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

4PRINCIPLESOFFOODCHEMISTRYassociatewitheachotherratherthanwiththethe apolar amino acid side chains in awater molecules. This concept of a hydro-polypeptide can do the same.The concentra-phobic bond has been schematically repre-tionof such side chains in proteins is high,sented byKlotz (1965),as shown in Figureand the combined effect of all these groups1-2. Under appropriate conditions apolarcan beexpected to result in the formation ofmolecules can form crystalline hydrates, ina stabilized and ordered water region aroundwhich the compound is enclosed within thethe protein molecule.Klotz (1965)has sugspace formed by a polyhedron made up ofgested the term hydrotactoids for these struc-watermolecules.Suchpolyhedronscanformtures (Figure 1-5).a large lattice, as indicated in Figure 1-3.The polyhedrons may enclose apolar guestSORPTIONPHENOMENAmolecules to form apolar hydrates (SpeedyWateractivity,whichisapropertyofaque-1984).These pentagonal polyhedra of waterous solutions, is defined as the ratio of themolecules are unstable and normally changevapor pressures of pure water and a solution:to liquid water above oC and tonormal hex-agonal ice below oC. In some cases, thehydratesmelt well above 30°C.Thereis a=卫awremarkable similarity between the smallPoapolar molecules that form these clathrate-like hydrates and the apolar side chains ofwhereproteins. Some of these are shown in Figureaw = water activity1-4. Because small molecules such as thep=partial pressure of water in afoodones shown in Figure 1-4 can form stableP. = vapor pressure of water at the samewater cages, it may be assumed that some oftemperatureAccordingtoRaoult's law,thelowering ofthe vapor pressure of a solution is propor-tional to the mole fraction of the solute: awcan then be related to the molar concentra-tions of solute (n,)and solvent (n2):niDawPon,+n2The extent to which a solute reduces aw is afunction of the chemical nature of the solute.The equilibrium relative humidity (ERH) inFigure 1-2 Schematic Representation.of thepercentage is ERH/100.ERH is defined as:Formation of a Hydrophobic Bond by ApolarGroup in an Aqueous Environment Open cir-equpcles represent water. Source: From I.M. Klotz,ERH =DsarRole of Water Structure in Macromolecules,Federation Proceedings,Vol.24, Suppl. 15,ppwhereS24-S33,1965

associate with each other rather than with the water molecules. This concept of a hydro￾phobic bond has been schematically repre￾sented by Klotz (1965), as shown in Figure 1-2. Under appropriate conditions apolar molecules can form crystalline hydrates, in which the compound is enclosed within the space formed by a polyhedron made up of water molecules. Such polyhedrons can form a large lattice, as indicated in Figure 1-3. The polyhedrons may enclose apolar guest molecules to form apolar hydrates (Speedy 1984). These pentagonal polyhedra of water molecules are unstable and normally change to liquid water above O0C and to normal hex￾agonal ice below O0C. In some cases, the hydrates melt well above 3O0C. There is a remarkable similarity between the small apolar molecules that form these clathrate￾like hydrates and the apolar side chains of proteins. Some of these are shown in Figure 1-4. Because small molecules such as the ones shown in Figure 1-4 can form stable water cages, it may be assumed that some of the apolar amino acid side chains in a polypeptide can do the same. The concentra￾tion of such side chains in proteins is high, and the combined effect of all these groups can be expected to result in the formation of a stabilized and ordered water region around the protein molecule. Klotz (1965) has sug￾gested the term hydrotactoids for these struc￾tures (Figure 1-5). SORPTION PHENOMENA Water activity, which is a property of aque￾ous solutions, is defined as the ratio of the vapor pressures of pure water and a solution: where aw = water activity p = partial pressure of water in a food po = vapor pressure of water at the same temperature According to Raoult's law, the lowering of the vapor pressure of a solution is propor￾tional to the mole fraction of the solute: aw can then be related to the molar concentra￾tions of solute (n{) and solvent (n2): = L = HI " W Po ni +n2 The extent to which a solute reduces aw is a function of the chemical nature of the solute. The equilibrium relative humidity (ERH) in percentage is ERH/100. ERH is defined as: equ ERH = "— sat P where Figure 1-2 Schematic Representation of the Formation of a Hydrophobia Bond by Apolar Group in an Aqueous Environment. Open cir￾cles represent water. Source: From LM. Klotz, Role of Water Structure in Macromolecules, Federation Proceedings, Vol. 24, Suppl. 15, pp. S24-S33, 1965

Water5Figure 1-3 Crytalline Apolar Polyhedrons Forming a Large Lattice.The space within the polyhedronsmay enclose apolarmolecules.Source:FromI.M.Klotz, Role of Water Structure in Macromolecules,FederationProceedings,Vol.24,Suppl.15,pp.S24-S33,1965.AminoAcidSideChainsCrystal HydrateFormersCH4-CH3(Ala)CH3CH3(Val)CHCHCH3CH3,CH3CH,CH3--CH(Leu)-CH2-CHCHCH3CH3-SH(Cys)-CH,-SH(Met)CHSCH3-CH2-CH2-S-CH3(Phe)Figure1-4Comparison of Hydrate-Forming Molecules and AminoAcid Apolar Side Chains.Source:FromI.M.Klotz,Role of Water Structure in Macromolecules,FederationProceedings,Vol.24,Suppl15,Pp.S24-S33,1965

Figure 1-4 Comparison of Hydrate-Forming Molecules and Amino Acid Apolar Side Chains. Source: From LM. Klotz, Role of Water Structure in Macromolecules, Federation Proceedings, Vol. 24, Suppl. 15, pp. S24-S33, 1965. Crystal Hydrate Formers Amlno Acid Side Chains (Ala) (VaI) (Leu) (Cys) (Met) (Phe) Figure 1-3 Crytalline Apolar Polyhedrons Forming a Large Lattice. The space within the polyhedrons may enclose apolar molecules. Source: From LM. Klotz, Role of Water Structure in Macromolecules, Federation Proceedings, Vol. 24, Suppl. 15, pp. S24-S33, 1965

6PRINCIPLES OFFOODCHEMISTRY100860400312MOISTURE CONTENTg/g solidsFigure 1-6 Water Activity in Foods at DifferentMoisture ContentsFigure 1-5 Hydrotactoid Formation Arounddistinction can be made between the adsorp-Apolar Groups of a Protein.Source:From I.M.tion and desorption isotherms by determin-Klotz,Role of Water Structure in Macromole-ing whether a dry product's moisture levelscules, Federation Proceedings, Vol. 24, Suppl.are increasing, or whether the product's15,Pp.S24S33,1965.moisture is gradually lowering to reach equi-librium with its surroundings,implying thatthe product is being dried (Figure 1-7).Gen-pequ=partial pressure of water vapor inerally, the adsorption isotherms are requiredequilibrium with the food at temper-fortheobservationofhygroscopicproducts,ature T and 1 atmosphere total pres-surepsat=the saturation partial pressure ofwater in air at the same temperatureandpressure%At high moisture contents,when theamount of moisture exceeds that of solids,desorptionthe activity of water is close to or equal to1.0. When the moisture content is lower thanthat of solids,water activity is lower thanadsorption1.0,as indicated in Figure 1-6.Below mois-ture content of about 50 percent the waterBCactivity decreases rapidly and the relation-ship between water content and relativeREL. HUM.%humidityisrepresentedbythe sorptioniso-therms.Theadsorptionanddesorption pro-Figure 1-7 Adsorption and Desorption Isocesses are not fully reversible; therefore, atherms

Figure 1-5 Hydrotactoid Formation Around Apolar Groups of a Protein. Source: From LM. Klotz, Role of Water Structure in Macromole￾cules, Federation Proceedings, Vol. 24, Suppl. 15, pp. S24-S33, 1965. p equ - partial pressure of water vapor in equilibrium with the food at temper￾ature T and 1 atmosphere total pres￾sure p sat = the saturation partial pressure of water in air at the same temperature and pressure At high moisture contents, when the amount of moisture exceeds that of solids, the activity of water is close to or equal to 1.0. When the moisture content is lower than that of solids, water activity is lower than 1.0, as indicated in Figure 1-6. Below mois￾ture content of about 50 percent the water activity decreases rapidly and the relation￾ship between water content and relative humidity is represented by the sorption iso￾therms. The adsorption and desorption pro￾cesses are not fully reversible; therefore, a MOISTURE CONTENT g/g solids Figure 1-6 Water Activity in Foods at Different Moisture Contents distinction can be made between the adsorp￾tion and desorption isotherms by determin￾ing whether a dry product's moisture levels are increasing, or whether the product's moisture is gradually lowering to reach equi￾librium with its surroundings, implying that the product is being dried (Figure 1-7). Gen￾erally, the adsorption isotherms are required for the observation of hygroscopic products, REL. HUM. % Figure 1-7 Adsorption and Desorption Iso￾therms desorption adsorption MOISTUR E % RELATIVE HUMIDITY %

Water7andthedesorptionisothermsareusefulforinvestigation of the process of drying. Asteeply sloping curve indicates that the material is hygroscopic (curve A, Figure 1-8); aflat curve indicates a product that is not verysensitive to moisture (curve B, Figure 1-8).Manyfoods show the type of curves given inFigure 1-9, where the first part of the curveis quiteflat,indicating alow hygroscopicity,and the end of the curve is quite steep, indi-cating highly hygroscopic conditions.Suchcurves are typical for foods with high sugaror salt contents and low capillary adsorption.Such foods are hygroscopic.The reverse ofREL.HUM. %this type of curve is rarely encountered.These curves show that a hygroscopic prod-Figure 1-9 Sorption Isotherms for Foods withHigh Sugar or Salt Content; Low Capillaryuct or hygroscopic conditions can be definedAdsorptionas the case where a small increase in relativehumidity causes a large increase in productmoisturecontent.Sorption isotherms usually have a sigmoidfirst part (A) of the isotherm, which is usu-shape and can be divided into three areas thatally steep,corresponds to the adsorption of acorespond to different conditions of themonomolecular layer of water; the second,flatter part (B)corresponds to adsorption ofwater present in the food (Figure 1-7). Theadditional layers of water;and thethird part(C)relates to condensation of water in capil-laries and pores of the material.There are nosharp divisions between these three regions,Aand no definite values of relative humidityexist to delineate these parts.Labuza (1968)%has reviewed the various ways in which theisotherms can be explained. The kineticapproach is based on the Langmuir equation,which was initially developed for adsorptionBof gases and solids.This can be expressed inthefollowingform:REL.HUM.%whereFigure 1-8 Sorption Isotherms of Hygroscopica=wateractivityProduct (A)and Nonhygroscopic Product(B)b=aconstant

and the desorption isotherms are useful for investigation of the process of drying. A steeply sloping curve indicates that the mate￾rial is hygroscopic (curve A, Figure 1-8); a flat curve indicates a product that is not very sensitive to moisture (curve B, Figure 1-8). Many foods show the type of curves given in Figure 1-9, where the first part of the curve is quite flat, indicating a low hygroscopicity, and the end of the curve is quite steep, indi￾cating highly hygroscopic conditions. Such curves are typical for foods with high sugar or salt contents and low capillary adsorption. Such foods are hygroscopic. The reverse of this type of curve is rarely encountered. These curves show that a hygroscopic prod￾uct or hygroscopic conditions can be defined as the case where a small increase in relative humidity causes a large increase in product moisture content. Sorption isotherms usually have a sigmoid shape and can be divided into three areas that correspond to different conditions of the water present in the food (Figure 1-7). The REL HUM. % Figure 1-9 Sorption Isotherms for Foods with High Sugar or Salt Content; Low Capillary Adsorption first part (A) of the isotherm, which is usu￾ally steep, corresponds to the adsorption of a monomolecular layer of water; the second, flatter part (B) corresponds to adsorption of additional layers of water; and the third part (C) relates to condensation of water in capil￾laries and pores of the material. There are no sharp divisions between these three regions, and no definite values of relative humidity exist to delineate these parts. Labuza (1968) has reviewed the various ways in which the isotherms can be explained. The kinetic approach is based on the Langmuir equation, which was initially developed for adsorption of gases and solids. This can be expressed in the following form: a _ r K -| _a_ ? = TO + ^ where a = water activity b = a constant MOISTUR E % REL HUM. % Figure 1-8 Sorption Isotherms of Hygroscopic Product (A) and Nonhygroscopic Product (B) MOISTUR E %

8PRINCIPLES OF FOODCHEMISTRYK=1/p.andp.=vaporpressureofwaterwhereS,= surface area, m°/g solidat T.MH,=moleular weigh of waerV=volume adsorbedN.=Avogadro'snumber,6×1023Vm = monolayer valueAH,0 = area of water molecule, 10.6 ×1020 m2When a/Vis plotted versus a, the result is astraight line with a slope equal to 1/Vm andThe BET equation has been used in manythe monolayer value can be calculated. Incases to describe the sorption behavior ofthis form, the equation has not been satisfac-foods.For example,note the work of Sarava-toryforfoods,because theheat of adsorptioncos (1967) on the sorption of dehydratedthat enters into the constant bis not constantappleand potato.Theform of BETequationover the whole surface, because of interac-used for calculation of the monolayer valuetionbetweenadsorbedmolecules,andwasbecause maximum adsorption is greater thanonly a monolayer.C-1 P1pAform of isotherm widely used forfoodsWCPW(P。-p)W,Cis the one described by Brunauer et al.(1938)andknown as theBET isotherm orequation.A form of the BET equation givenwherebyLabuza(1968)isW=watercontent (inpercent)p=vaporpressureof sampleP,= vapor pressure of water at same tem-a[a(C-1)1(l-a)v= vic+[peratureVCC=heatofadsorptionconstantW,=moisture consent corresponding towheremonolayerC = constant related to the heat of adsorptionThe BETplots obtained by Saravacos fordehydrated potato are presented in FigureAplot of al(1-a)V versus agives a straight1-11.line,as indicated inFigure1-10.Themono-Other approaches have been used to analayer coverage value can be calculated fromlyze the sorption isotherms, and these arethe slope and the intercept of the line.Thedescribed by Labuza (1968).However, theBET isotherm isonlyapplicableforvaluesofLangmuir isotherm as modified by Brunauera from 0.1 to 0.5. In addition to monolayeretal.(1938)has been most widely used withcoverage, the water surface area can be calcu-food products. Another method to analyzelated bymeans of thefollowing equation:the sorption isotherms is the GAB sorptionmodel described by van den Berg and Bruin(1981)andusedbyRoos(1993)andJouppila and Roos (1994).N..AH,OS.=VmmMHOAs is shown in Figure 1-7, the adsorptionand desorption curves are not identical. The=3.5×10°vhysteresis effect is commonly observed; note

K = l/p0 and p0 = vapor pressure of water at T0 V = volume adsorbed Vm = monolayer value When alV is plotted versus a, the result is a straight line with a slope equal to l/Vm and the monolayer value can be calculated. In this form, the equation has not been satisfac￾tory for foods, because the heat of adsorption that enters into the constant b is not constant over the whole surface, because of interac￾tion between adsorbed molecules, and because maximum adsorption is greater than only a monolayer. A form of isotherm widely used for foods is the one described by Brunauer et al. (1938) and known as the BET isotherm or equation. A form of the BET equation given by Labuza (1968) is a = J_ , Fa(C-I)I (l-a)V V1nC + V VmC J where C = constant related to the heat of adsorp￾tion A plot of a/(I - a) V versus a gives a straight line, as indicated in Figure 1-10. The mono￾layer coverage value can be calculated from the slope and the intercept of the line. The BET isotherm is only applicable for values of a from 0.1 to 0.5. In addition to monolayer coverage, the water surface area can be calcu￾lated by means of the following equation: S ° = Vm'M^>'N °'Att>° = 3.5 XlO3V1n where S0 = surface area, m2 /g solid M H Q = molecular weight of water, 18 N0 = Avogadro's number, 6 x 1023 ^H9O = area of water molecule, 10.6 x 1020m 2 The BET equation has been used in many cases to describe the sorption behavior of foods. For example, note the work of Sarava￾cos (1967) on the sorption of dehydrated apple and potato. The form of BET equation used for calculation of the monolayer value was p I C-I PO W(P0^p) ~ W 1C + W 1C' P where W = water content (in percent) p = vapor pressure of sample P0 = vapor pressure of water at same tem￾perature C = heat of adsorption constant W1 = moisture consent corresponding to monolayer The BET plots obtained by Saravacos for dehydrated potato are presented in Figure 1-11. Other approaches have been used to ana￾lyze the sorption isotherms, and these are described by Labuza (1968). However, the Langmuir isotherm as modified by Brunauer et al. (1938) has been most widely used with food products. Another method to analyze the sorption isotherms is the GAB sorption model described by van den Berg and Bruin (1981) and used by Roos (1993) and Joup￾pila and Roos (1994). As is shown in Figure 1-7, the adsorption and desorption curves are not identical. The hysteresis effect is commonly observed; note