[Advances in Food Research] Advances in Food Research Volume 13 Volume 13 || Fundamentals of Low-Temperature Food Preservation

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<ul><li><p>FUNDAMENTALS OF LO W-TEMPERATU RE FOOD PRESERVATION </p><p>BY 0 . FENNEMA ASD W . D . POWRIE </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . Introduction 220 221 </p><p>-4 Intermolecular Attractive Force of HOH Molecules 221 B . Proposed Structures for Water and Ice 222 C . Effect of Solutes on the Structure of Water 228 D . Gas Hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 E . Effect of m-ater on the Structure of Proteins 232 F . Bound Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 </p><p>I11 . Phase Diagram of Pure Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 238 </p><p>B . Freezing Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 B . Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 C . Specific Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 D . Latent Heat of Fusion E . Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F . Thermal Diffusirity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>V . Crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 A . Xucleation 244 B . Crystal Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 C . Ice Crystal Size D . Recrystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 E . Location of Ice Crystals in Cellular Suspensions and Tissue . . . . . . . . . </p><p>VI . Freezing Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 :I . Freezing Diagrams of Watrr and Simple Solutions . . . . . . . . . . . . . . . . B . Ireezing Diagrams of Food Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1 . Structure of kVater and Ice . . . . . . . . . . . . . . . . . . </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>. . . . . . . . . . . . . . . . . . . . </p><p>IV . Some Additional Physical Properties of Katcr and Ice . . . . . . . . . . . . . . . . . </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 241 243 </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 </p><p>257 </p><p>259 261 250 272 274 </p><p>B Factors Affecting Rates of Freezing and Thawing 258 C . Comparatire Rates of Freezing and Thawing of Normally Rigid </p><p>VII . Concentration of Nonaqneous Constituents during Freezing . . . . . . . . . . . VIII . Volume Changes during Freezing and Thawing . . . . . . . . . . . . . . . . . . . . . . . </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I X . Rate of Frerzing A . Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 </p><p>Aqucous Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 S . A Search for Protective Chemical Additives . . . . . . . . . . . . . . . . . . . . . . 288 </p><p>. . . . . . . . . . . . . . . . </p><p>X I . The Effect, of Freezing, Storage, and Tlr:nving on the Physical and Chrmical Prolwrtirs of Food . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 A . Classification and Characteristics of Food . . . . . . . . . . . . . . . . . . . . . . 289 3 . Changcs in Selected Food Systcnis Resulting from the Freezing Process 292 </p><p>219 </p></li><li><p>220 0. FENNEMA A S D W. D. POWRIE </p><p>SII. Conclusions Concerning Possible Causes of Frozen Food Deterioration . 31 1 A. Possible Causes of Damage during Freezing . . . . . . . . . . . . . . . . . . . 311 H. Possible Causes of Damage during Frozen Storage . . . . . . . . . . . . . . 315 C. Possible Causes of Damage during Thawing . . . . . . . . . . . . . . . . . . . . . 317 </p><p>SIII. General Aspects of Commercial Freezing Processes . . . . . . . . . . . . . . . . . . . . 317 8. Methods of Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 13. Selection of Fruits and Vegetables for Freezing . . . . . . . . . . . . . . . . . . . 319 C. Prcfrcezing Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 D. The Influencc of Freezing Rate on Quality . . . . . . . . . . . . . . . . . . . . . . . . 322 E. Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 F. Thawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 </p><p>S I V . Closing Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 References . . . . . . . . . . . . . . . . . . . . . . . . . . 330 </p><p>I. INTRODUCTION </p><p>The preservation of food by freezing is not new. Early man used ice and snow as one nieans of preserving his food. Even the commercial freez- ing of food by artificial methods had its origin nearly 100 years ago. During the last several decades, the quality of frozen foods has been vastly improved through the use of foods more suitable for freezing and the use of better processing and handling techniques. The widespread present-day acceptance of frozen foods can be attributed largely to these advances. </p><p>Recommended processing procedures for most foods are readily availa- ble in publications serving the food industry. Probably the most useful publications in this regard are Tressler and Eyers excellent two-volumc set, The Freezing Preservation of Foods (1957a, 1957b), and U.S.D.A. Bulletin ARS-74-21, Conference on Frozen Food Quality (Anonymous, 1960a), which partially summarizes an extensive research program on frozen foods conducted a t the Western Regional Research Laboratory, U.S.D.A. </p><p>Tlie value of the above works is indisputable, but a need for a more fundamental approach to the study of frozen foods nevertheless seems evident. As in any field, the technological advances of the future will dc- pend in a large measure on the effectiveness of our present prograni of fundamental research. Yet, anyone seeking knowledge of the fundamental factors involved in the freezing, storing, and thawing of foods will find i t difficult to obtain. A cursory review of the references a t the close of this paper illustrates part of the problem. A great many of these articles appear in publications unavailable to most food scientists. Furthermore, many of the more basic studies inyolve nonfood items such as bull sperma- tozoa, red blood cells, and microorganisms. There is a clear need to evalu- ate these studies, extract the parts pertinent to food materials, and present the results in a conimunication available to food scientists. That is our intent. </p></li><li><p>LOW-TEMPERATURE FOOD PRESERVATION 22 1 </p><p>I I . STRUCTURE OF WATER AND ICE </p><p>Kater, like the air we breathe, is so common that i t may hardly semi worthy of consideration. Szent-Gyorgyi (1960), in stressing the important and underemphasized role of water in biological materials, stated that "Biology has forgotten water as a deep-sea fish may forget about it." This is perhaps understandable, since biological materials contain such coni- plex and exciting constituents as proteins, carbohydrates, and lipids. Water appears to be an uninteresting, simple, and inert constituent, but is quite the contrary. Foods intended for freezing invariably contain a large percentage of water; otherwise there would be no need to freeze them. Many cuts of lean meat contain 65-70% water, and as much as 90% water is common in fruits and vegetables. Some water is adsorbed so strongly to food constituents such as proteins and polysaccharides that i t contributes to their native structure and even to the over-all nature of the product. Anything which modifies the normal relationship between water and the other food constituents will likely alter the typical character of the food. A change of state, such as freezing, most assuredly has this effect. Further- more, unfrozen water exists in nearly all frozen foods stored under normal commercial conditions. Although the per cent is small, i t appears to have a much greater influence than would be suspected. There are ample rea- sons, therefore, to undertake a detailed study of both water and ice. </p><p>A. IXTERMOLECULAR ATTRACTIVE FORCE O F HOH AIOLECULES Without prior knowledge, one would expect water and other coni- </p><p>pounds of similar molecular weight and electronic configuration to exhibit similar properties. When the properties of water are compared with those of other hydrides of elements near oxygen in the periodic table (CH,, KH3, HF, PH3, H2S, HCl) , water is found to have unusually high values for melting point, boiling point, heat capacity, latent heat of fusion, latent heat of vaporization, surface tension, and dielectric constant (Edsall and Wynian, 1958; Wells, 1950). To illustrate the striking differences involved, </p><p>TABLE I PHYSIFAL PROPERTIES OF SOME HYDRIDES YEAR OXYGEN I N THE PERIODIC TABLE </p><p>Melting point Boiling point Mold heat of vaporization Substan re ("C) ("C 1 (ral/mole) </p><p>CHI - 183 -161 2200 XH? - 78 - 3 3 5550 HF - n'L +19 7220 H?O 0 +100 9750 </p><p>~ ~ ~~~~~ </p></li><li><p>222 0. FENNESIA A S D \T. D. IO\TRIE </p><p>soiiie of the physical properties of n-atcr antl B fen- of tlic coriipoiintls men- tioned above are listed in Table I. </p><p>These properties indicate the existerice of an unusually strong iittrac- tive force between water molecules. This attractive force arises from n combination of circumstances (Edsall and IYyiiian, 1958; Wells, 1950) : </p><p>1 ) The HOH molecule, by virtue of its covalent hydrogcn-oxygen bonds, is ahlc to undergo intermolecular hydrogcn bonding. This condition results from the greaf, affinity of oxygen for electrons (second only to fluorine as the most, electronegntivc atom), enabling it t o gain electrons at the rxpense of tire nrighhoring hydrogen atoms. The oxygen atom therefore assumes a nct charge of minus two, and each hydrogen atom is left essentially as a positively charged proton. The unsymmetrical distribution of charges in the molecule (callcd a dipole) causes water molecules to orient in a fashion whereby the ncgativc charge of one molecule is associated with the positive charge of a neighboring molcculc. This type of molecular attraction antl orientation is particularly strong when hydrogen and the highly electronegative a t o m F, 0 , or N are involved, and has been given the name hydrogen bonding. The hydrides NHa, H20, and HF exhibit the strongest hydrogen bonding among molecules of low molecular weight. </p><p>The energies of hydrogen bonds are .sniwll (es~tiiniites range from 1.3 to 6.8 kilo- calories per niole ; Kkmethy anti Scheraga, 1962a) compared to covalent bonds but are 1:ti.g~ enough compared to the averagr kineiic encrgics of molecules, i Irat thcy significantly influence the physical 1)ropert ies of biological materials. [For conipai,ison, the enprgies of representative covalent bonds (in kcal/niole) a re : C-C, 80: C-H, 98.2; 0-H, 109.4; C=C, 145.1 </p><p>2 ) The atomic composition and groinetry of tlie water molecule facilitatr : a) :I maximum amount of hydrogen bonding (the charged atoms are sterically exposed), antl b) tlircc-diniensionR1 liydrogen bonding. Both factors contribute to the ahnor- i d l y sti.ong association of watcr molecules. </p><p>Tlic forces of attraction are best described by situating the riiolecule in an imaginary tetrahedron as indicated in Fig. 1. With oxygen a t the center of the tetrahedron, the hydrogen atoms may be positioned on any two of the lines originating a t the center of the tetrahedron and passing through any apex. The two lines chosen for the hydrogen at.oins represent tlie di- rection of the two positive forces. HOH molecules tend to orient and associate in accordance with these lines of force. </p><p>lu. PROPOSED STRUCTLRES EOR \VATER ~ N D ICE </p><p>The structure of wttcr and ice lias been inyestigated and discussed by a number of workers (Barnes, 1929; Barrcr and Stuart, 1957; Bernal, 1958; Buswell and Rodcbush, 1956; Claussen, 1951a,b; Edsall and W y - iiiaii, 1958; Eigen and IleMaeyer, 1958; Frank, 1958; Frank and Evans, 1945; Frank and Wen, 1957; Heslop and Robinson, 1960; Klotz, 1958; Lonsdale, 1951, 1958; Luyet and Rapate, 1958; Rlason, 1958a; Mullcr and yon Stackclhcrg, 1952; N h c t h y and Schcraga. 1962a,b,c; Omton, 1958; </p></li><li><p>LO\V-TE1LIPERATURE E 001) PREYERVATIOS 223 </p><p>FIG. 1. Orientation of c3liarpt.s arou~id :in HOH molecule. </p><p>Szont-Gyiirgyi, 1957; van P~KIth~h!OIl van Eck e t al.. 1958; Vogcl, 1921 : von Stackelberg and Muller, 1951). Proposed structures are bascd on in- foriiiation provided by X-ray, neutron, and electron diffraction patterns, and by infrared and ranian spcctra. </p><p>r p o n freezing, HOH inolecules associate in an orderly manncr to forin ii rigid structure which is more open (less dense) than the liquid forin. "Rigid," as used here, applies only in a inacro sense, since considerable nioreincnt of individual atoins and iiiolecules has been reported in ice, 1)articularly a t temperatures just below the freezing point (Lonsdale, 1951). At -10"C, for example, an HUH inolccule vibrates with an ainpli- tutle of approximately 0.44 A, which is nearly one-sixth the distance be- tween adjacent HOH niolecules (Owston, 1958). Furthermore, the hydro- gen atoms in ice arc apparently capable of wandering from one oxygen atoiii t o another, lending a certain degree of covalency to thc hydrogen bonds (Lonsdale, 1951; Frank, 1958). These comments should be kept in mind during the following presentation, where bond lengths are gircn as </p></li><li><p>224 0. FENNEMA AND W. D. POWRIE </p><p>fixed distances and the type of bonding is referred to solely as covalent or hydrogen bonding. </p><p>Each HOH molecule, by virtue of its four tetrahedrally spaced attrac- tive forces, 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 a t a distance of 0.96 A, and hydrogen-bonded with two other hydrogen atoms, each a t a distance of 1.80 A. This results in a quite open, tetrahedral structure, with adjacent oxygen atoms spaced approximately 2.76 A apart and separated by a single hydrogen atom. All bond angles are approximately 109 degrees. This structural arrangement is shown in Fig. 2. Molecules 1 and 2 are </p><p>\! </p><p>i </p><p>0 =oxygen 0 1 - - 1 - - 1 0 = 2.76 0 =Hydrogen = 0.96 </p><p>I-+* = Hydrogen bond C-...</p></li></ul>

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