettuce quality, it
was not known whether or not such an atmosphere
would be lost before the lettuce could be loaded.

Tests in the U.S. Department of Agriculture’s
pilot vacuum-cooling plant at Fresno, Calif.,
showed the speed at which the oxygen concentra-
tion returned to normal after the vacuum was
broken by nitrogen. Air samples were drawn with
a hypodermic needle from heads of lettuce from
various locations in the cartons at various times

NT )
Nn Wy DW
on Ff

yn & Oo @

OXYGEN CONCENTRATION ( PERCE
°

Oo 4 8 l2 I6 20

after the vacuum was broken by nitrogen. The
oxygen concentrations increased by about 1 per- |
cent per minute for the first 15 minutes and then
increased at a more gradual rate (fig. 6, appen-
dix). Within one-half hour or less, the oxygen
concentration was essentially back to normal.
Similar results were obtained in two com- |
mercial tests (trailers 11 and 13). Oxygen samples |
taken from a tube placed in the center of cartons
of lettuce at various times after the vacuum was |
broken with nitrogen showed an increase in the
oxygen level by about 1 percent per minute. The
oxygen level was essentially back to normal be-
fore the lettuce could be loaded in the trailer.

24 28 32 36 40 44 #448

TIME AFTER VACUUM BROKEN (MINUTES )

Figure 6.— Oxygen concentrations in cartons of vacuum-cooled iettuce after breaking the vacuum with nitrogen.

W328 7

+ U.S. GOVERNMENT PRINTING OFFICE: 1966—0 222-715

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