Moisture Sorption Isotherm Characteristics of Potatoes at Four Temperatures

7/28/2019 Moisture Sorption Isotherm Characteristics of Potatoes at Four Temperatures

1/19

Jour nal f Food Engineeri ng 4 (1991) 269-287

Moisture Sorption Isotherm Characteristics of Potatoesat Four TemperaturesN.Wang & J. G. Brent-m

Department of Food Science and Technology, University of Reading, Whiteknights,PO Box 226, Reading RG6 2AP, UK

(Received 14 June 1990; revised version received 20 November 1990;accepted 22 November 1990)

ABSTRACTM oist ure equil i brium data of potat oes by desorpt i on and adsorpt i on weredetermi ned at 40, 50, 60 and 70. The experimental procedur e w as agravimet ri c method. I sost eri c heat s of w at er desorpt i on and adsorpt i onw ere cal cul at ed by appl yi ng t he Clausi us-Clapeyron equati on t o thesorpt i on sot herms at dif l erent emperat ures. Seven mat hemat i cal model sw ere used t o i t t he experi ment al dat a. A nonl i near l east- squares egres-si on program w as used t o eval uat e he constant s of t he seven desorpt i onand adsorpt i on sot herm model s.

INTRODUCTIONThe importance of sorption isotherm data in considering dryingproblems is evident. Drying is a combined heat and mass transferprocess, in which the product temperature rises from room temperatureand approaches the drying air temperature. The sorption data may beused for the proper choice of the end-point of drying process (Gal,1987), that is the optimum residual moisture content of the final product.Too high a residual moisture content after too short a drying time leadsto reduced stability, whereas reduction of the final moisture contentbelow its optimum value wastes energy in the operation. Anotherapplication of sorption isotherms is in the calculation of the drying timeof hygroscopic substances (Gal, 1987).

The differential heat of sorption is a useful quantity because it yieldsinformation on the energetics of water sorption processes in foods. The269

Journal of Food Engineeri ng 0260-8774/91/$03.50 - 0 1991 Elsevier SciencePublishers Ltd, England. Printed in Great Britain


2/19

270 N . Wang, J. G. Brennan

level of moisture content at which the differential heat of sorptionapproaches the heat of vaporization of water is often taken as indicativeof the amount of bound water existing in the food (Duckworth, 1972).Knowledge of the differential heat of sorption is very important whendesigning equipment to be utilized in dehydration processes (Becker &Sallans, 1956; Heldman et al., 1965). The heat of vaporization of sorbedwater may increase to values well above the vaporization of pure wateras food is dehydrated to low moisture levels. The general case of dryingof food materials involves energy input to remove free water throughsublimation or evaporation and water associated with the food matrix(Rizvi, 1986).

Values of isosteric heats of adsorption were reported for spray-driedeggs by Makower (1945); for wheat (desorption) by Becker and Sallans(1956); for wheat flour, starch and gluten (adsorption) by Bushuk andWinkler (1957); for maize (desorption) by Rodriguez-Arias et al. (1963)and Chung and Pfost (1967a); for freeze-dried starch, gelatin gel, beefand peach (adsorption) by Saravacos and Stinchfield (1965). Iglesias andChirife (1976a, b) obtained values of the isosteric heat of water adsorp-tion and desorption of several foods, including fruits, protein foods,vegetables and spices.The objectives of this work are (a) to obtain data on desorption andadsorption isotherms of potato between 40 and 70C; (b) to report theisosteric heat of water sorption; and (c) to evaluate constants of sorptionisotherm models, using nonlinear least-squares regression analysis.

MATERIALS AND METHODSMaterialsFresh Desiree potatoes were purchased from a local market and kept in acold room at 10C for about 2 weeks prior to their use. Fresh potatoeswere used in desorption experiments. Samples used in adsorptionexperiments were sliced to 1 -mm thickness, frozen at - 18C and freeze-dried for 24 h to attain a low moisture content (Gutierrez-Lopez, 1989).Determination of sorption isothermsA gravimetric technique was used to determine the equilibrium moisturecontent of potatoes at several temperatures. Duplicate samples each of3 g + 0.00 1 g for desorption and O-3 g + 0.000 1 g for adsorption wereplaced in petri dishes which were placed inside sorption jars. Sulphuric


3/19

Moi st ure sorpt i on sot herms ofpot atoes 271acid solutions were used to maintain the relative humidity inside the jars.The jars were kept in temperature-controlled cabinets which were set at40,50,60 and 70C. The cabinet temperatures were monitored and con-trolled within + 1C. The concentrations of sulphuric acid at differenttemperatures are listed in Table 1 together with literature values for theirrelative humidities (Gutierrez-Lopez, 1989).

Sample were equilibrated for 3 weeks or more to constant weight( + 0.0001 g). The weight of dry matter was determined by drying at70C under vacuum for 24 h.Models for fitting to sorption isotherm dataThe equilibrium moisture data were fitted with seven mathematicalmodels shown in Table 2. The nonlinear regression analysis (SAS, 1982)was used to estimate the constants of the models from the experimentalresults.

To evaluate the goodness of fit of each equation, the mean relativepercentage deviation modulus (E) was used (Aguerre et cd.,1989). Thismodulus is defined by the following equation:

E% = -100 ~ InZi-m,jIN i=l mi (1)where lzti and mPi are experimental and predicted values, respectively,and N is the number of experimental data. This parameter was used inthe literature to evaluate the goodness of fit of different mathematicalexpressions as applied to experimental data (Boquet et al, 1978;Lomauro et al ., 1985). It is generally considered that E values below10% indicate a reasonably good fit for practical purposes.

TABLE 1Relative Humidities of Sulphuric Acid Solutions at the Temperatures UsedTemperature

CC) 20 30% w /w ) Sulphuric acid

40 50 55 60 65 7040 88-l 75.8 57.7 37.3 27-4 18.0 11.0 4.850 88.2 76.2 58.5 38-3 28.5 19.0 11.8 5.460 88.3 76.6 59-3 39.4 29.6 20-o 12.7 6-270 88.4 77-o 60.1 40-5 30.6 20.9 13.5 6.8


4/19

272 N. Wang, J. G . BrennanTABLE 2

Models for Fitting the Sorption Isotherms of PotatoesName of model M odels

GAB(Van Den Berg & Bruin, 198 1)

Chen(Chen, 1971)

M= CKMoA,/[( 1 - KA,)( 1 - KA, + CKA,)]

A, = exp[A- B exp( -CM)]

Oswin M= A(A,/( I - A,)lB(Oswin, 1946)

Chung-Pfost(Chung & Pfost, 1967b)

A, = exp[ - A/T exp( - BM)]

Henderson(Henderson, 1952)

A, = 1 - exp( ATMB)

Halsey(Halsey, 1948)

A, = exp( -A/MB)

BET(Brunauer etal., 1938)

M =hIoCA,/[(l-A ,)+(C-l)( l-A,)A,]

A, B, C and K =constants in sorption isotherm models; A,=water activity;M = moisture content (dry basis); MO = monolayer moisture content (dry basis); R = gasconstant (k&/mole K); T= absolute temperature (K).

Determination of the isosteric heat of sorptionThe application of the Clausius-Clapeyron equation to the isostericequilibrium pressures at different temperatures (sorption isotherms) is awidely used procedure for the calculation of the isosteric heat of sorption(e.g. Peri & De Cesari, 1974; Mazza & Le Maguer, 1978).

The isosteric heat of sorption is a differential molar quantity derivedfrom the temperature dependence of the isotherm (Iglesias & Chirife,1976~)

(dm P/dT),, = Q/RT* (2)where QSf is the isosteric heat of sorption (k&/g mole), T is the absolutetemperature, R the gas constant and P is the partial pressure of watervapour or equilibrium vapour pressure of the sorbate. At a constantamount of sorbed water (yla), the Clausius-Clapeyron equation for purewater is

dm P,,/dT= L/RT2 (3)


5/19

M oistur e sorpt i on sothenm of potat oes 273where L is the latent heat of vaporization at the specified temperature(average temperature between 40 and 70C) and PO the saturationpressure at temperature T. Substracting eqn (3) from eqn (2)

[dln(P&)/dT],,.=(dlnA,/dT),=Q;/RT2 (4)where A , is the water activity and Q;rt he net isosteric heat of sorption.

The net isosteric heat of sorption, at any value of nu, may becalculated from eqn (4) by plotting the sorption isotherm as In A, versus11T and determinin g the slope, which equals - Qf/R.y this method itis assumed that Q;l is invariant with T, but the application of this methodrequires the measurement of sorption isotherms at more than twotemperatures. The isosteric heat may also be calculated from theintegrated form of eqn (4) applied to sorption isotherms measured at twotemperatures:

QZ = R[( Tl Tz)/( Tz - ?)I bb%v, /A,~ )I (5)The isosteric heat is reported in the manner used by Iglesias and Chirife(1976a), that is in the form of a plot of Q versus moisture content. TheQ was calculated as

Qs=Qi t+L, (6)where L, is the latent heat of vaporization of pure water for desorptionor the heat of condensation of pure water vapour for adsorption at thespecified temperature (55C, average between 40 and 70C).

Young and Crowell(1962) and Ross and Olivier (1964) reviewed thequantitative relationships between the isosteric heat of sorption andother experimentally determined heats of sorption, like adiabatic andisothermal calorimetric heats and the integral heat of sorption.

In this work, eqn (4) was used to calculate the isosteric heats ofsorption. A least-squares analysis was performed to obtain the slope ofthe straight line representing ln A , versus l/ T.

The isosteric heats reported here were obtained from sorptionisotherms which show marked hysteresis. However, this procedure maybe questioned, since in systems exhibiting hysteresis either the adsorp-tion branch or the desorption one, or both, must involve thermo-dynamically irreversible phenomena. Evenett and Whitton (1955)suggested that, in systems exhibiting hysteresis, the Clausius-Clapeyronequation may have the form

dh P[ 1__ =AH(apparent)AH + Qd(l/T) na R R (71


6/19

274 N . Wang, J. G . Brennan

whereAHtapparenr)s the apparent differential molar heat of adsorptionand desorption, AH is the calorimetric isosteric heat of adsorption (ordesorption) and Q is the so-called uncompensated heat (which in anisothermal irreversible process is related to the entropy production (d,S)arising from the irreversibility, Q = T(diS); therefore, the isosteric heatdoes not strictly refer to heat transfer of the sorption process). Evenettand Whitton ( 1955) also suggested that if eqn (7) was valid, by combin-ing isotherm measurements with calorimetric measurements of the heatof adsorption, it should be possible to decide which part of the isothermcycle is associated with an irreversible process. The measurementsrequire high precision through strict and difficult control of experimentalvariables, due to the small value of (2.

From a practical standpoint, Iglesias and Chirife (1976a) concludedthat the heats of changes involved in irreversible processes are probablysmall compared with the overall energy changes, so they may beneglected in a general qualitative description or in the estimation of heatrequirements for the dehydration process. Accordingly, they added thatthe apparent heats of sorption calculated from the conventionalClausius-Clapeyron equation will be adequate for a qualitative thermo-dynamic discussion of the adsorption and desorption processes.

RESULTS AND DISCUSSIONDesorption and adsorption isothermsThe desorption and adsorption isotherms at 40, 50, 60 and 70C areshown in Figs 1 and 2. The points shown are average values from threedeterminations and coefficients of variation range from 1.5 to 5.4%. Theequilibrium moisture contents increase with decrease in temperature atconstant water activity, and increase with rise in water activity whentemperature was kept constant. The sorption isotherms give the charac-teristic S-shaped curve, typical of many sorption isotherms of food (Igle-sias & Chirife, 1982).Figure 3 shows that hysteresis between desorption and adsorption isexhibited over almost the entire range of relative humidity.Heats of desorption and adsorptionWhen water vapour is adsorbed on a surface, a quantity of heat (the heatof adsorption) is released. When adsorbed water vapour is desorbed, aquantity of heat is taken up (the heat of desorption) and is a measure of


7/19

Moist ure sorpt i on sotherms of potat oes30

- 7oc- 60-c- 50-c

c

0.0 0.2 0.4 0.6 0.8 1.0Water activity

Fig. 1. Desorption isotherms of potatoes at different temperatures.

I- 40%- 50-c- 60-c- 70-c

275

0.0 0.2 0.4 0.6 0.8 1.0Water activity

Fig. 2. Adsorption isotherms of potatoes at different temperatures.

the heat or energy that must be added to the adsorbed gas to break theintermolecular force (Chung & Pfost, 1967~). The heats of desorptionand adsorption indicate the binding energy or the intermolecular forcebetween the molecules of water vapour and the surface of adsorbent.Water sorption in foods is a complex phenomenon, The main water-sorbing constituents of foods are various polymers (proteins, starch,cellulose, hemicellulose, etc.) and sugars. The different polar groups ofthe polymers provide energetically different preferential sites for the


8/19

276 N . Wang, J. G . Brennan

0.0 0.2 0.4 0.6 0.8 1.0

-I0.4 0.6 0.8 1.0Water activityFig. 3. Desorption and adsorption isotherms of potatoes at 40C and 70C showing

hysteresis effect.

water. Besides, as a polymer sorbs water it undergoes changes of con-stitution, dimensions and other properties (McLaren & Rowen, 1951;Masuzawa & Sterling, 1968). Water sorption also leads to phase trans-formations of the sugars contained in the food (IgIesias et af., 1975). Ahthese facts make difficult the interpretation of the differential heatcurves; there are, however, some observations that could be made fromthe results.


9/19

Moisture sorption isotherm ofpotatoes 277

18 - - HeuofdaorpQn__-_r^ . Latent heati! - Heat of adsorpti on5 T3 14*o .0!z 12

% .a *= lo- _-_--___-_-_-_--_

S!. I. I. t.1.1. I. I. 1.1. I0 2 4 6 8 10 12 14 16 I8 20Moisture content (% dry basis)

Fig. 4. Isosteric heats of desorption and adsorption for potatoes at 55C.

The isosteric heats of desorption and adsorption are plotted as afunction of moisture content in Fig. 4. Plots of heats of desorption andadsorption demonstrate that heats of sorption decrease continually withincreasing moisture content. The horizontal dotted line indicates theheat of vaporization of pure water or heat of condensation of pure watervapour at 55C (the average temperature between 40 and 70C). Thedecrease in the isosteric heats with amount of water sorbed can bequalitatively explained by considering that, initially, sorption occurs onthe most active available sites giving rise to high interaction energy. Asthese sites become occupied, sorption occurs on the less active sites,giving rise to lower heats of sorption (Iglesias & Chirife, 1976~). It canalso be seen from Fig. 4 that the isosteric heats of desorption andadsorption at very low moisture content are still higher than the heats ofvaporization and condensation of pure water. McLaren and Rowen(1951) pointed out that the chemisorption on polar groups could beresponsible for the higher sorption heat at very low moisture content. Instudying the sorption of polar vapours on cellulose fibres, Colombo andImmergut (1970) observed an abnormahy high heat of sorption at lowcoverages indicating the presence of high-energy sites, which theyattributed to strained hydrogen bonds induced in the fibre on drying.Upon sorption these strained hydrogen bonds were broken with a netevolution of heat, in addition to the heat evolved from the formation ofstable hydrogen bonds between the cellulose and the vapour. An effectof this type could partially account for the high initial heats observed inFig. 4. Another observation from Fig. 4 is that the values of isosteric


10/19

278 N . Wang, . G. Brennanheats of desorption are greater than those of adsorption, indicating thatenergy involved in the desorption process is greater than that in theadsorption process. Iglesias and Chirife (1976~) compared the isostericheats of water adsorption and desorption for a large number of foods. Ingeneral, the desorption heats were significantly higher than the adsorp-tion heats at low moisture contents. Upon further sorption, the dif-ference rapidly decreased and practically disappeared at high moisturecontents. They concluded that there was no direct relationship betweenthe observed differences in the adsorption and desorption heats and thedistribution of hysteresis along the sorption isotherm. Several theorieshave been advanced to explain the phenomena of hysteresis. If hysteresisarises from a capillary condensation mechanism, two theories exist toexplain the form of the isotherm. These are the ink-bottle theory ofKraemer ( 193 1) and McBain (193 5), and the delayed meniscus theory ofFoster (1932) and Cohan (1938). It is believed that it is not possible togive a single explanation of the hysteresis phenomena in foods; each foodis a complex combination of various constituents which not only cansorb water independently, but can also interact among themselves givingrise to different water-sorbing capacities.The existence of higher values of desorption heats as compared withadsorption heats may be due to structural modifications which take placeon the substrate during desorption, modifying the overall energy ofbinding of the sorbate through co-operative binding or to entrapmenteffects.

Rao (1942) attributed such changes of sorption phenomena, resultingin the elimination of hysteresis, to the elastic properties of organogels. Insorption, the capillary pores of the adsorbent become elastic and swell.Upon desorption, the removal of water causes shrinkage and a generalcollapse of the capillary porous structure. Alteration of structure causeselimination of hysteresis due to the absence of capillary condensation.When considering the application of the Clausius-Clapeyron equation, itis obvious that anything that affects the sorption isotherm affects thethermodynamic quantities derived from it; this includes the prehistory ofthe sample, such as various preheat treatments, blanching, freezing anddrying. The treatment may change the polar and other groups that bindwater, along with changes in capillary and other configurations of thefood structure (Kapsalis, 1987).According to Chung and Pfost (1967a), the heats of sorption can beconsidered as indicative of intermolecular attraction forces betweensorptive sites and water vapour. Plotted values of Q versus moisturecontent can be used to study the magnitude of the binding energy or theavailability of polar sites to water vapour as sorption proceeds. Figure 4


11/19

M oist ure sorpt i on i sotherms of potat oes 279

TABLE 3Parameter Values for all Models in Desorption

M odel Temperature (C)40 50 60 70

GABCKMOE (%)

ChenABCE (%)

OswinABE (%)

Chung-PfostABE (%)

HendersonABE (%)

HalseyABE (%)

BETCMOE (%)

A , range (%)BETAll others

12.52 8.89 8.57 4.370.84 0.82 0.84 0.800.059 0.06 1 GO53 0.0575.68 6.74 4.59 9.15

- 0.097 - 0.1044.37 4.08

21.26 22.0010.14 11.18

-0.111 - 0.0943.95 3.15

24.27 23.448.09 15.78

0.092 0.087 0.078 0.0680.44 044 0.46 0.494.27 5.41 3.67 9.13

1 188.89 1 138.08 1119.92 991.4917.46 17.63 18.95 19.2216.09 17.04 15.28 19.47

0.084 0.075 0.075 0.06 11.57 1.49 1.44 1.29

13.34 13.52 11.54 14.30

0.049 0.046 0.042 0.0410.92 0.92 0.94 1.03

115.82 105.00 81.09 9.82

19.54 16.71 14.65 7.990.047 0.045 0.04 1 0.0387.33 6.11 4.78 8.71

4.8-57.7 5.4-58.5 6.2-59.3 6.8-60.14.8-88.1 5.4-88.2 6.2-88.3 6.8-88.4


12/19

280 N. Wang, J. G. Brennan

TABLE 4Parameter Values for all Models in Adsorption

Model40

Temperature (C)50 60 70

GABcK$h)

ChenABCE (%)

OswinABE (%)

Chung-PfostABE (%)

HendersonABE (%)

HalseyABE (%)

BETCMOE (%)

A, range (%)BETAll others

13.73 13.41 21.18 17.750.83 082 0.86 0.900.052 0.048 0.036 0.0295.62 4.08 3.41 5.69

- 0.083 - 0.086 - 0105 - 0.1094.47 4.60 5.46 4.26

23.79 27.06 35.90 38.599.92 7.24 4.63 10.97

0.082 0.074 0062 00510.43 0.42 0.43 0.494.13 3.31 4.07 586

1243.12 1301.08 1 466.60 1 244.8620.24 22.78 28.73 307714.72 11.93 9.15 16.50

0.117 0.142 0.22 1 0.1701.62 1.64 1.72 1.58

13.39 10.58 12.69 15.66

0.0025 0.002 1 0.0017 0.00122.64 2.6 1 2.50 2.4970.35 71.00 69.82 70.27

11.79 11.52 14.69 14.820.049 0.043 0037 0.0287.83 4.50 3.08 6.19

4.8-37.3 5.4-38.3 6.2-39.4 6.8-40.54.8-88.1 5.4-88.2 6.2-88.3 6.8-88.4


13/19

Moi st ure sorpt ion sot herm of potat oes 281

Adwqxion GAB at WC- Expcrimd_._.&_. Rediued

0.0 0.2 0.4 0.6 0.8 1.0water activity

Fig. 5. Comparison between experimental and predicted values using GAB equationat 60C.

also shows that the heats of sorption approach the latent heat of vaporiza-tion or condensation at 55C as the moisture content increases, indicatingthat adsorbed water vapour on the adsorbent may behave like purewater at either higher moisture content or higher relative humidity.Iglesias and Chirife (1976~) analysed the differential heat curves foradsorption. They stated that an isosteric heat value greater than the heatof condensation indicates that the energy of interaction between the


14/19

282 N . Wang, J. G . Brennan30- Dcmpt imlswm L oT

- Expaimmtal_._.& _. Rediced

0.6 0.8

10.0 0.2 0.4 0.6 0.8 1.0

Water activityFig. 6. Comparison between experimental and predicted values using Oswin equationat 60C.sorbate and sorption sites is greater than the energy that holds thesorbate molecules together in the liquid state.

The values of the isosteric heats of desorption and adsorption ofpotato were compared with those of vegetables reported by Iglesias andChirife ( 1976~): a good agreement was found.Fitting of equations to sorption isotherm dataThe estimated constants and the mean relative deviation modulus (E) forthe models in Table 2 are presented in Table 3 and Table 4. It can be


15/19

M oistur e sorpt ion kotherm of potat oes 283

seen that GAB and Oswin equations give smaller mean relativepercentage deviation (less than 10%); therefore, they give the best fit tothe experimental data for a wide range of water activities (O-88*4%). Theresults agree with those reported by Aguerre et al. (1989). Moreover, it isfound that the BET equation gives a better fit to the experimental datafor desorption of potato in the range of water activity 0.05-O-60, and foradsorption in the range of water activity 0.05-0.40. For adsorption, thereis a good agreement with Chirife and Iglesias (1978), who stated that theBET equation usually holds only between water activity from about 0.05

0.6Warn activity 0.8

-1 - Adsmption BlT .t WC

0.6 0.8

Fig. 7. Comparison between experimental and predicted values using BET equation at60C.


16/19

284 N. Wang, J. G. Br ennanto 0.45. For desorption, the water activity range is slightly larger thanthis range (o-0.45). It is obvious that the Halsey equation gives a poor fitto the data. However, this is different from the conclusion reached byBoquet et al. (1978). They concluded that the Halsey equation gave abetter fit for vegetables. Examples of the comparisons between experi-mental and predicted values at 60C using GAB, Oswin, BET andHalsey equations are shown in Figs 5-8.

30 DcmrptimHALSEY at WC-

25-Expaimslul

_._.&_. Pdictcd

0.4 0.6Water activity

Fig. 8. Comparison between experimental and predicted values using Halsey equationat 60C.


17/19

M oistur e sorpt i on sot herms of potat oes 285CONCLUSIONS

From the experiment, the following may be concluded.(1) Temperature has an effect on sorption isotherms. The equilibriummoisture content increases with decreasing temperature at

constant water activity. The water activity increases with increasein temperature when the moisture content is kept constant. Thisbehaviour is typical of many foods.

(2) The isosteric heats of sorption increase with decreasing moisturecontent. The heat of desorption is greater than that of adsorptionat low moisture contents. At higher moisture contents the dif-ference tends to disappear.(3) The GAB and Oswin equations are suitable for representing therelationship between water activity and equilibrium moisturecontent of potato in the range of water activity O-88%, and theBET equation in the range of water activity O-60%.

This work represents the preliminary stage of a study of the convectivedrying of potato and the solid-moisture relationships involved.

REFERENCESAguerre, R. J., Suarez, C. & Viollaz, P. Z. (1989). New BET type multiIayersorption isotherms. Part II. ModelIing water sorption in foods, Lebemm. -wirs.u-technol., 22, 192-5.Becker, H. A. & Sallans, H. R. (1956).study of desorption isotherms of wheatat 25 and 50C. Cereal Chem., 33,79.Boquet, R., Chirife, J. & Iglesias, H. A. (1978). Equations for fitting watersorption isotherm of foods. II. Evaluation of various two-parameter models.J. Food Techno l ., 13,319-27.Brunauer, S., Emmett, P. H. & Teller, E. (1938). Adsorption of gases in multi-molecular layer. J. Amer. Chem. Sot ., 60,309-19.Bushuk, W. & Winkler, C. A. (1957). Sorption of water vapor on wheat flour,starch, and gluten. Cereal Chem., 3473.Chen, C. S. (1971). Equilibrium moisture curves for biological materials. Trans.ASAE, 14,924-6.Chirife, J. & Iglesias, H. A. ( 1978). Equations for fitting water sorptionisotherms of foods. Part I. A review. J. Food Techno l ., 13,159-74.Chung, D. S. & Pfost, H. B. (1967a). Adsorption and desorption of water vaporby cereal grains and their products. Part I. Heat and free energy changes ofadsorption and desorption. Trans. ASAE, 10,549-5.Chung, D. S. & Pfost, H. B. (19676). Adsorption and desorption of watervapor by cereal grains and their products. Part II. Development of the generalisotherm equation. Trans. ASAE, 10, 52-5.


18/19

286 N. Wang, . G . BrennanCohan, L. H. (1938). Sorption hysteresis and the vapor pressure of concavesurfaces. J. Amer. Chem. Sot ., 60,433.Colombo, E. A. & Immergut, E. H. (1970). Interaction of cellulose with organic

liquids and vapours. J. Polymer Sci., (Part C), 31, 137-56.Duckworth, R. B. (1972). The properties of water around the surfaces of foodcohoids. Proc. Inst . Food Sci . Technol ., 5,50.Evenett, D. H. & Whitton, W. I. (1955). A thermodynamic study of the adsorp-tion of benzene vapour by active charcoals. Proc. Royal Sot ., A230,9 1.Foster, A. G. (1932). The sorption of condensible vapours by porous solids. PartI. The application of the capillary theory. Trans. Faraday Sot., 28,645.Gal, S. (1987). The need for, and practical applications of sorption data. InPhysical Propert i es of Foods - 2, ed. R. Jowitt, et al. Elsevier AppliedScience, London and New York, pp. 13 -25.Gutierrez-Lopez, G. E (1989). Convective drying and solid-moisture inter-actions. PhD thesis, University of Reading, UK.Halsey, G. ( 1948). Physical adsorption on non-uniform surfaces. J. Chem. Phys.,16,931-7.Heldman, D. R., Hall, C. W. & Hedrick, T. I. (1965). Vapor equilibrium rela-tionships of dry milk. J. Dairy Sci., 48,845.Henderson, S. M. (1952). A basic concept of equilibrium moisture. Agric. Erg.,33,29-32.Iglesias, H. A. & Chirife, J. (1976~). Isosteric heats of water vapour sorption ondehydrated foods. Part I. Analysis of the differential heat curves. Lebensm.-w i ss. u.-t echnol., 9, 116-22.IgIesias, H. A. & Chhife, J. (19766). Isosteric heat of water vapor sorption ondehydrated food. Part II. Hysteresis and heat of sorption comparison withB.E.T. theory. Lebensm.-w i ss. u.-t echnol ., 9,123-7.IgIesias, H. A. & Chirife, J. ( 1982). Handbook of Food Isotherms. AcademicPress, New York, pp. 170-5.Iglesias, H. A., Chirife, J. & Lombardi, J. L. (1975). Comparison of watervapour sorption by sugar root components. J. Food Technol ., 10,385.Kapsalis, J. G. (1987). Influences of hysteresis and temperature on moisturesorption isotherm. In Water Acti vi ty : Theory and Appl icati ons to Food, ed. L.B. Rockland & L. R. Beuchat. Marcel Dekker, Inc., New York, p. 173.Kraemer, E. 0. (1931). A Treatise on Physical Chemistry, van Nostrand, NewYork, p. 1661.Lomauro, C. J., Bakshi, A. S. & Labuza, T. P. (1985). Evaluation of foodmoisture sorption isotherm equations, Part I. Fruit, vegetable and meatproducts. Lebensm.-w i ss. u.-t echnol ., 18, 111-17.Makower, B. (1945). Vapor pressure of water adsorption on dehydrated eggs.Znd. Eng. Chem., 37,1018.Masuzawa, M. & Sterling, C. (1968). Gel-water relationships in hydrophilicpolymers: thermodynamics of sorption of water vapor. J. Polymer Sci., 12,2023.Mazza, G. & Le Maguer, M. (1978). Water sorption properties of globe onion. J.Can. Inst . Food Sci. Technol ., 11, 189.

McBain, J. W, (1935). An explanation of hysteresis in the hydration anddehydration of gels. 1. Amer. Chem. Sot ., 57,699.


19/19

Mo&ure sorptionsotherms of otatoes 287McLaren, A. D. & Rowen, J. W. ( 195 1). Sorption of water vapor by proteins andpolymers: a review. 1. Pol ymer Sci., 57,289-324.Oswin, C. R. (1946). The kinetics of packing life. III. The isotherm. J. Chem.

Znd., 65,419-23.Peri, C. & De Cesari, L. (1974). Thermodynamics of water sorption on &cc.cereviove and cell viability during spray-drying. Lebensm.-w i ss. u. t echnol ., 7,56.Rao, K. S. (1942). Disappearance of the hysteresis loop. The role of elasticity oforganogels in hysteresis in sorption. Sorption of water on some cereals. J.Phys. Chem., 45,5 17.

Ri zvi , S. S. H. (1986). Thermodynamics of foods in dehydration. In EngineeringProperties ofFoods, ed. M. A. Rao & S. S. H. Rizvi. Marcel Dekker, Inc., NewYork, pp. 196-201.Rodriguez-Arias, J. H., Hall, C. W. & Bakker-Arkema, F. W. (1963). Heat ofvaporization for shelled corn. Cereal Chem., 40,676.Ross, S. & Olivier, J. P. (1964). On Physical Adsorpti on. Interscience Publishers,New York.Saravacos, G. D. & Stinchfield, R. M. (1965). Effect of temperature andpressure on the sorption of water vapor by freeze-dried food material. J. FoodSci., 30,779.

SAS U ser s Gui de: St at i sti cs (1982). SAS Institute Inc., Cary, NC, USA.Van Den Berg, C. & Bruin, S. (1981). Water activity and its estimation in foodsystems: theoretical aspects. In Water Acti vi ty : Znfl uence on Food Qual it y ed.L. B. Rockland & G. F. Stewart. Academic Press, New York, p. 45.Young, D. M. & Crowell, A. D. (1962). Physical Adsorpt i on of Gases. Butter-worths, London, p. 64.

Moisture Sorption Isotherm Characteristics of Potatoes at Four Temperatures

Documents

Transcript of Moisture Sorption Isotherm Characteristics of Potatoes at Four Temperatures