SIMULATED MOISTURE LOSS ANDCOOLING TIME FOR BULK POTATOES

G.C. Misener

Research Station

Agriculture CanadaFredericton, New Brunswick

M.L. MacDonald

Research StationAgriculture Canada

Fredericton, New Brunswick

INTRODUCTION

The maintenance of potatoes attemperatures conducive to good tuberquality is one of the major considerationsof potato storage. During most of thestorage period heat must be removedfrom the structure to maintain propertemperatures. The heat removed is mainlyfrom two sources, the field heat of thepotatoes and the heat of respiration.Other minor sources of heat include thatwhich is transferred through the walls androof, outside air infiltration, and miscellaneous sources such as electric motors,tractors and trucks.

For satisfactory long-term storage, it isnecessary that the storage environment bekept at critical levels of temperature andrelative humidity (6, 7). However, whenpotatoes are first placed in storage it iswell to stimulate wound healing of thetubers. This interval, 10-14 days, requiredfor wound healing is known as the suberi-zation period. Growth of the new skin orperiderm is hastened when the potatoesare held at 20°C or higher and very highhumidity is maintained in the storage.After this preliminary period, thepotatoes are cooled to their holdingtemperature.

Loss of weight and volume of storedpotatoes may be regarded as falling intotwo main categories: (i) natural, loss dueto respiration, evaporation, etc.; and(ii) pathological, loss due to bacterial andfungus disease and physiological breakdown. Losses in the pathological categorywill vary from year to year, dependingupon the incidence of rot organisms andvarious other factors. As pointed out byWilson and Boyd (8), losses from pathological causes can generally be controlledor reduced by applying known methods.Loss in weight due to respiration is verysmall in comparison to the loss in weightcaused by evaporation of water. Smith(5) reported that in common storageduring a 7-mo period shrinkage from

RECEIVED FOR PUBLICATION MARCH 10,1975

72

respiration was only 0.51%, whereas fromtranspiration the shrinkage was 5.71%.Knudsen (2) stated that the loss of drymatter, nearly all as carbon, by respiration, amounted to about 0.07% of thestored weight per month. This was muchless than loss by the evaporation of water.

Much of the cumulative weight loss atthe end of the storage period due tomoisture loss from the potatoes occursduring the 1st weeks or even days afterharvest (4). Moisture loss is high not onlybecause the skin is very thin and permeable to water, but also because of thecuts and bruises which are inevitablypresent as a result of harvesting andtransportation. The tuber's susceptibilityto moisture loss during the initial storageperiod indicates that the rate of lossduring the temperature draw-down periodis very important with regard to tuberquality. The objective of the studyreported in this paper was to determinethe effect of ventilation rate on coolingtime and moisture loss during the initialcooling period and later during the holding period by means of a computersimulation model.

PUBLISHED WEIGHT LOSS

ESTIMATES

Moisture loss from tubers during thestorage period has been investigated byseveral researchers. Burton (1) found itpossible to express the rate of evaporation from tubers during continuous ventilation, after the 1st few weeks when thewounds in the skin were healed, in theform

dwt

dt '(1.2+0.65) VPD 10~ (1)

where

wt =

S =

VPD =

weight loss per unit of tuber (kgvapor/kg tuber);percentage by weight of sprouts;water-vapor pressure deficit of the airsurrounding the tubers (mm Hg);time (h).

For the 1st 2 wk after harvest, a coefficient of 4.2 should be used instead of 1.2.

Schippers (4) also suggested that thepercentage weight loss due to moistureremoved from the tubers is related to theproduct of vapor pressure deficit, expressed in millimeters mercury, and the duration of storage in weeks. Shortly afterharvest, the regression coefficient wasfound to range from 0.7 to 0.9 whichcorresponds to a weight loss of approximately 0.05 mg/cm2/mm hg/h. A regression coefficient of 0.125 was needed to

estimate weight losses after several weeksof storage. The coefficient was found tobe dependent on the maturity of thetuber.

Schaper (3) indicated that no linearrelationship between vapor pressure ortemperature and weight loss was apparentin his study. He also suggested that therelationship was at least partially time-dependent.

More recently, Misenera determinedthat the rate of moisture loss from

Kennebec potato tubers during the initialstorage period could be described by theequation

dwt _ 0.3 (VPD)059 10"dt (O035

(2)

Equation (1) was used to determinemoisture loss during the holding periodwhereas equation (2) was used to describethe phenomena during the initial coolingperiod.

THE COOLING MODEL

By using weather data as an input tocomputer programs developed to simulateheat and mass transfer during the ventilation of potatoes, the time for cooling,expected moisture loss and final potatotemperatures can be determined. Thesimulation models can be used to deter

mine the effects on the cooling phenomena of many variables including depthof potatoes, initial potato temperature,

a Misener, G.C. 1973. Simulated cooling ofpotatoes. Unpublished Ph.D. Thesis, University of Illinois.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 17 NO. 2, DECEMBER 1975

temperature and relative humidity of thesupplied air and ventilation rate.

The cooling model developed byMisener3 was utilized during the study todetermine cooling time and moisture lossat various ventilation rates. Essentiallythe model consists of a set of heat and

mass balances written for a single isothermal layer of potatoes to obtain equations describing the product temperature,the air temperature and specific humidity, and the moisture loss during thecooling process. The model utilizes afinite difference method to determine thecooling and drying performed on a deepbed of potatoes. A complete descriptionof the model is available in the thesisa.

Modifications to the original programwere necessary to accommodate the mixing of the fresh and storage air. When theoutside air is too cold to use alone forcooling, the ventilating air is mixed withthe recirculating air. A heat balance isnecessary to determine the ratio, X, offresh air to the total supply air.

MaCpmixTmix -X(MaCpiTi) -(1 - X)(MaCpsTs) =0

where

(3)

Ma = weight of air (kg);Cpmix ~ specific heat of mixed air (J/kg°C);Tmix = temperature of mixed air (°C);

= specific heat of ambient air (J/kg°C);= temperature of ambient air (°C);= specific heat of storage air (J/kg°C);= temperature of storage air (°C).

Ti

CpsTs

Once the fresh air to total air ratio isknown, the new absolute humidity of theventilation air can be determined.

PROCEDURE

Model Validation

To test the simulation model, experimental tests were carried out with laboratory equipment. The model describingthe initial cooling of the potatoes wasverified by Misenera during the developmental stages. During the present study,it was of interest to determine theaccuracy of the model for describing thecooling of potatoes that had been instorage for 2 mo. Equation (1) was usedto describe the mass transfer during thisportion of the study.

A 2.4-m deep bed of potatoes wassubjected to an air flow with velocity of1.3 m/min. Both the air and potatotemperatures were measured usingcopper-constantan thermocouples

connected to a Bristol multipoint recorder. The thermocouples were mounted inthe bed in order that the temperatures ofthe air and tubers could be sensed every0.15 m of depth. The temperature of thetubers was determined by inserting thethermocouple into the center of thetuber. The ventilation air was supplied byan Aminco-Aire unit. The conditions

under which the bed of potatoes wascooled were inputted to the computermodel in order that temperature gradientsin the bed of potatoes could be calculated. A comparison was then made

£ 8.0<l-

2

6.0

• -• SIMULATED

• • ACTUAL

0 0.5 1.0 1.5 2.0 2.5

DEPTH (m)

Figure 1. Comparison of actual and simulatedtemperature gradient in a bed ofpotatoes after 24 h cooling (initialpotato temperature 16.7°C; air flow1.3 m/min).

«!^-l

>-• SIMULATED

-• ACTUAL

^r^V^

0 0.5 1.0 1.5 2.0 2.5

DEPTH (m)

Figure 2. Comparison of actual and simulatedtemperature gradient in a bed ofpotatoes after 86 h cooling (initialpotato temperature 16.7°C; air flow1.3 m/min).

CANADIAN AGRICULTURAL ENGINEERING, VOL. 17 NO.2, DECEMBER 1975

between the experimental and calculatedvalues.

Simulations Using Weather Data

Weather data were obtained from the

Atmospheric Environment Services,Toronto, Ontario. The data describe a5-yr period, 1967-1971, inclusive forFredericton, New Brunswick. Hourlyobservations of the dry-bulb temperatureand relative humidity were recorded onmagnetic tape. The average hourlydry-bulb temperature and relative humid-

0 12 3 4 5

DEPTH (m)

Figure 3. Simulated temperature gradients in abed of potatoes (air flow 1.56m/min, initial potatoe temperature18.3°C, and cooling period November 1-15).

FINAL POTATO TEMPERATURE-»- 4.5°C TOTAL COOLING TIME-A- 4.5°C ACTUAL FAN HOURS-•- 7.2°C TOTAL COOLING TIME

AIR VELOCITY (m/min)

Figure 4. Simulated cooling times for a 4.8-m(16-ft) deep bed of potatoes withvarious air flow rates (cooling periodNovember 1-15); initial potato temperature 18.3°C).

73

•*- 4.5°C FINAL POTATO TEMPERATURE-•- 7.2°C FINAL POTATO TEMPERATURE

AIR VELOCITY (m/min)

Figure 5. Simulated moisture loss from a 4.8-m(16-ft) deep bed of potatoes withvarious air flow rates (cooling periodNovember 1-15; initial potatotemperature 18.3°C).

_«_ 4.5°C FINAL POTATO TEMPERATURE-•- 7.2°C FINAL POTATO TEMPERATURE

AIR VELOCITY (m/min)

Figure 6. Simulated cooling times for a 4.8-m(16-ft) deep bed of potatoes withvarious air flow rates (cooling periodDecember 1-15; initial potatotemperature 12.8°C).

Figure 7.

74

MR VELOCITY (m/min)

Simulated moisture loss from a 4.8-m(16-ft) deep bed of potatoes withvarious air flow rates (cooling periodis December 1-15; initial potatotemperature 12.8°C).

ity were calculated for the periodsNovember 1-15 and December 1-15,1967-1971. These values were used to

supply the hourly weather data requiredfor the cooling model. Although theactual temperatures for any given yeafrnever follow these profiles exactly, theaveraged values do give a good indicationof what can be expected.

Simulations were carried out for a

4.8-m (16-ft) deep bed of potatoes exposed to air flows varying from 0.52 to5.75 m/min. The simulation was doneusing the average hourly ambient temperatures and relative humidities calculated

for the periods November 1-15 andDecember 1-15. Temperature profiles ofthe deep bed of potatoes were determined, as well as cumulative moisture lossfrom the potatoes.

RESULTS AND DISCUSSION

Model Validation

The temperature gradients within abed of potatoes were measured during thecooling period with an air flow of 1.3m/min. Temperature distributions werepredicted by simulated cooling ofpotatoes under similar conditions. Figures1 and 2 depict the degree of agreementbetween experimental and simulatedvalues. It is apparent from Figures 1 and2 that reasonable agreement existsbetween the two sets of values.

Simulations Using Weather Data

Figure 3 shows the predicted temperature distribution in a 4.8-m (16-ft) deepbed of potatoes as cooling progresses.Eighty-six hours are required to cool themass of potatoes from 18.3 C to 7.2 Cwith an air flow of 1.56 m/min. It is ofinterest to note that although the minimum cooling air temperature is 6.7 C,the potatoes do cool to 5.8°C because ofevaporative cooling.

Simulated cooling times duringNovember 1-15 for the 4.8-m (16-ft) deepbed of potatoes are depicted in Figure 4.The values presented include the timewhen the cooling fan was not operatingbecause of high ambient temperatures.The actual hours that the cooling fanoperated are also presented. The warmambient air conditions during this periodcaused the cooling time to be extendedconsiderably when cooling the potatoesto4.5°C.

The simulated moisture loss from thepotatoes during this initial cooling period,November 1-15, ranged from 0.016 kg/kgof potatoes down to 0.009 kg/kg (Figure

5). The results indicate the importance ofventilation rate with regard to mass transfer. High moisture losses correspond tothe low air flows as indicated in Figure 5.

A similar trend was apparent from thestudy using the weather data forDecember 1-15. Cooling time andmoisture loss (Figures 6 and 7) bothincreased at the lower air flows. Theresults stress the point that in order toreduce the temperature of the entire bedof potatoes, the ventilation must continue over several hours. This should betaken into consideration when planning atime schedule for the ventilation ofpotatoes during the holding period.

SUMMARY

The ventilation of potatoes withambient air was simulated using acomputer model based on the finitedifference method. Heat and mass transfer between the potatoes and coolingmedium can be predicted for variousambient conditions.

Weather data for Fredericton, NewBrunswick were supplied to the model inorder that the effect of ventilation rateon cooling time and moisture loss couldbe studied. Low air flows resulted inincreased cooling time and moisture lossduring both the initial cooling period andthe holding period. Cooling times andmoisture loss are presented in the paperfor the various air flows and coolingperiods.

REFERENCES

1. Burton, W.G. 1973. The basic principles ofpotato storage as practiced in GreatBritain. Eur. Potato J. 6(2): 77-92.

2. Knudsen, E. 1965. Estimation of weightloss in stored potatoes by determinationof their ash content. Eur. Potato J. 8(3):150-153.

3. Schaper, L.A. and E.E. Hudson. 1971.Biological and engineering factorsaffecting white potato losses in storage.ASAEPap.no. 71-377.

4. Schippers, P.A. 1971. Quality of potatoesas related to storage environment. ASAEPap.no. 71-375.

5 Smith, O. 1933. Studies of potato storage.Cornell Univ. Agric. Exp. Sta. Bull. 553.

6. Sparks, W.C., V.T. Smith and J.G. Garner.1968. Ventilating Idaho potato storages.Idaho Agric. Exp. Sta.,Bull. 500.

7. United States Department of Agriculture.1963. Shell ventilation systems for potatostorage. Marketing Res. Rep. no. 579.

8. Wilson, A.R. and A.E.W. Boyd. 1945.Potato wastage in clamps. Agriculture 51:507-512.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 17 NO. 2, DECEMBER 1975