FROZEN FOOD SCIENCE AND TECHNOLOGY
CONTENTS THERMAL PROPERTIES AND ICE CRYSTAL DEVELOPMENT IN FROZEN FOODS EFFECTS OF FREEZING ON NUTRITIONAL AND MICROBIOLOGICAL PROPERTIES OF FOODS MODELLING OF FREEZING PROCESSES SPECIFYING AND SELECTING REFRIGERATION AND FREEZER PLANT EMERGING AND NOVEL FREEZING PROCESSES FREEZING OF MEAT FREEZING OF FISH FREEZING OF FRUITS AND VEGETABLES FREEZING OF BAKERY AND DESSERT PRODUCTS DEVELOPING FROZEN PRODUCTS FOR THE MARKET AND THE FREEZING OF READY-PREPARED MEALS FROZEN STORAGE FREEZE DRYING FROZEN FOOD TRANSPORT FROZEN RETAIL DISPLAY CONSUMER HANDLING OF FROZEN FOODS INFLUENCE OF FREEZING AND FROZEN STORAGE ON QUALITY OF FOODS FOOD PRODUCTS THAWED AFTER COLD STORAGE SHOULD IDEALLY BE INDISTINGUISHABLE FROM THE FRESH PRODUCT (THIS OBVIOUSLY DOES NOT APPLY TO PRODUCTS SUCH AS ICE CREAM THAT ARE CONSUMED IN THE FROZEN STATE) THIS REQUIREMENT IS EASIER TO ACHIEVE IN SOME FOODS THAN IN OTHERS FOODS WITH A DELICATE STRUCTURE ARE MORE LIKELY TO SUFFER CELL DAMAGE HOWEVER, FOR THE MAIN FOOD COMMODITIES (BREAD, MEAT, ?SH, VEGETABLES) THE QUALITY OF THE THAWED PRODUCT IS INDEED KIM ET AL (2007) NOTE: AWATER CONTENT OF FISH IS APPROXIMATELY (80% – FAT CONTENT), LOVE (1982) COMPARABLE WITH THE FRESH PRODUCT (AND IN SOME CASES, APPLYING CERTAIN CRITERIA, FOR EXAMPLE, VITAMIN CONTENT, ENHANCES THE QUALITY OF FRESH FOOD SOLD AS CHILLED) THE FORMATION OF ICE CRYSTALS CAN DOWNGRADE THE QUALITY OF THE FOOD BY ONE OF THE FOLLOWING THREE MECHANISMS: (A) MECHANICAL DAMAGE TO THE FOOD STRUCTURE THE SPECI?C VOLUME OF ICE IS GREATER THAN THAT OF WATER (GREATER BY ABOUT 10%) AND THEREFORE THE EXPANDING ICE CRYSTALS COMPRESS THE FOOD MATRIX ICE CRYSTAL EXPANSION IN SOME FRUITS SUCH AS STRAWBERRY DAMAGES THEM SEVERELY, BECAUSE OF THEIR DELICATE STRUCTURE (THE FRUIT BECOMES ‘SOGGY’ ON THAWING) ON A MACROSCOPIC SCALE, DURING RAPID CRYOGENIC FREEZING, THERMAL STRESSES DUE TO EXPANSION MAY CRACK THE FOOD (B) CROSS-LINKING OF PROTEINS (IN ?SH AND MEAT) DECREASE IN THE AMOUNT OF LIQUID WATER AVAILABLE TO THE PROTEINS AND INCREASE OF ELECTROLYTE CONCENTRATION DURING FREEZING LEAD TO AGGREGATION AND DENATURATION OF ACTOMYOSIN (CONNELL, 1959; BUTTKUS, 1970) (C) LIMITED RE-ABSORPTION OF WATER ON THAWING THIS IS CONNECTED WITH MECHANISM (B) AGAIN, WE CAN TAKE THE EXAMPLE OF ANIMAL TISSUE IN WHICH THE MUSCLE PROTEINS, DURING FROZEN STORAGE, BECOME ‘DENUDED’ OF THEIR HYDRATION WATER AND CROSS-LINKED ON THAWING, THE TISSUE MAY NOT RE-ABSORB THE MELTED ICE CRYSTALS FULLY TO THE WATER CONTENT IT HAD BEFORE FREEZING THIS LEADS TO UNDESIRABLE RELEASE OF EXUDATE – ‘DRIP LOSS’ – AND TOUGHNESS OF TEXTURE IN THE THAWED MUSCLE, THE MAIN ATTRIBUTES DETERMINING QUALITY (MACKIE, 1993) MECHANISMS (B) AND (C) ARE USUALLY THE MAIN CAUSES OF DETERIORATION OF QUALITY OF FROZEN FOODS, WHICH MEANS DETERIORATION OF QUALITY IS CAUSED MAINLY BY PROCESSES TAKING PLACE IN FROZEN STORAGE RATHER THAN DURING THE INITIAL FREEZING RAPID FREEZING IS POSSIBLE ONLY FOR SMALL SAMPLES, NOT COMMERCIAL ONES THE RATE OF FREEZING ACHIEVABLE FOR LARGE COMMERCIAL ‘SAMPLES’ IS SO SMALL THAT THE QUALITY OF FOODS WOULD NOT BE GREATLY AFFECTED BY THE FREEZING RATE (EXTRACELLULAR ICE INVARIABLY FORMS FOR ALL SAMPLES OTHER THAN THOSE WHICH ARE SMALL AND FROZEN IN A LABORATORY BY SPECIAL TECHNIQUES) BOTH DAMAGE TO FOOD AND ITS CONSEQUENCES FOR CONSUMER-ASSESSED QUALITY DEPEND ON THE TYPE OF FOOD (ITS BIOLOGICAL MAKEUP AND STRUCTURE) FOR EXAMPLE, MEAT IS LESS PRONE TO DAMAGE FROM FREEZING AND FROZEN STORAGE THAN ?SH IS THIS IS BECAUSE MEAT PROTEIN ?BRES ARE MORE ‘ROBUST’ AND, MOREOVER, MEAT IS COOKED FOR LONGER THAN ?SH FISH, A COLD-BLOODED ANIMAL, STARTS COOKING AT 35?C – THE BODY TEMPERATURE OF MAMMALS – WHEREAS MEAT PROTEINS ARE MORE STABLE (THERE SEEMS TO BE A CORRELATION BETWEEN THE TEMPERATURE OF THE LIVING ANIMAL AND THE STABILITY OF PROTEINS, E G TROPICAL SEA ?SH AS COMPARED WITH NORTH SEA ?SH) ADDING THERMAL PROPERTIES AND ICE CRYSTAL DEVELOPMENT IN FROZEN FOODS FREEZING CURVES FREEZING OF FOOD STARTS WHEN THE FOOD IS PLACED IN CONTACT WITH A COLD MEDIUM, WHICH CAN BE SOLID (FOR EXAMPLE, HEAT EXCHANGER PLATES AT ?30 TO ?40?C, SOLID CARBON DIOXIDE (DRY ICE) AT SUPERCOOLING BELOW ITS INITIAL FREEZING POINT, A LIQUID IS SAID TO BE SUPERCOOLED THIS IS A METASTABLE STATE OF THELIQUID;THELIQUIDCANCONTINUETOBEINTHISSTATEFORAVERYLONGTIME,BEFORENUCLEATIONOFTHE ?RST CRYSTAL TAKES PLACE FOLLOWING THIS THE CRYSTALS GROW AND SPREAD THROUGHOUT THE VOLUME RAPIDLY PURE WATER (FREE OF IMPURITIES SUCH AS DUST PARTICLES THAT WOULD ACT AS NUCLEATION CENTRES) CAN BE SUPERCOOLED TO AROUND ?40?C AT LOWER TEMPERATURES WATER FREEZES DUE TO HOMOGENEOUS ICE NUCLEATION AND GROWTH IN FOODS THE DEGREE OF SUPERCOOLING IS MUCH SMALLER THAN IN PURE WATER BECAUSE OF HETEROGENEOUS ICE NUCLEATION SUPERCOOLING IS IMPORTANT IN NATURE SINCE THIS IS ONE OF THE MECHANISMS BY WHICH LIVING PLANTS AND ANIMALS COPE WITH SUB-ZERO TEMPERATURES OR MINIMIZE THE DAMAGE OF THEIR TISSUE THAT ICE FORMATION CAN CAUSE THERMAL PROPERTIES AND ICE CRYSTAL DEVELOPMENT IN FROZEN FOODS ICE FRACTION FROZEN OUT PURE WATER FREEZES AT 0?C (SAVE FOR THE PHENOMENON OF SUPERCOOLING), BUT WATER SOLUTIONS (IN FOOD SODIUM CHLORIDE OR OTHER SALT SOLUTIONS) HAVE A LOWER FREEZING POINT, THE DEPRESSION BEING APPROXIMATED BY RAOULT’S EQUATION (MILES ET AL , 1997) DURING COOLING BELOW TF, THE EXTRACELLULAR REGION FORMS ICE ?RST AND THEN THE INTRACELLULAR REGION BEGINS TO CHANGE STATE THIS CAN BE ATTRIBUTED TO THE FACT THAT THE CELL (TYPICAL DIAMETER 50 ?M) MEMBRANE PREVENTS GROWTH OF EXTERNAL ICE INTO THE REGION INSIDE THE CELL (CALLED INTRACELLULAR REGION) MAKING THE INTRACELLULAR REGION SUPERCOOLED (??8?C) FIGURE 1 2 SHOWS A SCHEMATIC DIAGRAM OF AN AQUEOUS BINARY SOLUTION THE EQUILIBRIUM BETWEEN ICE FROZEN OUT BELOW TF AND THE REMAINING SOLUTION REQUIRES THE CHEMICAL POTENTIAL OF THE TWO TO BE THE SAME (PIPPARD, 1961) THIS LEADS TO A RELATION BETWEEN THE WATER ACTIVITY AW OF THE SOLUTION AND THE MOLECULAR MASSES OF THE COMPONENTS AND THEIR FRACTIONS IT IS POSSIBLE TO SHOW FROM THESE THERMODYNAMIC CONSIDERATIONS (FOR EXAMPLE, MILES, 1991) THAT THE AMOUNT OF ICE XI FROZEN OUT AT EACH TEMPERATURE T < TF, IS IN THE ?RST APPROXIMATION AQUEOUS SOLUTION SOLUTE + SOLUTION ICE + SOLUTION 0°C TEMPERATURE TE E PURE WATER PURE SOLUTE ICE + SOLID SOLUTE CONCENTRATION OF SOLUTE (%) A STATE DIAGRAM, SHOWING SCHEMATICALLY THE BEHAVIOUR OF AN AQUEOUS BINARY SOLUTION WITH EUTECTIC POINT E AND EUTECTIC TEMPERATURE TE FROZEN FOOD SCIENCE AND TECHNOLOGY (ASSUMING AN IDEAL BINARY SOLUTION AND SMALL TEMPERATURE DIFFERENCES TF ?T ) GIVEN BY XI = (XW ?XU)(1 ?TF/T ) (1 1) WHERE TF AND T ARE IN DEGREES CELSIUS, XW IS THE TOTAL WATER CONTENT OF THE FOOD AND XU IS THE UNFREEZABLE WATER CONTENT THE LAST ONE IS TYPICALLY 5% AND INCLUDES THE SO-CALLED BOUND WATER, SO THAT XU > XB WHERE XB IS THE CONTENT OF BOUND WATER THE TERM ‘BOUND WATER’ IS NOT UNDERSTOOD WELL AND NOT DE?NED CLEARLY FENNEMA (1985) DE?NES IT IN PRACTICAL TERMS AS WATER WHICH EXISTS IN THE VICINITY OF SOLUTES AND OTHER NON-AQUEOUS CONSTITUENTS, EXHIBITS REDUCED MOLECULAR MOBILITY AND OTHER SIGNI?CANTLY ALTERED PROPERTIES AS COMPARED WITH “BULK WATER” IN THE SAME SYSTEM, AND DOES NOT FREEZE AT ?40?C THIS DE?NITION HAS TWO DESIRABLE ATTRIBUTES ONE, IT PRODUCES A CONCEPTUAL PICTURE OF BOUND WATER, AND TWO, IT PROVIDES A REALISTIC APPROACH TO QUANTIFYING THE BOUND WATER WATER UN- FREEZABLE AT ?40?C CAN BE MEASURED WITH EQUALLY SATISFYING RESULTS BY EITHER PROTON NMR OR CALORIMETRIC PROCEDURES FIGURE 1 3 SHOWS THE GRAPH OF XI FOR TF = ?1?C AND XU = 5% RIEDEL (1957, 1978) MADE THE ?RST SYSTEMATIC EXPERIMENTAL DETERMINATION OF THE ICE FRACTION XI BY CALORIMETRIC MEASUREMENTS OTHER EXPERIMENTAL INVESTIGATIONS, FOR EXAMPLE BY NMR, CON?RM THAT THE APPROXIMATION OF XI BY EQUATION (1 1) IS ACCEPTABLE FOR ENGINEERING PURPOSES SUCH AS THE CALCULATION OF THERMAL PROPERTIES OF FROZEN FOOD, REQUIRING ACCURACY OF ABOUT ±10% (NOVIKOV, 1971) EQUATION 1 1 IS DERIVED FROM THERMODYNAMIC CONSIDERATIONS (SEE FOR EXAMPLE MILES (1991)) THAT DO NOT TAKE INTO ACCOUNT THE FACT THAT EVEN AT CONSTANT TEMPERATURE THE FRACTION OF ICE INCREASES WITH TIME, AS WAS OBSERVED, FOR EXAMPLE, BY KENT (1975) THE TIME DEPENDENCE IS DUE TO KINETICALLY HINDERED MOBILITY OF THE WATER MOLECULES FROZEN FOOD IS NOT AN EQUILIBRIUM PROPORTION OF WATER FROZEN OUT IN FOOD AS A FUNCTION OF TEMPERATURE, CALCULATED FOR A FOOD WITH WATER CONTENT XW OF 80% AND UNFREEZABLE WATER CONTENT XU OF 5% THERMAL PROPERTIES AND ICE CRYSTAL DEVELOPMENT IN FROZEN FOODS AQUEOUS SOLUTION GLASS ICE + SOLUTION 0°C TEMPERATURE TG´ PURE WATER PURE SOLUTE CONCENTRATION OF SOLUTE (%)