Item |
Temperature management of apple fruit for maintenance of quality |
Controlling postharvest decays on apples |
DPA effects on McIntosh and Empire |
Rapid CA establishment using nitrogen |
Storage tips |
Storage trivia |
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We have been carrying out a project funded by the Statewide Program Committee Research/Extension Grants program to investigate temperature management of apple fruit for maintenance of quality for local and export markets. Problems in meeting firmness standards for fruit, particularly those exported to Europe, has been a major factor in deciding to put research and extension effort in to this area. A key objective of this study is to measure the effects of current temperature management strategies on apple fruit quality. Our initial approaches have involved obtaining base information on temperatures during different parts of the handling chain.
1. Effects of current packing operations on fruit temperatures
Skin and core temperatures of fruit from various positions on
the packing line have been measured in nine packing houses (Table
1). Fruit that were tested included McIntosh, Empire, Delicious
and Idared, and packing lines were operating with and without
waxing. The data show that the warming of fruit can be rapid,
even while in bins prior to entering the water dumper. The effects
of the water dump process varied according to whether water was
heated or not. However, by the post-dryer step, skin temperatures
averaged 67oF (individual operations ranged from 59
to 77oF), while core temperatures averaged 43oF
(range from 39 to 50oF). These temperatures were reached
even when fruit were not actually being waxed. While on the sorting
tables, core temperatures of fruit continued to increase. Technically,
only heat moves. Surfaces warm first due to water and dryer temperatures
being greater than apple temperatures. Later, as temperatures
equilibrate, the core warms as the surface cools, and both become
equal. In some cases, especially on rotary tables where fruit
may revolve for some time before being packed, very high fruit
temperatures were obtained. Temperatures of packed apples commonly
exceeded 60oF.
Packing is obviously necessary for the industry and warming of
the fruit during this process is unavoidable. These data do, however,
highlight a couple of important steps that can be taken to minimize
warming and the time that fruit sit in warm conditions. The most
important of these starts in the storage room prior to moving
bins of fruit to the water dump. Temperatures in these rooms should
be maintained at 32oF for all varieties. Secondly,
only the required number of bins to maintain the packing operation
should be brought from the storage rooms. Operations that remove
a morning's run at one time, for example, will cause undesirable
warming of fruit to occur. Now that we have established the warming
cycles that occur during packing operations the question that
arises is, "What happens to temperatures of packed fruit?"
2. Cooling rates of fruit in packed cartons
Cardboard is a great insulator, so a warm product inside a sealed
carton on a pallet will take a long time to cool. So far, our
data for temperature changes over time in standard cartons are
limited to one test in which fruit temperatures of 100 count McIntosh
apples were measured over 10 days (Table 2). We put temperature
probes (Figure 2) in single fruit in the center of cell packed
cartons. These electronic probes can record temperatures over
time and the information is downloaded by computer (Figure 3).
The cartons were positioned in a middle carton of the bottom,
top and middle layers.
We measured two factors.
1. The time that it took for fruit to cool from the initial packing
temperature. Sometimes, fruit actually warmed up during this period
because heat was produced faster by fruit respiration than it
was removed by the surrounding cold air. Interior fruit temperatures
always exceeded exterior exposed fruit temperatures during cool-down.
2. The half cooling time is a standard index for expressing cooling
effectiveness. This measures the time that is required to remove
50% of the heat from the apple. For example, if fruit temperatures
are at 72oF in a storage room at 32oF, the
temperature difference between the fruit and final desired temperature
is 40oF. A half cooling time of 8 hours means that
it will take that time to reduce fruit temperatures by 20oF
to 52oF, and a further 8 hours to reduce fruit temperatures
by 10oF to 42oF. Fruit are generally considered
"cooled" after three half cooling times (75% of heat
removed). In the example, three half cooling times would equal
24 hours and the fruit temperature is 37oF. Fruit will
continue to cool to 32oF if left in the 32oF
room.
In our trial, we found that the fruit in a center carton in the
bottom layer of the pallet stack took 13 hours before any cooling
occurred, and the half cooling time was 69 hours (Table 2). In
other words, fruit in this carton took almost 9 days to "cool"
after packing! Fruit in other positions cooled faster, but clearly
still slower than desirable. These temperatures probably reflect
commonly found cooling rates, although cooling in tray packed
cartons is probably faster than that observed in the present study
for cell packed cartons.
3. Methods by which cooling rates of fruit can be increased
We are investigating ways to improve cooling rates of apple fruit
in packed cartons. Cartons with vents designed to allow air from
the cold room to flow over the fruit are commonly used for apples
from some growing regions. These cartons have been a standard
for produce like grapes and tomatoes for many years. Figure 4,
for example, shows New Zealand apple cartons with venting designed
to maximize cooling rates. However, such cartons have not been
used in New York. We compared the effectiveness of vented and
standard cartons with handholds only under both passive and forced-air
systems. Passive cooling was defined as cooling palletized cartons
only in a traditional cold storage, while forced-air cooling utilizes
a fan to actively pull cold air through and around cartons of
fruit. Two tests for each carton/cooling combination were carried
out using the 60 x 40 export carton for Empire. We inserted temperature
probes into an apple placed in the center of the top tray in the
10th layer from the bottom (14 layer pallet) in positions indicated
in Figure 5.
Fruit half cooling times were affected by location, but the effects of venting and cooling method were the most dramatic (Table 3). Note that in non-vented cartons, long delays occurred before fruit cooling was begun. In addition, two features of fruit temperature responses are particularly interesting. First, forced air cooling was about ten times faster than passive cooling, regardless of the presence of vents. Even with no vents, cooling of fruit would be complete within 24 hours (half cooling time x 3). Second, the presence of vents resulted in much faster cooling (approximately 2.5x faster) regardless of cooling method, although half cooling times still averaged 29 hours in passively cooled cartons.
4. Summary
This project is ongoing and further research is required before
firm recommendations about carton design can be made to the industry.
Although we have found in separate experiments that CA-stored
fruit packed in April were 0.5 lb firmer when kept at 32oF
than 41oF for only two weeks, the relationship between
carton temperatures and fruit quality has not been established.
This is a priority for future research, especially for export
fruit where problems with firmness maintenance continue to plague
the industry. An important outcome of this study may be that fruit
with lower flesh firmness can be exported and still meet quality
standards overseas. Carton venting, however, also brings an additional
set of issues including faster warming of packed fruit if the
cold chain is interrupted. Also, successful development of better
cartons will require collaboration with manufacturers to ensure
that carton strength is not compromised.
Nevertheless, our studies to date highlight the need to consider
temperature management at all stages of the operation from harvest
to market. The industry is actively concerned with cooling of
fruit before storage, but this concern must also address the issues
of temperature control during packing and transport.
Table 1: Skin and flesh temperatures (oF) of fruit at different packing houses in New York State. | |||||||
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Packing house No | Variety |
Water dump temp. (oF) |
Fruit location | Bin | Post-water dump | Post-waxer | Sorting tables |
1 |
McIntosh (no wax) |
46 |
Skin Core |
48 43 |
47 38 |
66 44 |
59 48 |
2 |
Empire (no wax) |
67 |
Skin Core |
44 34 |
56 35 |
50 39 |
53 40 |
3 |
Delicious (wax) |
52 |
Skin Core |
53 40 |
60 36 |
69 41 |
63 46 |
4 |
McIntosh (wax) |
80 |
Skin Core |
50 42 |
66 45 |
59 50 |
61 54 |
5 |
Empire (no wax) |
63 |
Skin Core |
51 38 |
51 38 |
63 39 |
63 42 |
6 |
Delicious (wax) |
44 |
Skin Core |
52 39 |
51 38 |
68 42 |
64 43 |
7 |
McIntosh (wax) |
68 |
Skin Core |
53 39 |
53 40 |
77 44 |
67 48 |
8 |
Empire (no wax) |
50 |
Skin Core |
49 36 |
60 39 |
77 41 |
71 40 |
9 |
Idared (wax) |
53 |
Skin Core |
55 36 |
58 37 |
69 41 |
69 46 |
Average |
Skin Core |
51 38 |
56 38 |
67 43 |
63 45 |
Table 2: Delays in cooling (hr) and half cooling times (hr) for McIntosh fruit in bushel cartons stacked on a pallet. | ||
Position on pallet1 | Cooling delay (hr) | Half cooling time (hr) |
Bottom | 13 | 69 |
Middle | 0 | 38 |
Top | 0 | 34 |
1 Center cartons on pallet, 7 cartons high on rows 1 (bottom), 4 (middle), and 7 (top). |
Table 3: Half cooling time (hr) values for 60 X 40 export cartons of Empire apples in normal or vented cartons, either passive or force air cooled. Each value is the average of two independent experiments. | ||||||
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Treatment | 1 | 2 | 3 | 4 | 5 | Average |
Passive, no ventsb, c | 72 | 92 | 64 | 101 | 89 | 84 |
Forced, no vents | 6 | 6 | 8 | 10 | 8 | 8 |
Passive, vents | 30 | 34 | 23 | 30 | 27 | 29 |
Forced, vents | 2 | 3 | 2 | 3 | 4 | 3 |
a See Fig. 1 for sensor location and air flow direction. | ||||||
b "Unvented" carton contained hand holds in ends only | ||||||
c Delays in start of cooling process in this treatment were 6 hr, 21.6 hr, 18.5 hr, 41.4 hr, 12 hr, and 15.5 hr for locations 1, 2, 3, 4, and 5, respectively. |
Recommendations for controlling postharvest decays on apples remain largely unchanged from previous years and are summarized at the end of this article. Thiabendazole (TBZ, sold as Mertect 340F) and captan continue to be the only chemical fungicides with postharvest labels for apples. Several biocontrol fungicides are registered, but they are either unavailable in New York or ineffective under commercial conditions. Effectiveness of TBZ has been decreasing in recent years because of fungicide-resistance problems. Captan provides good control of postharvest decays when used at the full label rate of 2.5 lb per 100 gallons of drench solution, but use of captan is limited by zero-residue tolerances for captan in some export and US processing markets. In the absence of fully-effective postharvest fungicides, sanitation measures become increasingly important for controlling losses to storage decays.
Fungicide-resistance problems with apple postharvest pathogens
have gradually increased in severity over the past 25 years. Many
strains of Penicillium and Botrytis developed resistance to TBZ
and related benzimidazole fungicides (Benlate, Topsin M) soon
after these products were introduced in the 1970's. However, the
benzimidazole fungicides continued to provide acceptable control
of postharvest decays for at least 10 years after resistant strains
of the pathogens were present in storages. Research in the mid-1980's
showed that most of the benzimidazole-resistant strains of Penicillium
expansum and Botrytis cinerea were unusually sensitive to diphenylamine
(DPA), an anti-oxidant used to control storage scald on apples.
In commercial practice, TBZ and DPA are usually applied together,
and that combination provided adequate control of both benzimidazole-sensitive
and benzimidazole-resistant pathogens. Effectiveness of the benzimidazole-DPA
combination decreased, however, as strains of Penicillium expansum
with resistance to both chemicals gradually emerged in the 1990's.
These doubly-resistant strains have caused up to 15% decay in
some lots of Empire fruit held in controlled atmosphere (CA) storage
for 9-10 months after harvest.
Several biocontrol fungicides have federal registrations for postharvest
use on apples. Biocontrol fungicides are formulations of bacteria
or yeasts- living organisms that actually grow on the fruit after
they are applied. Biocontrols do not act by killing pathogen spores
or by inhibiting spore germination. Instead, they arrest decays
by colonizing the wounds on apple fruit where decays are usually
initiated. The biocontrol organisms presumably utilize all of
the available nutrients in the wounds, leaving nothing to support
initial growth of the decay fungi. The decay fungi utilize the
apple juice and damaged cells in wounds as a source of nutrients
for initial growth of spores. When this "start-up fuel"
is consumed by the biocontrol fungi, the pathogens are left without
the nutrients needed to initiate growth.
A series of experiments was conducted with the biocontrol product,
Decco I-182, during the 1997 harvest season to determine if it
could be used alone or in combination with TBZ to control mixed
populations of TBZ-sensitive and TBZ-resistant P. expansum. Decco
I-182 is a formulation of the yeast Candida oleophila that was
formerly marketed as 'Aspire'. Results from the 1997 trials showed
that although Decco I-182 was sometimes as effective as the standard
DPA/TBZ combination, it was never superior and its effectiveness
seemed to fade as the duration of storage increased. Furthermore,
there was no additive effect or benefit from combining Decco I-182
with the standard DPA/TBZ treatment. With continued research,
more effective ways of applying and using Decco I-182 might be
devised. In the meantime, however, there seems to be little reason
for including this expensive product in postharvest treatments
of apples.
Research we conducted over the past 10 years has repeatedly shown
that the predominant postharvest pathogen in apples receiving
postharvest treatments is Penicillium expansum. In apples that
are moved to storage without treatment, decays caused by Botrytis
cinerea often predominate. (Apples with Botrytis decay come out
of CA storage as firm, tan, completely-decayed fruit that look
very much like baked apples.) The biology and epidemiology of
Botrytis decays in apples has not been adequately researched under
east-coast conditions. However, it seems likely that Botrytis
decays are uncommon in fruit receiving postharvest treatments
because relatively few strains of Botrytis have developed resistance
to both DPA and TBZ whereas such resistance is common in Penicillium.
Empire apples from western NY may require postharvest treatment
to control the Botrytis decays that predominate when Empire fruit
are stored without any postharvest treatment. We do not know why
Botrytis decays are more common in Empire apples from Western
NY than in those from the Hudson Valley. However, the prevalence
of Botrytis in western NY may relate to growing conditions in
the field. In some other crops (grapes, kiwi), researchers have
shown that Botrytis infections that occur early in the season
remain dormant until fruit begin to ripen. It is quite possible
that latent Botrytis infections on apples are favored by the cooler
summers (and perhaps cooler and damper conditions during bloom
and petal fall) that generally prevail in western NY as compared
to the Hudson Valley. If early-season conditions contribute to
latent infections of Botrytis on apples, and if these latent infections
are a primary cause of postharvest Botrytis decays under NY conditions,
then 1998 could be a bad year for Botrytis decays in stored apples
because the extended wet periods that prevailed during and immediately
after bloom would have favored higher-than-usual levels of infection.
Following are suggestions for controlling postharvest decays of apples for the 1998 harvest season:
1. Apply postharvest fungicide treatments only to when needed.
For example, most New York growers have found the Golden Delicious
are best stored without any postharvest treatment. Empire produced
in the Hudson Valley are frequently stored without any postharvest
treatment, but Empire fruit from western NY may require treatment
to prevent Botrytis. Fruit held for less than two months in regular
cold storage seldom require any postharvest treatment. If DPA
must be applied to control storage scald, then TBZ or TBZ plus
captan should be applied to control pathogens that will accumulate
in the drench solution.
2. Always use DPA and TBZ together because that combination provides
better control of both TBZ-sensitive and TBZ-resistant isolates
than either product used alone. Follow label recommendations for
rates, replenishing solutions, and maximum quantities of fruit
that can be treated before the drencher solution must be replaced.
3. Captan used at the full label rate will help to control isolates
of P. expansum that are resistant to DPA and TBZ. However, captan-treated
fruit may be unacceptable in some markets. Furthermore, captan
is again under close regulatory scrutiny because of the Food Quality
Protection Act. The captan label might be revised in the near
future. Using captan might reduce losses to postharvest decays
but could significantly limit marketability of treated fruit.
4. Only use application equipment that has an effective agitation
system in the holding tank. Otherwise, the fungicide(s) in the
drench solution will settle out of solution on the bottom of the
tank.
5. Use the cleanest bins available for fruit that is most at-risk
for decay. Thus, where possible, use new bins or sanitized bins
for Empire fruit that will be held in long-term storage and be
especially careful of drench-water sanitation when these fruit
are being treated.
6. Rapid cooling can significantly reduce the incidence of decay
that develops in storage. If CA rooms are being filled rapidly,
fruit should be pre-cooled in separate rooms before it is loaded
in the CA room so as to reduce the total cooling time.
Last year we reported that McIntosh apples were firmer when treated with a prestorage drench with diphenylamine (DPA), regardless of the presence of calcium salts. This result supported earlier results obtained with Empire apples in 1995. There are two reasons for being interested in these responses. First, the apparent benefit obtained on firmness (and reduction of carbon dioxide injury) of these varieties may sometimes be critical in meeting market requirements. Second, and perhaps as important, is the observation that current handling and storage protocols have developed during a time that DPA application has become standard for the industry. If the chemical is lost to the industry, there may be implications for storage operations that we are not expecting. An excellent example of this possibility, were the increased fruit losses in Empire apples a few years ago because of external carbon dioxide injury. High losses occurred when many operators stopped using DPA, and ironically, the worst problems occurred in those operations with optimal storage conditions such as rapid CA. Historically, carbon dioxide injury used to be a standard problem for McIntosh, but incidence of the disorder now seems low. One reason may be the widespread use of DPA.
During the 1997 season we harvested fruit from 4 orchards in the
major apple growing regions during the commercial harvest season.
McIntosh apples were harvested from the Champlain, Hudson Valley
and Western New York, and Empire only from Hudson Valley and Western
New York. On the day of harvest, fruit were dipped in water, DPA,
calcium chloride, Mertec 340-F, or a combined cocktail of all
chemicals.
McIntosh apples were stored in 2% carbon dioxide and 2% oxygen,
or 5% carbon dioxide (in 2% oxygen) after one month at the lower
concentration as commercially recommended. Empire apples were
stored in 2% carbon dioxide and 2% oxygen. Both varieties were
stored for seven months and quality factors measured after one
and seven days at 68 oF.
For McIntosh, firmness of DPA-treated fruit was always higher
than water- or TBZ-treated fruit, but not always higher than calcium-treated
fruit (Table x). However, the cocktail of all chemicals resulted
in consistently firmer fruit in all growing regions. For Empire,
DPA-treated fruit, alone or in combination, were always firmer
than any other treatment (Table 1).
External carbon dioxide injury occurred only in Hudson Valley-grown
McIntosh, being as high as 13% in fruit from one of the test orchards.
Injury was absent in both DPA treatment. Senescent breakdown was
found in both varieties and all regions, but was not affected
by treatment. Superficial scald was found only in two orchards
in the Champlain region, averaging 10%, but was completely controlled
by DPA.
The bottom line is that there are clear advantages to use of DPA,
over and above, control of scald. However, it is labeled only
as a scald-controlling chemical. If you are not currently using
DPA we recommend that you continue to avoid its use. However,
if you are using it, you need to know that it may be resulting
in effects over and above prevention of scald development during
storage. If the industry loses DPA for scald control, recognition
of the impact of these other effects may become important especially
as they relate to firmness and susceptibility of fruit to carbon
dioxide injury.
Table 3: Effect of postharvest drench of water, DPA, CaCl2 and/or TBZ1 on firmness of 'McIntosh' and 'Empire' apples.2, 3 | |||||
Cultivar | Treatment |
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Champlain | Hudson Valley | Western NY | Average | ||
McIntosh | |||||
Water | 13.4 | 13.5 | 13.8 | 13.6 c | |
DPA | 13.5 | 13.8 | 14.2 | 13.8 ab | |
CaCl2 | 13.3 | 13.7 | 14.0 | 13.7 bc | |
TBZ | 13.3 | 13.4 | 13.9 | 13.5 c | |
DPA,CaCl2,TBZ | 13.8 | 13.8 | 14.4 | 14.0 a | |
Empire | |||||
Water | - | 15.5 | 15.1 | 15.3 b | |
DPA | - | 15.8 | 15.5 | 15.7 a | |
CaCl2 | - | 15.4 | 15.1 | 15.2 b | |
TBZ | - | 15.4 | 15.1 | 15.3 b | |
DPA,CaCl2,TBZ | - | 16.0 | 15.6 | 15.8 a | |
1 1000 ppm DPA; 25 lb/100gal CaCl2; 16 fl oz/100gal Mertec 340-F | |||||
2 For McIntosh, fruit were stored at 2% CO2 throughout or in 2% CO2 for one month then 5% CO2 (in 2% O2). Fruit were measured after 7 months and firmness readings are the average of 1 and 7 days shelf life at 68oF | |||||
3 Four orchards harvested in each region. |
Air separation equipment and cryogenic Nitrogen have been in common use in the northeast for over ten years However, we still receive questions on the use of liquid nitrogen and the operation of air separators, so a review article on the subject seems to be in order. The two topics of the day are safe and effective application of liquid nitrogen, and operation of the air separator for rapid pulldown of oxygen.
Liquid Nitrogen
In 1986 we summarized our recommendations for liquid nitrogen
use in rapid CA. Purging or "flushing" CA rooms is a
very quick way of reducing the Oxygen level and it also eliminates
the problems of CO2 accumulation and hydrocarbon contamination
experienced with catalytic burners. There are, however, some cautions
to be observed when using liquid nitrogen.
The boiling point of liquid nitrogen is -320°F and it must
be handled carefully to prevent injury to people and apples. We
originally recommended introducing the liquid nitrogen directly
into the CA room. This was because
external nitrogen vaporizers were not commonly available and without
the vaporizer, the flow of gas from the Dewar was slow. With direct
injection of liquid, a Dewar containing 45 gallons of liquid (3600
cubic feet of nitrogen gas) could be safely emptied in 20 to 30
minutes, but this procedure always left some freeze-damaged apples
in the room. The loss of fruit and concern about ethylene produced
by the spoiling apples caused most operators to use a vaporizer
and convert the cold liquid to a gas before piping it to the CA
room. Bulk truckloads of nitrogen are now commonly offloaded as
vapor and nitrogen from Dewars may need to be fed through a vaporizer
in order to achieve an acceptable purge rate for rapid CA.
Operators who use nitrogen routinely generally know how much nitrogen they need for each CA room. Those contemplating using nitrogen for the first time may wish to estimate the volume needed so that they can determine the cost or reserve a supply. Estimated liquid and gaseous nitrogen use per 1,000 bushels of storage is shown in Table 1
Table 1. Nitrogen Requirements for Rapid CA. (Pure, 100% Nitrogen.) | ||
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21 |
0 |
0 |
10 |
1,110 |
56 |
5 |
2,160 |
108 |
4 |
2,490 |
125 |
3 |
2,930 |
147 |
2 |
3,530 |
177 |
1 |
4,560 |
228 |
Liquid nitrogen is sold by the liter. One liter of liquid expands
to 20 cubic feet of gas when it vaporizes. It is imperative that
you monitor the room pressure when the nitrogen is being applied.
Gaseous nitrogen is safer from a room pressure standpoint than
direct injection of liquid, but it is still possible to over pressurize
the room and destroy the gas seal if the nitrogen flow into the
CA room exceeds the capacity of the port hole or exhaust vent.
Room pressure should never exceed one inch of water column pressure.
Before establishing the atmosphere, lock or seal all gas tight windows and doors and placard all access points with "Danger-Low Oxygen" signs. Defrost the evaporator(s) and fully open the porthole before initiating the purging process. Nitrogen should be injected into the room near the evaporator and all fans should be turned on to facilitate mixing. Make absolutely sure that the hallway space near the CA room porthole is well ventilated and that people are kept out of the area while purging is taking place. The cool, oxygen depleted effluent gas from the CA room may pool in the hallways, analysis rooms, loading docks, etc. and there may not be enough oxygen present to sustain human life. When the desired atmosphere concentration is achieved and the room pressure has dropped to near zero, close the porthole and continue to ventilate the hallway areas.
Air Separators
Air separators are now widely used to establish the low oxygen
atmosphere and to a lesser extent for scrubbing CO2
and maintaining the atmosphere in leaky or partially empty CA
rooms. The same precautions associated with liquid nitrogen should
be followed when purging with an air separator. Gas from an air
separator will not cause freeze damage but it can over pressurize
the CA room or cause asphyxiation. The oxygen-rich effluent from
the air separator should be vented out doors and away from equipment.
The quantity of purge gas and rate of oxygen depletion during purging depends on the capacity of the air separator and the purity of gas being delivered. In Table 2, the quantity and purity of purge gas needed to achieve various storage atmospheres is shown. A lesser quantity of a purer gas is needed to achieve the same final oxygen level in the CA room. However, the cost of producing the purer gas is higher and the delivery is lower than for the less pure gas. In addition, because all air separators deliver higher capacities at lower purities, it is possible to achieve faster pull down rates at certain stages of the process by using a less pure gas.
Table 2. Volume of Purge Gas From Air Separator Required To Achieve Indicated Oxygen Level. (Volume of gas per 1,000 bushels of apples.) | |||
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Desired Oxygen Concentration In CA Room, % | 99% Nitrogen | 98% Nitrogen | 95% Nitrogen |
21 |
0 |
0 |
0 |
10 |
1,199 |
1,298 |
1,740 |
7 |
1,800 |
2,010 |
3,120 |
5 |
2,415 |
2,775 |
- |
4 |
2,850 |
3,375 |
- |
3 |
3,450 |
4,410 |
- |
2 |
4,500 |
- |
- |
All air separators have a unique output rate versus purity relationship making it impossible to accurately generalize the performance of all these systems. The manufacturer should be able (and willing) to specify in writing exactly what their machine will do for you. All air separators deliver less gas per hour as the purity of the gas increases. Typically, atmosphere establishment with these machines is faster if you start the purging process on a room at 21% oxygen with an air separator purity setting in the 95 to 96% nitrogen range and then increase the purity to 98% when the room oxygen has dropped below 10%. If you are purging to a final concentration below 5% oxygen, it is generally most efficient to change the purity setting from 98% to 99% when the room oxygen reaches approximately 5.5%. The optimum set points are unique to each machine, and if the manufacturer has other recommendations, follow those instead.
With the electronics available today, I cannot understand why the manufacturers do not simply add a $25 microprocessor to their control panel to provide the optimum purity during the entire purging process. Aside from faster atmosphere establishment, you will save money on electricity by making the indicated manual adjustments.
Separator Configurations:
In the past some growers experimented with returning the vent
gas from the CA room to the air separator intake. Where the distances
were short this could be accomplished without causing excessive
back pressure in the CA room. The lower oxygen in the vent gas
improved the separator efficiency, although considerable fresh
air still had to be fed into the compressor along with the effluent
from the CA room. Several years ago one of the air separator companies
patented this process and it is now illegal to configure air separators
in this manner.
To my knowledge, you can still legally "cascade" two
rooms when using air separators or liquid nitrogen to establish
the atmosphere. The output of one room is fed into the second
CA room the begin the purging process. This procedure is also
helpful in situations where harvest rates briefly exceed machine
capacity. If machine capacity is constantly lagging, it would
be wise to purchase some nitrogen to permit rapid CA establishment
in all rooms.
Several people have called me over the past year to inquire about
on site storage of nitrogen from their air separators. While this
is an appealing concept, it is a very difficult thing to do on
a large scale in a practical way.
If you require small quantities of nitrogen for purging carbon
scrubbers after the regeneration cycle, then a tank with a capacity
of several hundred cubic feet will suffice. This quantity of low
pressure gas will supply the CO2 scrubber for several hours, and
will prevent the air separator from "short cycling"
each time the CO2 scrubber is regenerated. Larger quantity
storage is more difficult, especially if one wishes to have enough
reserve gas to purge a CA room.
The situation is best examined in light of the information in
Table 2. It requires from 2,415 to 2,775 cubic feet of 99% and
98% purity nitrogen respectively, to purge 1000 bushels of storage
capacity down to 5% oxygen. A 10,000 bushel CA room would require
roughly 25,000 cubic feet of nitrogen. In terms of volume, this
is approximately equal to the empty volume of a 10,000 bushel
CA room. Since the nitrogen from the air separator is produced
at a relatively low pressure (5 to 6 atmospheres), the storage
vessel(s) have to be large to accommodate significant volumes.
(At 5 atmospheres of pressure, the 25,000 cubic feet would fit
into a space 1/5th the size of the empty 10,000 bushel CA room.)
Higher pressure storage would require robust tanks and booster
pumps to compress the gas. While these things are all technically
feasible, they are not cost efficient in a one time use per season
application.
Respiration: The respiration rate of apples at 40°F
is double that at 32°F. This fact underscores the need for
rapid cooling at harvest. This also means that if temperatures
are not closely maintained, significant deterioration can occur
during long term storage. Store all varieties at the safe recommended
temperature and make sure the temperature is being accurately
maintained. Do not store chill-sensitive varieties at colder than
recommended temperatures.
Leak tests: All CA rooms should be leak tested to insure
gas tightness. One of the recommendations for successful CA is
a pressure test standard whereby the room loses half of its pressure
in 20 to 30 minutes. We suggest starting from a positive pressure
of one inch water column and recording the time it takes to reach
one-half inch. This should take at least 20 minutes. The starting
pressure may be less than one inch, but the time to reach half
the starting pressure should still be 20 minutes.
In some cases rooms may test less than 20 minutes and still function
satisfactorily. We recommend 20 minutes because this gives a factor
of safety and rooms always tend to get less gas tight with age
and abuse.
Some growers expressed concern about testing metal clad "tin
rooms". They feel one inch of negative pressure could loosen
the metal from the walls in old rooms. If the metal and the fasteners
have seriously deteriorated this could happen as an alternative
test we recommend using less pressure, e.g. 0.3 to 0.4 inches
of water column when testing these room under "vacuum".
This is sufficient pressure to use
soap solution to find small leaks that do not make audible noise.
'Soap': 'Soap' solution for leak testing CA rooms can be improved by adding glycerin to the detergent-water mixture. The glycerin creates a stronger bubble that persists longer and is more visible. Add the same volume of glycerin as dish detergent to your solution. This may be brushed or spritzed on the areas being tested. This solution will wash off easily with water.
Voids: Fully loaded CA rooms actually contain more voids than solids. A 20 bushel hardwood bin occupies about 34 cubic feet of space in the CA room. There is about 3 cubic feet of solid wood in the bin and the 20 bushels of fruit displace about 17 cubic feet. The total solids volume of the bin plus the fruit is approximately 20 cubic feet, or one cubic foot of solids inside the room per bushel. The volume of most CA rooms ranges from about 2.1 to 2.6 cubic foot per bushel of capacity. This means there are somewhere between 1.1 to 1.6 cubic feet of empty space for every 1.0 cubic foot of solids inside the filled CA room.
Many growers and storage operators contributed fruit and allowed
us to obtain data from their storage and packing operations. Special
thanks go to Walt Blackler and Bob Rigdon at Apple Acres who allowed
us to obtain much of the data on forced air cooling in their facility.
The Statewide Program Committee Research/Extension Grants program
has funded the temperature project. Thanks also to Bill Gerling
at Lake Ontario Fruit for assisting with commercial-scale trials
of Decco I-182. Tom Clark assisted with collection of temperature
information. We thank Jackie Nock and Melany Budiman (Ithaca),
and Fritz Meyer, Catherine Ahlers and Albert Woelfersheim (Hudson
Valley Lab.) for excellent technical assistance.
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