EFFICACY OF 'I'EMPERA1URETRE'KIMENTSFOR lliSEcrDISINFESTATION OF DRIED FRUITS AND NUI'S

J. A. Johnson, H. R. Bolin, G. Fuller, and J. F. Thompson

In order to determine the potential of various temperature treabnents fordisinfesting dried fruits and nuts of postha1:vest insect pests, hightemperature/exposure response cw:ves for navel oran;Jeworm eggs and pupaeand Indiarnneal moth pupae were estimated. For navel oran;Jeworm eggs, 99%mortality was achieved after exposure to 490e for 14 minutes. For navelorange worm pupae, 99% mortalities were prc:duced after a 12 minute exposureto 490e. Indianrneal moth pupae were less heat tolerant, similarmortalities were obtained after 13 minutes at only 46oe. Freezingpoints for various stages of the Indianrneal moth and navel orangeworm werealso determined. Pupae had the lowest freezing points for both species,with freezing points of -24oe for Indianrneal moth and -22oe for navelorangeworm. DiapausiJ:lg Indianrneal moth lcu:vae also had low freezingpoints, averaging -230e. The freezing points for non-diapausing lcu:vaewere highest, -lSoe for Indianrneal moth and -10oe for laboratory-reared navel oran;Jeworm. Wild-type NOWlcu:vae had lower freezing points(-16oe) than laboratory-reared lcu:vae, but freezing points for wild-typepupae (-22°C) were similar to those for laboratory-reared pupae. Ageeffected the response of both ll1M and NOWeggs to sub-freezingtemperatures. Eggs appear to be the most cold tolerant stage of both ll1Mand NOW, and the ll1M is more tolerant than the NOO. studies on the effectof high temperatures on prc:duct quality showed that the taste of prunesand raisins are only slightly effected by exposure to sooe for 1 hour.Walnuts showed little change in taste for up to 9 months of storage, butan increase in rancidity was associated with heat treatments. Informationon the engineering and economic aspects of both high and low temperaturetreabnents was gathered and an analysis begun.

OBJECTIVES

The purpose of the study is the development of temperature treabnents asalternatives to fumigation for insect disinfestation of post.ha1:vest driedfruits and nuts. In accomplishing this goal the following obj ecti ves havebeen set.

(a) Determine the temperature extremes and associated exposure timesrequired to kill various stages of the Indianrneal moth (ll1M), thenavel oran;Jeworm (NOO) and the driedfruit beetle (DFB).(b) Compare control efficacy of different heat application methodsagainst the above insects in infested raisins, prunes and walnuts.(c) Evaluate the effect of the above treatments on prc:duct quality.(d) Estimate and compare the costs and feasibility of the abovetreabnents to currently used fumigants and other alternatives.

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The mortality of pupae of the IMM, arrl eggs arrl pupae of the NOOexposedto temperatures ranging from 42 to 490C for different times wasdetermined. Treatments were applied in a water bath capable ofmaintaining constant temperatures with :!:0.20C accuracy. For IMMpupae,both direct hot water irmrersion arrl indirect irmrersion in srrall watertightcells were used to predict the efficacy of different heating methcds. ForNCMeggs and pupae, only direct iImnersion was used. Data were analyzed toplot thenna1. death rate curves arrl predict UI'SOS' UI'9Ss and LT99sfor each temperature.

Freezing points for IMMpupae, non-diapausing larvae and diapausing laxvaearrl NCMpupae and larvae were determined by CCX)ling test insects atconstant rates in dewar flasks containing liquid nitrogen. Freezingevents were observable as sudden increases in insect surface temperaturesas detected by thermocouples. Mortality of different ages of IMMand NOWeggs exposed to sub-freezing temperatures, along with those of NOWlarvaeand pupae, was determined.

To determine the effect of high ~tures on product quality, prunes,raisins and walnuts were heated at SSOC for 1 hour. Walnuts were alsoheated at 400C for 48 hours arrl 4SoC for 4 hours. The heat treatedsamples and untreated controls were divided into subsarnples, sealed inglass jars and stored at 200C. samples were removed every 2 months,processed and submitted to a duo-trio taste panel for flavor evaluation.Replicate samples were also evaluated for colo1;' and moisture.

Researchers have gathered much of the necessary information for analysisof the economic feasibility of temperature treatments for raisins, prunesand walnuts. A prel:iJninary report on their findings has been prepared.Also, equipment that will be used to apply precise temperatures toinfested cormnodities has been designed arrl built by proj ect engineers.

RESULTS

Efficacy studies:

Exposure times that would provide so, 9S and 99% mortality (UI'SO'ill 5' and LT99) of IMMarrl NCMpupae at various temperatures wereesfllnated us1l1g simple regression analysis, and are presented inTable 1-For both insects, a single degree cl1arBe in temperature resulted inconsiderable differences in UI' values. NCWpupae proved to be more heatresistant than IMMpupae. The UI'99 for NCMat 460C was nearly threetlines that for IMM. To obtain NCMUI'99S that were cgmpara}?le to those ofIMMat 460C, the temperature had to be raised to 490C.

Tests using different heating methcds against IMMpupae, showed thatdifferences in heating rate account for differences in mortalities,because a slower heating rate increases the exposure times necessary forthe desired mortality. For this reason, only direct ilmnersion methcdswill be used for the remainder of these studies.

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Simple regression analysis gave fairly acx:urate estiltIations of Ill'50S'IrI'95s an:l IrI'99s for NCMeggs, an:l are presented in Table 2. At lowertemperatures, NCMeggs were fairly heat tolerant: 194 minutes wererequired to prcx:iuce 99% ItDrtality at 45°C. At 49°C, the sameItDrtality is obtained at 14 minutes. Eggs proved much ItDre heat tolerantthan pupae at lower temperatures, but this difference decreased astemperatures increased, with eggs less tolerant than pupae at 49°C.

IMMlcu:vae an:l pupae were capable of surviving exposure to temperatureswell below OoC without freezing (Table 3). IMMpupae proved IrOSt freezeresistant, the average temperature at which IMMpupae froze was-23.90C. Diapausing larvae, the natural overwintering stage of theinsect, had an average freezing point of -22.6oC. Non-diapausinglarvae, with an average freezing point of only -15.2oC, were much lessfreeze tolerant than either diapausing larvae or pupae.

Laboratory-reared NOWlarvae also had much higher freezing points(-9.7°C) than NOWpupae (-22. 3°C). Freezing points for wild-type NOWlarvae were much lower (-16.1oC) than for laboratory-reared larvae.Wild-type NOWpupae had freezing points very close to laboratory-rearedpupae.

Effects of freezing temperatures on hatch of IMMeggs varied with the ageof egg (Fig. 1). Eggs ~ to -16°C at 12 hour inteJ:vals an:l to-20°C at 6 hour intervals showed an increase in cold tolerancethroughout the first 47 to 53 hours of egg development, followed by arapid increase in susceptibility over the next 24 hours. A similarpattern has been identified in NCMeggs (Fig. 2), an:l additional work isplanned to determine the ItDSt tolerant egg age for this insect.Preli1nina1:y work using -20°C as a treatment temperature for NOWeggsresulted in very high ItDrtalities, in:licating that NOWeggs are ItDresusceptible than IMMeggs.

Mortality of NOWlarvae and pupae exposed to -12, -16, -20 and -24°Cshowed that pupae were m:>re cold tolerant than larvae (Table 4). '!hisconfinns predictions made from freezing" point studies, which identifiedthe pupal stage as the ItDSt cold tolerant. However, a comparison of Table4 with Fig. 2 indicates that NOWeggs are even m:>re cold tolerant.

Quality Studies:

Results from the quality studies showed that, after 9 months in storage,heat treatment did not cause accelerated darkening of raisins, prunes andwalnuts. Taste panel results showed no discernable taste differencesbetween treated and control raisins for up to 6 ItDnths in storage. At 9months there was a flavor difference, with 20% of the panelists in:licatingan off flavor in the heated raisins. No taste differences were detectedin prunes up to 9 ItDnths in storage. Treated walnuts showed no tastedifferences for up to 8 to 9 m:>nths in storage, when the study was stoppedbecause of rancidity. Table 5 shows thatthe degree of the taste panelindicating rancidity seemed correlated to the severity of the treatment.

180

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Economic am Engineer:in;J:

'!he economic analysis of high terrp=rature clisinfestations was begun, amwill be based on the current a::>st of driers for pra:lucts such as pistachionuts, walnuts, am dried prunes. '!he complete financial analysis of hightemperature disinfestation will probably be based on us:in;J these driers intheir off season. '!hese driers are idle for 10 to 11 months Per year andwould be available for use. Also, they will probably require very littleIOOdification in order to be used for disinfestation because they arealready set up to harxUe the crops that we are consider:in;J.

If::Mtemperature disinfestation costs will be based on forced air cool:in;Jam storage operations which are COImOC>nlyused in Califo:rnia's fresh fruitam vegetable industry. '!he off season for many of these facilities issix to nine months am we may assume that existiD;J installations can beused for disinfestation in our financial analysis. We have collected somecapital cost am energy use information for recently constructedfacilities. '!he possibility of utiliz:in;J night air recirculation toreduce cooling costs is also be:in;J examined. Figs. 3, 4 am 5 giveSUI1Ut1ariesof current am proposed insect control practices for prunes,walnuts am raisins, resPectively.

CONCI.IJSIONS

'!he results from the efficacy tests in:licate that temperatures as low as460C for as little as 13 minutes would effectively control IMMpupae.Both NCWeggs am pupae were more heat resistant; 490C for 14 am 12minutes would be needed to control eggs am pupae, respectively. Whenboth insects are present, a treabrent terrp=rature of 500C for 15 minutesshould insure complete control. Before final recommendations can be made,the thennal death points for larval am adult stages of both NCWani IMM,along with eggs of the IMMam all stages of the DFB, need to bedeteI:mined.

'!he freezing point deteminations in:licate that the pupal stage of the IMMam NCWis more freeze tolerant than the larval stage. Prelllninaryresults from mortality studies with NCMconfinn these fin:lings. Coldtolerance of eggs of both IMMam NCWvaried with age. 'Ihe more resistantNCWegg ages were more cold tolerant than pupae. Early fin:lings irrlicatethat DFB may be the most cold tolerant of the three target insects.

Quality studies detected same off-flavors in heat-treated raisins after 9months of storage. Heat-treated prunes showed no flavor differences after9 months. Heat-treated walnuts showed no differences up to 9 months ofstorage, but increased rancidity was associated with seyerity oftreatment. If::Mtemperature methods may be rore suitable for walnuts.

Preliminary engineer:in;J am economic studies in:licate that modificationsof existing dehydration procedures may provide the most economical heatdisinfestation of carrlidate COImOC>dities. '!he use of exist:in;J lowtemperature facilities for disinfestation may also prove acceptable.

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182

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Table 1. Lethal Times (in minutes) for IMM and NOW Pupae

Temp IMM NOW

(C) LT50 LT95 LT99 LT50 LT95 LT99

42 27.7 40.6 41.8

43 19.1 28.4 29.2

44 13.2 19.7 20.3

45 9.5 16.5 11.2

46 7.4 12.4 12.9 24.0 35.6 36.6

47 - - - 16.9 23.4 23.9

48 - - - 10.8 17.1 17.7

49 - - - 7.2 11.3 11.6

Table 2. Lethal Times (in minutes) for NOW Eggs

TempLT50 LT95 LT99

(C)-45 143.0 189.9 194.1

46 57.4 106.9 111.3

47 31.4 57.8 60.2

48 11.1 28.1 29.6

49 5.0 10.9 14.2

Table 3. Supercooling points for INN and NOW larvae and pupae

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Insect and Stage n x +/- SE Minimum tvlaximum

Non-diapausing 159 -15.2 +/- 0.2 -20.0 -8.0IMN larvae

Diapausing HIM 172 -22.6 +/- 0.2 -27.0 -12. O'larvae

INN Pupae 184 -23.9 +/- 0.2 -28.0 -11.0

Laboratory-reared 55 -9.7 +/- 0.2 -14.0 -7.0NOW larvae

Wild-type NOW 40 -16.1 +/- 0.5 -21.0 -9.0larvae

Laboratory-reared 241 -22.3 +/- 0.5 -26.0 -9.0NOW pupae

Wild-type NOW 40 -21.5 +/- 0.6 -27.0 -11.0pupae

Table 4. Response of NOW Larvae and Pupae to Subfreezing

Temperature

TEMP EXPOSURE % MORTALITY

(min) Larvae Pupae

-16 10

25

40

-20 10 96.5 77.6

20 100.0 96.6

30 100.0 100.0

-24 5 96.5 56.8

10 100.0 84.5

15 100.0 100.0

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-12 20 89.3 8.5

40 85.8 18.8

60 91.2 32.6

89.4 17.1

98.3 74.1

100.0 98.3

Table 5. Percentage of Panel Indicating Samples Tasted Rancid

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STORAGE TIME HEATED AT 40°C HEATED AT 45°C HEATED AT 55°C

0 0 0 0

2 8 15 20

4 8 23 30

6 - 23 38

8 28

9 - 55 60

Fig. 1 Effect of Age on 1MM Egg Hatch afterExposure to -16°and -20°C for 25 minutes

% Eaa Hatch100 ~""

,,,,I

°-20 C

o

\ -16 C\

'\

~I

80

60

.,t:::.

{it... ,.'I "

;' ~"".~'

,.

,

a17 23 29 35 41 47 53 59

Age of Eggs (hrs)

65 71 77

Fig. 2 Effect of Age on NOW Egg Hatch afterExposure to -16°C for 30 minutes

% Hatch100

~ 0

80

\\\\

\

\.

60

I

40 l-

I

20 l-

I

I

I

0124 48 72

Age of Eggs (hrs)

\ \.\\.\

\\1~94

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PRUNES

Process Current InsectControLPr~ctices

-190°F drying air temperaturefor 18 hrs.

Short termStorage

1-2XLong Term .Storage

- MeBr fumigation in Nov. andoften once more in spring

- mist application of contactinsecticide

I I I

~ III I

Juice I II II I

Ilngre~ I i

tIPitted I

- high temperature processing

- high temperature processing

- high temperature processing I

inert atmosphere packaging

- med. temperature processing

Alternative InsectControLPmctices

- improve sanitation toreduce reinfestation

- some processors eliminateNov. fumigation

- insulate roof and usenight ventilation

- improve plant sanitation

- increase temperatureexposure duringrehydration of naturalcondition fruit

- refrigerated storage ofproduct

-MA packaging

Level of infestation Number of fumiqations+ none. low ._X. high

Figure 3. Current and proposed insect control practices for prunes.

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Process

~xLong TermStorage

WALNUTS

Current InsectControLPractices

- hulled and washed

- 110°F drying air temperaturefor 2- 48 hrs.

- MeBr fumigation at receivingand as needed later

- some nuts stored at 40 - 45°F,especially shelling stock

some shelled nuts

packaged in inertatmosphereinshell nuts are

bleached in hypochlorite

Alternative InsectControLPractices

- reduce storage temperatureto 28°F

- insulate roof and usenight ventilation inunrefrigerated storage

- seal storage and use MAfumigation

- seal inshell nuts in nitrogen-flushed bag and disinfest intransit

- refrigerated storage ofproduct or refrigeratedtransport

- MA packaging

Level of infestation+ none. low~ high

Figure 4. Current and proposed insect control practices for walnuts.

188

Number of fumiqations

SUNDRIED RAISINS

Process Current Insect~(")ntr(")' Pr~r.tir.p~

Alternative Insect~(")ntr(")1 Pr~r.tir.p~

initial trash removal - solar disinfestFarmStoraae

some PH3 fumigation

10- 20XLongTerm .

Storage - monthly PH3 fumigation inpaper covered stacks

-solar disinfest

- insulate roof and usenight ventilation instorage rooms

- sorting

- MeBr fumigation in storage .

rooms or fumigators

- improve plant sanitation

- continuous flow hi- temp.disinfest during packing

- refrigerated storage ofproduct

- MA packaging

Level of infestationnone. low. high

Figure 5. Current and proposed insect control practices for raisins.

Number of fumiaations

_X

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