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

14PRINCIPLESOFFOODCHEMISTRY40%。6/1D335%230%23.225.3%ES20.9%0%18.3%Q上-20-10010-30ocTEMPERATUREFigure 1-16 Specific Heat of Bread of Different Water Contents (Indicated as %) as a Function ofTemperature.Source:From L.Riedel, Calorimetric Studies of theFreezing of White Bread and OtherFlour Products, Kaltetechn, Vol. 11,pp.41-46, 1959.FREEZINGANDICESTRUCTUREfrom another hydrogen atom.When icemelts, some of the hydrogen bonds are broken and the water molecules pack togetherA water moleculemaybind four others inmore compactly in a liquid state (the averagea tetrahedral arrangement. This results in aligancy of a water molecule in water is abouthexagonal crystal lattice in ice,as shown in5 and in ice,4).There is some structural dis-Figure I-17.The lattice is loosely built andorder in the ice crystal. For each hydrogenhas relatively large hollow spaces; thisbond, there are two positions for the hydro-results in a high specific volume. In thegen atom: O-H+O and O+H-O. Withouthydrogen bonds, the hydrogen atom is o.1restrictions on the disorder, there would benmfromoneoxygenatomand0.176nm4N ways of arranging the hydrogen atoms inan ice crystal containing N water molecules(2N hydrogen atoms). There is one restric-tion, though: there must be two hydrogenatoms near each oxygen atom. As a resultthere are only (3/2)MVwaysof arrangingthehydrogen atoms in the crystal.The phase diagram (Figure 1-18) indicatesthe existence of three phases: solid, liquid,and gas. The conditions under which theyexist are separated by three equilibriumlines: the vapor pressure line TA, the meltingFigure 1-17 Hexagonal Pattern of the Latticepressure line TC,and the sublimation presStructureinIcesure line BT. The three lines meet at point T

FREEZING AND ICE STRUCTURE A water molecule may bind four others in a tetrahedral arrangement. This results in a hexagonal crystal lattice in ice, as shown in Figure 1-17. The lattice is loosely built and has relatively large hollow spaces; this results in a high specific volume. In the hydrogen bonds, the hydrogen atom is 0.1 nm from one oxygen atom and 0.176 nm from another hydrogen atom. When ice melts, some of the hydrogen bonds are bro￾ken and the water molecules pack together more compactly in a liquid state (the average ligancy of a water molecule in water is about 5 and in ice, 4). There is some structural dis￾order in the ice crystal. For each hydrogen bond, there are two positions for the hydro￾gen atom: O-H+O and O+H-O. Without restrictions on the disorder, there would be 4^ ways of arranging the hydrogen atoms in an ice crystal containing TV water molecules (2N hydrogen atoms). There is one restric￾tion, though: there must be two hydrogen atoms near each oxygen atom. As a result there are only (3/2)^ ways of arranging the hydrogen atoms in the crystal. The phase diagram (Figure 1-18) indicates the existence of three phases: solid, liquid, and gas. The conditions under which they exist are separated by three equilibrium lines: the vapor pressure line TA, the melting pressure line TC, and the sublimation pres￾sure line BT. The three lines meet at point T, Figure 1-17 Hexagonal Pattern of the Lattice Structure in Ice TEMPERATURE 0C Figure 1-16 Specific Heat of Bread of Different Water Contents (Indicated as %) as a Function of Temperature. Source: From L. Riedel, Calorimetric Studies of the Freezing of White Bread and Other Flour Products, Kdltetechn, Vol. 11, pp. 41-46, 1959. SPEC. HEA T Cal/g.° C

15Waterform at higher temperature and in larger vol15cAumes.Inultrapure water,1mL canbe super-cooledto-32C;dropletsof0.1mmdiame-liquidter to-35; and droplets of 1μm to-41℃Sbefore solidificationoccurs.10The speed of crystallization-that is,thesolidprogress of the ice front in centimeters persecond-isdeterminedbytheremoval oftheheat of fusion from the area of crystalliza-5tion. The speed of crystallization is low at ahighdegreeofsupercooling(Merymanvapor1966).This is important because it affectsBthe size of crystals in the ice. When large-上0watermassesare cooled slowly,there issuf0101020-20ficient time for heterogeneous nucleation inTEMPERATURE°Cthe area of the ice point. At that point thecrystallization speed is very large so that aFigure1-18PhaseDiagram of Waterfew nuclei grow to a large size, resulting in acoarse crystalline structure. At greater cool-where all three phases are in equilibrium.ing speed, high supercooling occurs; thisFigure 1-18 shows that when ice is heated atresults in high nuclei formation and smallerpressures below 4.58 mm Hg,it changesgrowth rate and, therefore, a fine crystaldirectly into the vapor form. This is the basisstructureof freezedryingUpon freezing,HOH molecules associateIt is possible to supercool water.When ain an orderly manner to form a rigid structuresmall ice crystal is introduced, the supercool-that is more open (less dense)than the liquiding is immediately terminated and the tem-form.There still remains considerable move-perature rises to oC. Normally the presencement of individual atoms and molecules inof a nucleus is required. Generally, nucleiice,particularly just below the freezingform around foreign particles (heterogeneouspoint. At 10C an HOH molecule vibratesnucleation).It is difficult to study homoge-with an amplitudeof approximately 0.044neous nucleation. This has been studied innm, nearly one-sixth the distance betweenthe case of fat crystallization, by emulsifyingadjacent HOH molecules.Hydrogen atomsthe fat so that it is divided into a large num-may wanderfromoneoxygen atomtober of small volumes, with the chance of aanother.EachHOHmoleculehas fourtetrahedrallyglobule containing a heterogeneous nucleusbeing very small (Vanden-Tempel 1958).Aspaced attractive forces and is potentiallyhomogeneous nucleus forms from the chanceable to associate by means of hydrogenagglomeration of water molecules in the icebonding withfour otherHOH molecules.Inconfiguration. Usually, such nuclei disinte-this arrangement each oxygen atom isgrate above a critical temperature.Theprob-bonded covalently with two hydrogen atoms,ability of such nucleiforming depends on theeachat adistanceof 0.096nm,and eachvolume of water; they are more likely tohydrogen atom is bonded with two other

TEMPERATURE 0C Figure 1-18 Phase Diagram of Water where all three phases are in equilibrium. Figure 1-18 shows that when ice is heated at pressures below 4.58 mm Hg, it changes directly into the vapor form. This is the basis of freeze drying. It is possible to supercool water. When a small ice crystal is introduced, the supercool￾ing is immediately terminated and the tem￾perature rises to O9C. Normally the presence of a nucleus is required. Generally, nuclei form around foreign particles (heterogeneous nucleation). It is difficult to study homoge￾neous nucleation. This has been studied in the case of fat crystallization, by emulsifying the fat so that it is divided into a large num￾ber of small volumes, with the chance of a globule containing a heterogeneous nucleus being very small (Vanden-Tempel 1958). A homogeneous nucleus forms from the chance agglomeration of water molecules in the ice configuration. Usually, such nuclei disinte￾grate above a critical temperature. The prob￾ability of such nuclei forming depends on the volume of water; they are more likely to form at higher temperature and in larger vol￾umes. In ultrapure water, 1 mL can be super￾cooled to -320C; droplets of 0.1 mm diame￾ter to -350C; and droplets of 1 |im to -410C before solidification occurs. The speed of crystallization—that is, the progress of the ice front in centimeters per second—is determined by the removal of the heat of fusion from the area of crystalliza￾tion. The speed of crystallization is low at a high degree of supercooling (Meryman 1966). This is important because it affects the size of crystals in the ice. When large water masses are cooled slowly, there is suf￾ficient time for heterogeneous nucleation in the area of the ice point. At that point the crystallization speed is very large so that a few nuclei grow to a large size, resulting in a coarse crystalline structure. At greater cool￾ing speed, high supercooling occurs; this results in high nuclei formation and smaller growth rate and, therefore, a fine crystal structure. Upon freezing, HOH molecules associate in an orderly manner to form a rigid structure that is more open (less dense) than the liquid form. There still remains considerable move￾ment of individual atoms and molecules in ice, particularly just below the freezing point. At 1O0C an HOH molecule vibrates with an amplitude of approximately 0.044 nm, nearly one-sixth the distance between adjacent HOH molecules. Hydrogen atoms may wander from one oxygen atom to another. Each HOH molecule has four tetrahedrally spaced attractive forces and is potentially able to associate by means of hydrogen bonding with four other HOH molecules. In this arrangement each oxygen atom is bonded covalently with two hydrogen atoms, each at a distance of 0.096 nm, and each hydrogen atom is bonded with two other vapor solid liquid PRESSUR E m m H g

16PRINCIPLESOFFOODCHEMISTRYhydrogen atoms, each at a distance of 0.18that the greater density of water must beachieved by each molecule having somenm. This results in an open tetrahedral struc-neighbors.Acubic structure witheach HOHture with adjacent oxygen atoms spacedmolecule surrounded by six others has beenabout 0.276 nm apart and separated by singlehydrogen atoms. All bond angles are approx-suggested.At o,water contains ice-like clustersimately 109 degrees (Figure 1-19)Extension of the model in Figure 1-19averaging 90 molecules per cluster. Withincreasing temperature, clusters becomeleads to the hexagonal pattern of ice estab-smallerand more numerous.At OC,approx-lished when several tetrahedrons are assem-bled (Figure1-17),imately half of the hydrogen bonds present at-183Cremainunbroken,and evenat100℃Upon change of state from ice to water,rigidity is lost, but water still retains a largeapproximately one-third are still present.Allnumber of ice-like clusters.The term ice-likehydrogen bonds are broken when watercluster does not imply an arrangement iden-changes into vapor at 100C. This explainstical to that of crystallized ice.The HOHthe large heat of vaporization of water.bond angle of water is several degrees lessthan that of ice, and the average distanceCrystal Growth and Nucleationbetween oxygen atoms is 0.31 nm in waterCrystal growth,in contrast to nucleationand 0.276 nm in ice. Research has not yetoccurs readily at temperatures close to thedetermined whether the ice-like clusters offreezing point. It is more difficult to initiatewater exist in a tetrahedral arrangement, ascrystallization than to continue it. The rate ofthey do in ice. Since the average intermolec-ice crystal growth decreases with decreasingular distance is greater than in ice, it followstemperature. A schematic graphical repre-sentation of nucleation and crystal growthXrates is given in Figure 1-20. Solutes ofmany types and in quite small amounts willgreatly slow ice crystal growth. The mecha-nismof thisactionisnotknown.Membranesmay be impermeabletoice crystal growthand thus limit crystal size. The effect ofmembranes on ice crystal propagation wasstudied by Lusena and Cook (1953), whofound that membranes freely permeable toliquidsmaybeeitherpermeable,partlyper-meable, or impermeable to growing ice crys-tals.In a given material,permeability to iceO:OxygenO---O -2768crystal growth increaseswithporosity,but is?"HydrogenO:096also affected by rate of cooling,membrane1-1-FHydrogenbondO--- :1.80composition and properties,and concentra-:Chemicalbondtion of the solute(s) present in the aqueousphase. When ice crystal growth is retardedFigure1-19HydrogenBondedArrangementofWaterMoleculesinIceby solutes,the icephase maybecome dis

hydrogen atoms, each at a distance of 0.18 nm. This results in an open tetrahedral struc￾ture with adjacent oxygen atoms spaced about 0.276 nm apart and separated by single hydrogen atoms. All bond angles are approx￾imately 109 degrees (Figure 1-19). Extension of the model in Figure 1-19 leads to the hexagonal pattern of ice estab￾lished when several tetrahedrons are assem￾bled (Figure 1-17). Upon change of state from ice to water, rigidity is lost, but water still retains a large number of ice-like clusters. The term ice-like cluster does not imply an arrangement iden￾tical to that of crystallized ice. The HOH bond angle of water is several degrees less than that of ice, and the average distance between oxygen atoms is 0.31 nm in water and 0.276 nm in ice. Research has not yet determined whether the ice-like clusters of water exist in a tetrahedral arrangement, as they do in ice. Since the average intermolec￾ular distance is greater than in ice, it follows that the greater density of water must be achieved by each molecule having some neighbors. A cubic structure with each HOH molecule surrounded by six others has been suggested. At OT!, water contains ice-like clusters averaging 90 molecules per cluster. With increasing temperature, clusters become smaller and more numerous. At O0C, approx￾imately half of the hydrogen bonds present at -1830C remain unbroken, and even at 10O0C approximately one-third are still present. All hydrogen bonds are broken when water changes into vapor at 10O0C. This explains the large heat of vaporization of water. Crystal Growth and Nucleation Crystal growth, in contrast to nucleation, occurs readily at temperatures close to the freezing point. It is more difficult to initiate crystallization than to continue it. The rate of ice crystal growth decreases with decreasing temperature. A schematic graphical repre￾sentation of nucleation and crystal growth rates is given in Figure 1-20. Solutes of many types and in quite small amounts will greatly slow ice crystal growth. The mecha￾nism of this action is not known. Membranes may be impermeable to ice crystal growth and thus limit crystal size. The effect of membranes on ice crystal propagation was studied by Lusena and Cook (1953), who found that membranes freely permeable to liquids may be either permeable, partly per￾meable, or impermeable to growing ice crys￾tals. In a given material, permeability to ice crystal growth increases with porosity, but is also affected by rate of cooling, membrane composition and properties, and concentra￾tion of the solute(s) present in the aqueous phase. When ice crystal growth is retarded by solutes, the ice phase may become dis￾Figure 1-19 Hydrogen Bonded Arrangement of Water Molecules in Ice Oxygen Hydrogen Hydrogen bond Chemicol bond

17Watertemperature on the linear crystallizationvelocity of water is given in Table 1-6.If thetemperatureisloweredtobelowtheCRYSTALGROWTHFP (Figure 1-20), crystal growth is the pre-Vdominant factor at first but, at increasingrate of supercooling,nucleation takes over.NUCLEATIONTherefore,at low supercooling large crys-tals are formed; as supercooling increases,manysmallcrystalsareformed.Control ofcrystal size is much more difficult in tissues2SUPERCOOLINGthan in agitated liquids. Agitation may promote nucleation and,therefore,reducedFP = temperature at which crystals start to form.crystal size. Lusena and Cook (1954) sugFigure 1-20 Schematic Representation of thegested that large ice crystals are formedRateof Nucleationand Crystal Growthwhen freezing takes place above the criticalnucleation temperature (close to FP in Fig-ure 1-20).When freezing occurs at the crit-ical nucleation temperature, small ice cry-continuous eitherbythe presence of amem-stals form. The effect of solutes on nucle-braneorspontaneously.ation and rate of ice crystal growth is aIce crystal size at the completion of freezmajor factor controlling the pattern of prop-ing is related directly to the number of nuclei.agation of the ice front. Lusena and CookThegreater the number ofnuclei, the smaller(1955)also found that solutes depress thethe size of thecrystals.In liquid systemsnucleation temperature to the same extentnuclei can be added.This process is knownthat they depress the freezing point. Solutesas seeding.Practical applications of seedingretard ice growth at 10°C supercooling,withinclude adding finely ground lactose to evap-organic compounds having a greater effectorated milk in the evaporator, and recirculat-than inorganic ones. At low concentrations,ing some portion of crystallized fat in a heatexchanger during manufacture of margarine.If the system is maintained at a temperatureclose to the freezing point (FP), where crys-Table1-6 Effect of Temperature on Linear CrystallizationVelocityofWatertallization starts (Figure 1-20),only a fewnuclei form and each crystal grows exten-sively.The slow removal of heat energy pro-TemperatureatOnsetLinearCrystallizationofCrystallization (C)Velocity (mm/min)duces an analogous situation, since the heatof crystallization released by the few grow-0.9230ing crystals causes the temperature to remain-1.9520near the melting point, where nucleation is-2.0580unlikely. In tissue or unagitated fluid sys-2.26803.5tems, slow removal of heat results in a con-1,2205.0tinuous ice phase that slowly moves inward,1,750with little if any nucleation. The effect of7.02,800

FP = temperature at which crystals start to form. Figure 1-20 Schematic Representation of the Rate of Nucleation and Crystal Growth continuous either by the presence of a mem￾brane or spontaneously. Ice crystal size at the completion of freez￾ing is related directly to the number of nuclei. The greater the number of nuclei, the smaller the size of the crystals. In liquid systems nuclei can be added. This process is known as seeding. Practical applications of seeding include adding finely ground lactose to evap￾orated milk in the evaporator, and recirculat￾ing some portion of crystallized fat in a heat exchanger during manufacture of margarine. If the system is maintained at a temperature close to the freezing point (FP), where crys￾tallization starts (Figure 1-20), only a few nuclei form and each crystal grows exten￾sively. The slow removal of heat energy pro￾duces an analogous situation, since the heat of crystallization released by the few grow￾ing crystals causes the temperature to remain near the melting point, where nucleation is unlikely. In tissue or unagitated fluid sys￾tems, slow removal of heat results in a con￾tinuous ice phase that slowly moves inward, with little if any nucleation. The effect of temperature on the linear crystallization velocity of water is given in Table 1-6. If the temperature is lowered to below the FP (Figure 1-20), crystal growth is the pre￾dominant factor at first but, at increasing rate of supercooling, nucleation takes over. Therefore, at low supercooling large crys￾tals are formed; as supercooling increases, many small crystals are formed. Control of crystal size is much more difficult in tissues than in agitated liquids. Agitation may pro￾mote nucleation and, therefore, reduced crystal size. Lusena and Cook (1954) sug￾gested that large ice crystals are formed when freezing takes place above the critical nucleation temperature (close to FP in Fig￾ure 1-20). When freezing occurs at the crit￾ical nucleation temperature, small ice cry￾stals form. The effect of solutes on nucle￾ation and rate of ice crystal growth is a major factor controlling the pattern of prop￾agation of the ice front. Lusena and Cook (1955) also found that solutes depress the nucleation temperature to the same extent that they depress the freezing point. Solutes retard ice growth at 1O0C supercooling, with organic compounds having a greater effect than inorganic ones. At low concentrations, Table 1-6 Effect of Temperature on Linear Crys￾tallization Velocity of Water Temperature at Onset Linear Crystallization of Crystallization (0C) Velocity (mm/min) ^09 230 -1.9 520 -2.0 580 -2.2 680 -3.5 1,220 -5.0 1,750 -7.0 2,800 SUPERCOOLING NUCLEATION CRYSTALGROWTH RATE

18PRINCIPLES OFFOOD CHEMISTRYproteins are as effective as alcohols andTable1-7VolumeChangeofWaterandSucrosesugars in retarding crystal growth.Solutions on FreezingOnceformed,crystalsdonot remainunchanged during frozen storage; they haveVolume Increase DuringTemperature Changea tendency to enlarge. Recrystallization isSucrose (%)from70°Fto09F(%)particularly evident when storage tempera-08.6tures are allowed to fluctuate widely. There108.7is a tendency for large crystals to grow at the208.2expenseof small ones.306.2Slow freezing results in large ice crystals405.1located exclusively in extracellular areas.503.9Rapid freezing results in tiny ice crystals60Nonelocated both extra- and intracellularly. Not70too muchisknown about therelation-1.0 (decrease)between ice crystal location and frozen foodquality.During the freezing of food, water ishas been described by Roos and Kareltransformed to ice with a high degree of(1991a,b,c).purity, and solute concentration in the unfro-zen liquid is gradually increased. This isaccompanied by changes in pH,ionicThe Glass Transitionstrength, viscosity, osmotic pressure, vaporIn aqueous systems containing polymericpressure, and other properties.substances or some low molecular weightWhen water freezes,it expands nearly9materials including sugars and other carbo-percent. The volume change of a food that ishydrates, lowering of the temperature mayfrozen will bedetermined byits water con-result in formation of a glass. A glass is antent and by solute concentration.Highly con-amorphous solid material rather than a crys-centrated sucrose solutions do not showtalline solid. A glass is an undercooled liquidexpansion (Table 1-7).Air spaces maypar-tially accommodate expanding ice crystals.Volume changes in some fruit products uponfreezing are shown in Table 1-8.The effectTable1-8 Expansion of Fruit Products Duringof air space is obvious. The expansion ofFreezingwater on freezing results in local stresses thatundoubtedlyproduce mechanical damage inVolume Increasecellular materials.Freezing maycauseDuring Tempera-ture Changefromchanges in frozen foods that make the prod-Product70Fto0°F(%)uct unacceptable.Such changes may include8.3destabilization of emulsions,flocculation ofApple juice8.0proteins,increase in toughness of fish flesh,Orange juice4.0loss of textural integrity, and increase in dripWhole raspberries6.3loss of meat.Iceformation can be influencedCrushed raspberries3.0by the presence of carbohydrates. The ef-Wholestrawberries8.2fect of sucrose on the ice formation processCrushed strawberries

proteins are as effective as alcohols and sugars in retarding crystal growth. Once formed, crystals do not remain unchanged during frozen storage; they have a tendency to enlarge. Recrystallization is particularly evident when storage tempera￾tures are allowed to fluctuate widely. There is a tendency for large crystals to grow at the expense of small ones. Slow freezing results in large ice crystals located exclusively in extracellular areas. Rapid freezing results in tiny ice crystals located both extra- and intracellularly. Not too much is known about the relation between ice crystal location and frozen food quality. During the freezing of food, water is transformed to ice with a high degree of purity, and solute concentration in the unfro￾zen liquid is gradually increased. This is accompanied by changes in pH, ionic strength, viscosity, osmotic pressure, vapor pressure, and other properties. When water freezes, it expands nearly 9 percent. The volume change of a food that is frozen will be determined by its water con￾tent and by solute concentration. Highly con￾centrated sucrose solutions do not show expansion (Table 1-7). Air spaces may par￾tially accommodate expanding ice crystals. Volume changes in some fruit products upon freezing are shown in Table 1-8. The effect of air space is obvious. The expansion of water on freezing results in local stresses that undoubtedly produce mechanical damage in cellular materials. Freezing may cause changes in frozen foods that make the prod￾uct unacceptable. Such changes may include destabilization of emulsions, flocculation of proteins, increase in toughness of fish flesh, loss of textural integrity, and increase in drip loss of meat. Ice formation can be influenced by the presence of carbohydrates. The ef￾fect of sucrose on the ice formation process Table 1-7 Volume Change of Water and Sucrose Solutions on Freezing Volume Increase During Temperature Change Sucrose (%) from 70 0F to O 0F (%) ~oa s 10 8.7 20 8.2 30 6.2 40 5.1 50 3.9 60 None 70 -1.0 (decrease) has been described by Roos and Karel (1991a,b,c). The Glass Transition In aqueous systems containing polymeric substances or some low molecular weight materials including sugars and other carbo￾hydrates, lowering of the temperature may result in formation of a glass. A glass is an amorphous solid material rather than a crys￾talline solid. A glass is an undercooled liquid Table 1-8 Expansion of Fruit Products During Freezing Volume Increase During Tempera￾ture Change from Product 70 0F to O 0F (%) Apple juice 8.3 Orange juice 8.0 Whole raspberries 4.0 Crushed raspberries 6.3 Whole strawberries 3.0 Crushed strawberries 8.2