《食品化学》课程教学资源（书籍文献）Principles of Food Chemistry，Third Edition，John M. deMan, PhD

9Water1slopeCV.(I-a)vminterceptCV0.5Figure1-1oBETMonolayerPlot.Source:From T.P.Labuza, Sorption Phenomena in Foods,FoodTechnol.,Vol.22,pp.263-272,1968.forexample,thesorptionisothermsofwheatlaries,andtheeffectoccursnotonlyinregionflourasdeterminedbyBushukandWinklerC of Figure 1-7 but also in a large part of(1957)(Figure 1-12).The hysteresis effect isregion B. The best explanation for this phe-explained bywatercondensinginthecapil-nomenonappearstobetheso-calledinkbot-1210AIR-DRIEPUFF-DRIED100pW (PoP)REEZE-DRIED1020304050P100(%R.H.)PoFigure1-11BET Plots for Dehydrated Potato.Source:From G.D.Saravacos,Effect of the DryingMethod on theWater SorptionofDehydrated Apple andPotato,J.Food Sci.,Vol.32,pp.81-84,1967

for example, the sorption isotherms of wheat flour as determined by Bushuk and Winkler (1957) (Figure 1-12). The hysteresis effect is explained by water condensing in the capil￾laries, and the effect occurs not only in region C of Figure 1-7 but also in a large part of region B. The best explanation for this phe￾nomenon appears to be the so-called ink bot￾Figure 1-11 BET Plots for Dehydrated Potato. Source: From G.D. Saravacos, Effect of the Drying Method on the Water Sorption of Dehydrated Apple and Potato, / Food ScL, Vol. 32, pp. 81-84, 1967. 100-&- (%R.H.) K o FREEZE-DRIED PUFF-DRIED AIR-DRIED 10Op W(P0-P) Figure 1-10 BET Monolayer Plot. Source'. From TP. Labuza, Sorption Phenomena in Foods, Food TechnoL, Vol. 22, pp. 263-272, 1968. Q (l-a)V intercept slope 0.5 . _ ! " CV m C -I "CV m

10PRINCIPLES OFFOODCHEMISTRY16080FLOURSTARCH(0/OW)X16080FREEZE-DRIEDSPRAY-DRIEDGLUTENGLUTEN080.40.80.4P/PoFigure 1-12 Sorption Isotherms of Wheat Flour, Starch, and Gluten.Source:From W.Bushuk andC.A.Winkler, Sorption of Water Vapor on WheatFlour, Starch and Gluten,Cereal Chem.,Vol.34,pp.7386,1957.tle theory (Labuza 1968). It is assumed thatThe position of the sorption isothermsthe capillaries have narrow necks and largedepends on temperature: the higher the tem-bodies, as represented schematically in Figperature, the lower the position on the graphThis decrease in the amount adsorbed ature 1-13. During adsorption the capillarydoes not fill completely until an activity ishigher temperatures follows the Clausiusreached that corresponds to the large radiusClapeyron relationship,R. During desorption, the unfilling is con-Qsd(lna)trolled by the smaller radius r, thus loweringd(1/T) =-R-the water activity. Several other theories havebeen advanced to account for the hysteresiswhereinsorption.ThesehavebeensummarizedbyO,=heat of adsorptionKapsalis (1987)

tie theory (Labuza 1968). It is assumed that the capillaries have narrow necks and large bodies, as represented schematically in Fig￾ure 1-13. During adsorption the capillary does not fill completely until an activity is reached that corresponds to the large radius R. During desorption, the unfitting is con￾trolled by the smaller radius r, thus lowering the water activity. Several other theories have been advanced to account for the hysteresis in sorption. These have been summarized by Kapsalis (1987). The position of the sorption isotherms depends on temperature: the higher the tem￾perature, the lower the position on the graph. This decrease in the amount adsorbed at higher temperatures follows the Clausius Clapeyron relationship, d(lna) _ _Qs d(l/T) ~~ ~~R where Q8 = heat of adsorption P/Po Figure 1-12 Sorption Isotherms of Wheat Flour, Starch, and Gluten. Source: From W. Bushuk and C.A. Winkler, Sorption of Water Vapor on Wheat Flour, Starch and Gluten, Cereal Chem., Vol. 34, pp. 73-86, 1957. FREEZE-DRIED GLUTEN SPRAY-DRIED GLUTEN FLOUR STARCH X(MGXG )

11Water2.000and 10,000cal permole,demonstrating the strong binding of this water.21According to the principle of BET iso-therm, the heat of sorption Q,should be con.stant up to monolayer coverage and thenshould suddenly decrease.Labuza (1968)2R2Rhas pointed out that the latent heat of vapor.ization△H.,about10.4kcalpermole,shouldbe added to obtain the total heat value. TheFigure 1-13 Ink Bottle Theory of Hysteresis inplot representing BET conditions as well asSorption.Source:From T.P.Labuza, Sorptionactual findings are given in Figure 1-15.ThePhenomena in Foods, Food Technol., Vol. 22,observed heat of sorption at low moisturePp.263272,1968.contents is higher than theory indicates andfalls off gradually, indicating the gradualchange from Langmuir to capillary water.R =gas constantT =absolutetemperatureTYPESOFWATERBy plotting the natural logarithm of activityThe sorption isotherm indicates that differ-versus the reciprocal of absolute tempera-ent forms of water may be present in foods.ture at constant moisture values, straightIt is convenient to divide the water into threelines are obtained witha slope of -QJRtypes: Langmuir or monolayer water, capil(Figure 1-14).The values of Q,obtained inlary water, and loosely bound water. Thethis way for foods having less than fullbound water can be attracted strongly andmonolayer coverage are between aboutheld in a rigid and orderly state, In this formm5m4O)m3m2m1I/TFigure 1-14 Method for Determination of Heat of Adsorption.Moisture content increases from M,toMs.Source:FromT.P.Labuza,SorptionPhenomena inFoods,Food Technol.,Vol.22,pp.263-2721968

Figure 1-13 Ink Bottle Theory of Hysteresis in Sorption. Source: From T.P. Labuza, Sorption Phenomena in Foods, Food TechnoL, Vol. 22, pp. 263-272, 1968. R = gas constant T = absolute temperature By plotting the natural logarithm of activity versus the reciprocal of absolute tempera￾ture at constant moisture values, straight lines are obtained with a slope of -QJR (Figure 1-14). The values of <2S obtained in this way for foods having less than full monolayer coverage are between about 2,000 and 10,000 cal per mole, demonstrat￾ing the strong binding of this water. According to the principle of BET iso￾therm, the heat of sorption Qx should be con￾stant up to monolayer coverage and then should suddenly decrease. Labuza (1968) has pointed out that the latent heat of vapor￾ization Af/v, about 10.4 kcal per mole, should be added to obtain the total heat value. The plot representing BET conditions as well as actual findings are given in Figure 1-15. The observed heat of sorption at low moisture contents is higher than theory indicates and falls off gradually, indicating the gradual change from Langmuir to capillary water. TYPES OF WATER The sorption isotherm indicates that differ￾ent forms of water may be present in foods. It is convenient to divide the water into three types: Langmuir or monolayer water, capil￾lary water, and loosely bound water. The bound water can be attracted strongly and held in a rigid and orderly state. In this form In (ACTIVITY) VT Figure 1-14 Method for Determination of Heat of Adsorption. Moisture content increases from M1 to M5. Source: From T.P. Labuza, Sorption Phenomena in Foods, Food Technol, Vol. 22, pp. 263-272, 1968

12PRINCIPLESOFFOODCHEMISTRYobservedS.-iBET1--VmMOISTURE%Figure1-15Relationship of Heatof Sorption and MoistureContent as ActuallyObserved and Accord-ing to BET Theory.Source:From T.P. Labuza, Sorption Phenomena in Foods, Food Technol.,Vol.22pp.263-272,1968.the water is unavailable as a solvent and doessponds to 11.4 percent of total water in leannot freeze. It is difficult to provide a rigidmeat. Most fruits and vegetables contain lessdefinition of bound water because muchthan 6 percent unfreezable water; wholedepends on the technique used for its mea-graincorn,34percent.surement.Two commonly used definitionsThe free water is sometimes determined byare as follows:pressing a food sample between filter paper,by diluting with an added colored substance,1.Boundwateristhe waterthatremainsor by centrifugation. None of these methodsunfrozen at some prescribed temperapermits a distinct division between free andturebelow0℃,usually-20℃bound water, and results obtained are not nec-2. Bound water is the amount of water in aessarilyidentical betweenmethods.Thisissystem that is unavailable as a solvent.not surprising since the adsorption isothermindicatesthatthedivisionbetweenthediffer-The amount of unfreezable water,based onentformsof wateris gradual rather thanprotein content, appears to vary only slightlysharp.A promising new method is the use offrom one food to another. About 8 to 10 per-nuclear magnetic resonance, which can becent of the total water in animal tissue isexpected togive resultsbased on thefreedomunavailable for ice formation (Merymanof movement of the hydrogen nuclei.1966).Egg white, egg yolk, meat, and fishThe main reason for the increased waterall contain approximately 0.4g of unfreez-content at high values of water activity mustable water per g of dry protein. This corre-be capillary condensation. A liquid with sur-

the water is unavailable as a solvent and does not freeze. It is difficult to provide a rigid definition of bound water because much depends on the technique used for its mea￾surement. Two commonly used definitions are as follows: 1. Bound water is the water that remains unfrozen at some prescribed tempera￾ture below O0C, usually -2O0C. 2. Bound water is the amount of water in a system that is unavailable as a solvent. The amount of unfreezable water, based on protein content, appears to vary only slightly from one food to another. About 8 to 10 per￾cent of the total water in animal tissue is unavailable for ice formation (Meryman 1966). Egg white, egg yolk, meat, and fish all contain approximately 0.4 g of unfreez￾able water per g of dry protein. This corre￾sponds to 11.4 percent of total water in lean meat. Most fruits and vegetables contain less than 6 percent unfreezable water; whole grain corn, 34 percent. The free water is sometimes determined by pressing a food sample between filter paper, by diluting with an added colored substance, or by centrifugation. None of these methods permits a distinct division between free and bound water, and results obtained are not nec￾essarily identical between methods. This is not surprising since the adsorption isotherm indicates that the division between the differ￾ent forms of water is gradual rather than sharp. A promising new method is the use of nuclear magnetic resonance, which can be expected to give results based on the freedom of movement of the hydrogen nuclei. The main reason for the increased water content at high values of water activity must be capillary condensation. A liquid with sur￾Figure 1-15 Relationship of Heat of Sorption and Moisture Content as Actually Observed and Accord￾ing to BET Theory. Source: From TR Labuza, Sorption Phenomena in Foods, Food TechnoL, Vol. 22, pp. 263-272,1968. MOISTURE % Vm BET observed HEAT O F SORPTIO N

13Waterface tension in a capillary with radius r isTable1-4Capillary Radius and Water Activitysubject to a pressure loss,the capillarypressure p,= 2o/r, as evidenced by the rising ofRadius (nm)Activity (a)the liquid in the capillary. As a result, there is0.50.116a reduction in vapor pressure in the capillary,10.340which can be expressed by the Thomson20.583equation,50.806100.898InP=-20.V200.948-RTPo500.9791000.989where10000.999p=vaporpressureof liquidP。=capillaryvapor pressureo=surfacetensionV-mole volume of liquidand for fish and meat, 0.40 g per g proteinR=gas constantThe nonfreezable and Langmuir water areT=absolutetemperatureprobably not exactly the same.Wierbicki andDeatherage (1958)used apressuremethodtoThis permits the calculation of water activitydetermine free water in meat. The amount ofin capillaries of different radii, as indicatedfree water in beef, pork, veal, and lamb var-in Table 1-4. In water-rich organic foods.ies from30 to 50percent of total moisture,such as meat and potatoes,the water isdepending on thekind of meat and the periodpresent in part in capillaries with a radius ofof aging.A sharpdrop in bound water occurs1 μm or more.The pressure necessary toduring the first day after slaughter, and is fol-remove this water is small.Calculated valueslowed by a gradual, slight increase. Hammof this pressure are given in Table 1-5 forandDeatherage (1960b)determined thewater contained in capillaries ranging fromchanges in hydration during the heating of0.1 μm to 1 mm radius. It is evident thatmeat. At the normal pH of meat there is awater from capillaries of 0.1 μm or largerconsiderable reduction of bound water.can easily drip out.Structural damagecaused, for instance, by freezing can easilyresult in drip loss in these products. The factthat water serves as a solvent for many sol-Table1-5PressureRequiredToPressWaterutes such as salts and sugars is an additionalfromTissueat20℃factorinreducingthevaporpressure.The caloric behavior of water has beenRadiusPressure (kg/cm2)studied by Riedel (1959), who found thatwater in bread did not freeze at all when0.1 μm14.84moisture content was below 18 percent (Fig-1.4841 μmure 1--16).With this method it was possible10μm0.148to determine the nonfreezable water.For0.1 mm0.0148bread, the value was 0.30 g per g dry matter,0.00151mm

face tension a in a capillary with radius r is subject to a pressure loss, the capillary pres￾sure p0 = 2a/r, as evidenced by the rising of the liquid in the capillary. As a result, there is a reduction in vapor pressure in the capillary, which can be expressed by the Thomson equation, !„£.-_ 22. Jl P0 ~ r RT where p = vapor pressure of liquid P0 = capillary vapor pressure a = surface tension V = mole volume of liquid R = gas constant T = absolute temperature This permits the calculation of water activity in capillaries of different radii, as indicated in Table 1-4. In water-rich organic foods, such as meat and potatoes, the water is present in part in capillaries with a radius of 1 (urn or more. The pressure necessary to remove this water is small. Calculated values of this pressure are given in Table 1-5 for water contained in capillaries ranging from 0.1 |im to 1 mm radius. It is evident that water from capillaries of 0.1 |0,m or larger can easily drip out. Structural damage caused, for instance, by freezing can easily result in drip loss in these products. The fact that water serves as a solvent for many sol￾utes such as salts and sugars is an additional factor in reducing the vapor pressure. The caloric behavior of water has been studied by Riedel (1959), who found that water in bread did not freeze at all when moisture content was below 18 percent (Fig￾ure 1-16). With this method it was possible to determine the nonfreezable water. For bread, the value was 0.30 g per g dry matter, Table 1-4 Capillary Radius and Water Activity Radius (nm) Activity (a) O5 0.116 1 0.340 2 0.583 5 0.806 10 0.898 20 0.948 50 0.979 100 0.989 1000 0.999 and for fish and meat, 0.40 g per g protein. The nonfreezable and Langmuir water are probably not exactly the same. Wierbicki and Deatherage (1958) used a pressure method to determine free water in meat. The amount of free water in beef, pork, veal, and lamb var￾ies from 30 to 50 percent of total moisture, depending on the kind of meat and the period of aging, A sharp drop in bound water occurs during the first day after slaughter, and is fol￾lowed by a gradual, slight increase. Hamm and Deatherage (196Ob) determined the changes in hydration during the heating of meat. At the normal pH of meat there is a considerable reduction of bound water. Table 1-5 Pressure Required To Press Water from Tissue at 2O0C Radius Pressure (kg/cm2 ) 0.1 jim 14.84 1 ^m 1.484 10 ^m 0.148 0.1 mm 0.0148 1 mm 0.0015