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Freezing characteristics of fresh water fish (Labeo rohita) by liquid nitrogen vapor
CRYOBIOLOGY
24, 58-64 (1987)
Freezing
Characteristics of Fresh Water Fish (Labeo Liquid Nitrogen Vapor
SUVENDU BHATTACHARYA,’ Department
of Food Technology
rohita)
by
SILA BHATTACHARYA,* AND D. R. CHOWDHURY
and BioChemical
Engineering,
Jadavpur
University,
Calcutta,
700032, India
The lean fish Laheo rohita was subjected to liquid nitrogen vapor in a batch freezer to determine its freezing characteristics. The moisture content of the fish muscle tissue was about 75%. The initial and average freezing points were found to be - 1.34 and -5.25”C, respectively. The fish slab sample showed a decreased rate of temperature change in that region. The suggested cut-off freezing temperature was - 3O”C, corresponding to about 90% of total water frozen. Continued freezing below ~ 30°C resulted in negligible freezing of residual liquid water in the sample. The effects of low-density polyethylene (LDPE) bags and aluminum foil on the freezing times and the overall heat transfer coefficients were sufficiently small that the authors recommended thicknesses of up to 0.1 and 0.5 mm for LDPE bags and aluminum foil, respectively. In these cases there was only 5% increase in freezing time compared to unpackaged fish samples. o 1987 Academic press, Inc.
Fillets of rohu fish (Labeo rohita) were frozen in liquid nitrogen vapor both with and without common packaging materials including low-density polyethylene (LDPE) bags and aluminum foil of different thicknesses. The effect of these packaging materials on freezing time and on the heat transfer coefficient were studied. For unpackaged fish fillets, the initial and average freezing points, the amount and fraction of frozen and unfrozen water, and the freezing times were determined theoretically. Theoretical freezing times were found to be comparable to the experimental values. Freezing is considered to be the simplest and safest method for long-term preservation of fish. Radiation, antibiotics, and canning all have advantages, but freezing is regarded as the most effective option. The quality of the preserved food depends on the rate of freezing. Rapid freezing in freon- 12, propylene glycol, liquid nitrogen (LN,), liquid carbon dioxide, etc., give the best quality product, and the current trend
is to use liquid nitrogen. The use of liquid nitrogen in food freezing has been summarized by Chattopadhyay and Debling (4) and advantages of cryofrozen products were discussed by Wagner (13). The most frequently adopted techniques are immersion, spraying, or freezing in LN, vapor. The present communication deals with the vapor phase freezing characteristics of the common freshwater lean fish Rohu (La&o vohita) using liquid nitrogen. The objectives of the present work are to determine (1) the initial and average freezing points and total freezing time of fish muscle tissue, and (2) the effects of common packaging materials like aluminum foil and low-density polyethylene bags on quick freezing of individual samples. MATERIALS
Preparation
58 Copyright All riehts
0 1987 by Academic Press. Inc. of reoroduction in anv form reserved.
of j?sh sample.
Live rohu fish, about 4 kg in weight, were collected from a local market and brought to the laboratory at the Jadavpur University. Fillets from the dorsum of the fish (including red meat) with known dimensions and temperature were placed in a well-insulated evaporating LN, vapor chamber designed by Advanced Centre of Cryogenic Research, Jadavpur, Calcutta. The samples
Received December 12, 1985; accepted September 5, 1986. ’ Present address: Department of Agricultural Engineering, I.J.T., Kharagpur Pin 721302, India. 2 Present address: Post Harvest Technology Centre, I.I.T., Kharagpur Pin 721302, India.
001l-2240/87 $3.00
AND METHODS
and freezing
FREEZING CHARACTERISTICS
were wrapped in a minimum of packaging material of appropriate thickness. Temperature measurement. All temperatures, including the temperature of the fish slab at its geometric center, were measured with copper-constantan thermocouples. Density of fish slab. Density was measured at - 17°C by the liquid (toluene, sp gr 0.871) displacement technique after coating the fish slab with thin layer of wax. Composition determination. Moisture, ash, carbohydrate, fat, and protein content (N x 6.25) were determined according to AOAC methods (1). Initial freezing point. The equilibrium phase change temperature for the muscle was calculated using the Clausius-Clapeyron equation (Eq. [ 11)for freezing point depression as described by Heldman (6).
l,X,=$
0
[f-i 1 0
m&G m,lM,
f m,lM,
’
PI
Average freezing point. This parameter was calculated using the method described by Charm (3). Overall
heat transfer
coefficient,
U. U
was calculated by Eq. [3], neglecting smaller terms (8): 1 1 -z-+U h
d K,,,’
Z = C, (T, - T,) + L, + Ci (T, - T*) [5] RESULTS
(a) Composition
The approximate composition of Rohu fish muscle tissue is given in Table 1. (b) Time-Temperature Relationship Fish Muscle during Freezing
of
The time-temperature relationships of fish slab during freezing for determination of the initial freezing point are given in Table 2. Assuming 100 kg of fish slab as the basis of calculation, the values of nonfat solids (NFS), bound water [25% of NFS (12)], and free water are 20.44 kg, 5.11 kg, and 70.1 kg, respectively. Using Eq. [2], the value of X, is calculated to be 0.987 which, when used in Eq. [l], gives the initial freezing temperature as - 1.34”C. In this calcultion the ash content of fish muscle tissue was modeled as NaCl. A factor of 2 was used to account for the two ions of NaCl in aqueous solution. In calculating the mole fraction of water, X,, the carbohydrates, fats, and proteins were neglected
times,
TABLE 1 Approximate Composition of Rohu Fish Muscle Tissue Water Protein Fat Ash Carbohydrate
-30°C.
The theoretical freezing time was calculated by using Levy’s modification (11) of the Nagaoka model for freezing of fish (Eq. [41):
of Fish Muscle
[31
8,. The experimental freezing time was determined from Table 2, corresponding to the time required for bringing the center of slab from 16 to Freezing
where
111 (c) Initial Freezing Point
where A, is the latent heat of fusion and the mole fraction of water in the fish slab, X,, is given by Eq. [2].
x, =
59
OF ROHU IN LN,
75.2 k 0.1 18.0 f 0.3 8.79 k 0.18 1.48 k 0.09 0.96 k 0.26
Note. The values are means k standard deviation
(n = 4).
60
BHATTACHARYA,
BHATTACHARYA.
TABLE 2 Time-temperature Relationship of Fish Slab during Freezing by LN, Vapor for Determination of the Initial Freezing Point Time (min) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2s 26 27 28 29 30 31 32
Center temperature (“Cl 16.0 k 0.1 11.3 2 0.2 8.4 -c 0.3 6.1 k 0.2 5.6 k 0.3 5.2 k 0.2 5.0 k 0.4 1.2 2 0.2 -1.9 t 0.4 -3.6 !I 0.5 -6.2 k 0.3 -9.7 * 0.3 -13.6 2 0.4 -17.7 * 0.5 -22.6 t 1.0 -27.9 ” 0.6 -34.2 k 1.7 -39.8 k 2.01 -47.2 ” 1.27 -55.0 2 1.59 -63.2 i 2.0 -70.5 2 0.7 -78.2 2 2.0 -88.0 +- 1.8 -93.1 2 2.3 -105.2 t 3.2 -118.2 ‘- 2.5 -129.6 -+ 3.7 -140.1 ” 3.7 -152.6 2 3.1 -163.0 + 5.8 - 173.4 +- 3.4 -180.3 2 6.1
AND CHOWDHURY
the Appendix. From Fig. 1, the value of average freezing temperature is - 5.2X. (e) Effect of Common Packaging Materials on 8, and U The effects of packaging materials on freezing time and overall heat transfer coefficient during freezing by LN, vapor are shown in Table 3. (f) Calculation of Total Freezing Time, 0, (Theoretical) Dimension of fish slab: 9.5 cm x 5 cm x 4 cm; average temperature of freezing chamber = - 110.7”C; density of fish slab at - 17°C = 966 kg/m3; initial center temperature of fish slab = 16°C; final center temperature of fish slab = -30°C. Using Eq. [4], 0r = 860 set (detailed calculation is given in Appendix 2). DISCUSSION
The fat content of fish muscle (Table 1) was slightly high due to its fatty red meat portion. During freezing of the fish slab (Table 2), the center temperature fell sharply except in the region of the initial and average freezing points. The theoretical initial phase change temperature of the fish muscle came out to be - 1.34”C. It is
Note. Dimensions of fish slab: 9.5 cm x 5 cm X 4 cm; average temperature of freezer = - 190°C; initial center temperature of fish slab = 16°C; height of fish slab above LN, liquid surface = 2 cm. The center temperature values are the means 4 standard deviation (n = 5).
due to their low solubility in water and their high molecular weight. (d) Determination of Average Freezing Point of Fish Slab Details of the calculations for fraction of frozen and of unfrozen water are shown in
0.51
0
’
-51 125’C
-10
-15
TEMPERATURE
-20
-25
t-C )
FIG. 1. Determination of average freezing point of
fish slab.
FREEZING CHARACTERISTICS
61
OF ROHU IN LNZ
TABLE 3 Effect of Packaging Materials on Freezing Time and Overall Heat Transfer Coefficient during Freezing by LN, Vapor
Thickness of packaging material (mm)
Observed freezing time,= 8, (xc)
Calculated overall heat transfer coefficient, U (W/m2 “K)
Changes in the values for packaged fish slab compared to unpackaged slab 8, m
u mo)
919
70.4b
LDPE bag
0.041 0.050 0.062 0.072 0.085 0.090 0.099 0.112 0.140 0.148 0.183 0.207 0.248 0.310 0.354 0.401 0.489
932 933 934 937 940 948 960 974 987 1005 1010 1016 1021 1037 1047 1068 1117
69.8 69.7 69.5 69.3 69.1 69.0 68.9 68.7 68.3 68.2 67.7 67.4 66.8 65.9 65.3 64.7 63.6
1.34 1.53 1.61 1.89 2.20 3.09 4.38 5.91 7.36 9.31 9.88 10.5 11.1 12.8 13.9 16.2 21.5
-0.85 -0.99 - 1.28 - 1.56 - 1.84 - 1.94 -2.13 -2.41 -3.99 -3.15 -3.87 -4.36 -5.18 - 6.39 -7.23 -8.11 -9.72
Aluminum foil
0.012 0.035 0.090 0.103 0.209 0.296 0.412 0.508
920 926 932 934 841 949 952 969
70.4 70.4 70.4 70.4 70.4 70.4 70.4 70.3
0.12 0.69 1.34 1.54 2.39 3.25 3.52 5.39
- 0.000 -0.001 - 0.003 - 0.004 - 0.007 - 0.009 -0.014 -0.018
Unpackaged
Note. Dimension of fish slab: 9.5 cm x 5 cm x 4 cm; initial temperature of slab: 16°C; height of slab above LN, liquid surface: 7 cm. The observed 0r values are the means ? standard deviation (n = 5). a Freezing time was the time noted to lower the center temperature of fish slab from 16 to - 30°C. b The heat transfer coefficient is the value for gas recirculation zone of a cryotransfer liquid nitrogen freezer. Converted from British unit of source (13).
worth mentioning that many food freezing ture thus differed from the theoretically processes begin between -0.5 and -2°C calculated initial freezing temperature by depending on the product composition. For about 4°C. This difference suggested that fruits, vegetables, and meat, the reported maximum ice formation did not occur at value is about - 1°C (7). that initial freezing temperature but at From Fig. 1, the average freezing tem- somewhat lower temperatures. perature was found to be -5.25”C, correFigure 2 shows an additional factor for sponding to the state for which the rate of fish freezing; the fraction of frozen water change of fraction of frozen water was changes little below the temperature of maximum. The average freezing tempera- -30°C where 90.1% of total water was
62
BHATTACHARYA,
BHATTACHARYA,
AND CHOWDHURY
times, 8,, from the theoretical values was about 7% for the modified Nagaoka equation (Levy modification), indicating agreement between this model and the experimental results. APPENDIX
Thickness of fish fillet, m Specific heat of fish below freezing ci point, J/kg “K Specific heat of fish above freezing G point, J/kg “K d Thickness of packaging material, m Fraction of frozen water Fraction of unfrozen water 2 h Heat transfer coefficient for LN2 vapor, W/m2 “K K Thermal conductivity of fish fillet below freezing point, W/m “K KlIl Thermal conductivity of packaging material, W/m “K Latent heat of fusion of water Lf multiplied by fraction of freezable water, J/kg m’ Fraction of freezable water frozen at - 30°C dimensionless mA Mass fraction of water in fish slab, dimensionless MA Molecular weight of water mB Mass fraction of fish solute, dimensionless Molecular weight of fish solute MB P, R Geometric constants for rectangular slab, dimensionless Universal gas constant, Cal/g-m01 RO Initial freezing temperature of fish T muscle, “K Freezing point of water, “K To Average temperature of freezer, “K T’ Initial temperature of fish muscle, “K Tl Final temperature of frozen product, T2 “K Average freezing point of fish Tf muscle, “K Overall heat transfer coefficient, u W/m2 “K Mole fraction of water in fish slab, xA dimensionless a
Q Q
1” ‘t
FIG. 2. Change in fraction of unfrozen and frozen water (f” and fr) with temperature during LN, vapor freezing.
frozen, indicating that it might not be economical to freeze rohu fish beyond a center temperature of - 30°C. Persson (10) has shown that the storage life of lean fish is about 2 years at that temperature; thus, a cut-off freezing temperature of -30°C for lean fish like rohu is recommended. Table 3 shows the change in observed freezing time Gr (for lowering center temperature from 16 to -30°C) and the calculated overall heat transfer coefficient, U, during the process. It is noted that experimental 0r values are higher for both the LDPE bag and aluminum foil than for unpackaged specimens. The variation in 8, and U values due to packaging thickness was low for aluminum foil packed samples due to the relatively high conductivity of aluminum compared to LDPE. If we allow an increase in 0, values of 5% over those of unpackaged fish, the use of LDPE bags and aluminum foil up to the thickness of 0.1 and 0.5 mm, respectively, is acceptable. These data might also be useful in the individual quick freezing technique of fish freezing. The deviation of experimental freezing
1: NOMENCLATURE
FREEZING
CHARACTERISTICS
Heat to be removed from fish in lowering it from T, to T2, J/kg Total freezing time (16 to - 3O”C), set Density of fish slab, below freezing point, kg/m3
Z % P
APPENDIX
Calculation Freezing
2
of Initial Temperature
OF ROHU
IN LN2
63
mA = 0.305.
So, at -3°C mass of frozen water = mass of total water - mass of unfrozen water mass of bound water = 0.396 kg. Fraction of frozen water, mass of frozen water ” .tf = mass of total water = o’396 kg = 0 526 0.752 kg ’ ’
Fraction of unfrozen water, f,, = 1 - ff Basis of calculation: 100 kg of fish slab. Nonfat solids (NFS) = (0.96 + 1.48 + = 0.474. Figures 1 and 2 were obtained similarly. 18.0) kg = 20.44 kg. Bound water = 25% of NFS = 5.11 kg. Free water = total water - bound water Calculation of U Sample calculation: Packaging with = (75.2 - 5.11) kg = 70.1 LDPE bag of thickness 0.041 mm. kgPutting K, = 0.329 W/m “K (9), d = 4.1 Using Eq. [l], x lop5 m and h = 70.4 W/m* “K in Eq. [3], x, = U = 69.8 W/m* “K. 0.701/18 Changes in eT values = 0.987. 0.70108 + 0.0148 x2135.5 + 23 = or, packaged - 8,, unpackaged x loo 8,, unpackaged Using Eq. [2], -
In 0.987 =
--
or T = 271.66”K = - 1.34”C. Calculation Freezing
of Average Temperature
Basis of calculation: 1 kg of fish slab. Sample calculation: at -3°C using Eq.
PI, lnX,
1 273 - 3
=
or x,
= 0.971.
Now, using Eq. [l], 0.971 =
or
X
;19.8 x 100 = 1.34%
Changes in U values = U, packaged - U, unpackaged x loo U, unpackaged 69.8 70.4 = x 100 = -0.85%. 70.4 K, for aluminum foil = 202 W/m “K (5). Calculation
ofe,
(Theoretical)
Putting C, = 3766 J/kg “K (2), Ci = 1674 J/kg “K (2), T, = 16”C, T, = -5.25”C, T2 = -3O”C, and Lf = 3.015 x IO5 J/kg, where m’ = 0.9 in Eq. [5], the value of Z becomes 4.23 x lo5 J/kg. From Eq. 141,or becomes 860 set when Z = 4.23 x lo5 J/kg, p = 966 kg/m3, P = 0.225, R = 0.063, a = 0.04 m, h = 70.4 W/m* “K, and K = 1.628 W/m “K (2). ACKNOWLEDGMENT
m,/18
m,/18 + 0.0148
= ““.‘,,,
2/(35.5 + 23)
The authors are grateful to the Advanced Centre of Cryogenic Research (Jadavpur University, Calcutta) for use of their liquid nitrogen freezing facilities.
64
BHATTACHARYA,
BHATTACHARYA,
REFERENCES 1. Association of Official Analytical Chemists (AOAC) “Official Methods of Analysis,” 1lth ed. AOAC, Washington, D.C., 1970. 2. American Society of Heating, Refrigeration and Air Conditioning Engineers “ASHRAE Handbook and Product Directory.” ASHRAE Inc. New York, 1977. 3. Charm, S. E. “The Fundamentals of Food Engineering,” p. 622. AVI Publ., Westport, Conn., 1971. 4. Chattopadhyay, P., and Debling, G. B. Effect of operating variables on the performance of a J. liquid nitrogen freezing tunnel. Indian ctyog. 2, 93-100 (1977). 5. Geankoplis, C. J. “Transport Processes and Unit Operations,” p. 650. Allyn & Bacon, Boston, 1978. 6. Heldman, D. R. Prediction of food product freezing rates. In “Food Process Engineering” (P. Linko, Y. Malkki, J. Olkku, and J. Larinkari, Eds.), Vol. 1, pp. 58-64. Applied Science Publ., London, 1980. 7. Kessler, H. G. “Food Engineering and Dairy Technology,” p. 335. Kessler, Freising, 1981.
AND CHOWDHURY
8. McCabe, W. L., and Smith, J. C. “Unit Operations of Chemical Engineering,” 2nd ed., p. 315. McGraw-Hill, New York, 1967. 9. Perry, J. H. “Chemical Engineers Handbook,” 4th ed. McGraw-Hill, New York, 1969. 10. Persson, P. 0. Frozen storage, refrigeration equipment and freezing systems. In “Advances in Technology in the Chilling, Freezing Processing, Storage and Transport of Fish, Especially Underutilized Species,” pp. 49-60. International Institute of Refrigeration, Paris, 1981. 11. Ramaswamy, H. S., and Tung, M. A. A review on predicting freezing times of foods. J. Food Puocess. Eng. 7, 169-203 (1984). 12. Schwartzberg, H. G. Effective heat capacities for the freezing and thawing of foods. In “Proceedings of the Commissions Cl, C2,” pp. 303-309. International Institute of Refrigeration, Karlsruhe, 1977. 13. Wagner, R. C. Engineering considerations for the design of a cryogenic food freezer. In “Applications of Cryogenic Technology” (R. W. Vance, Ed.), Vol. 4, pp. 202-214. The Aerospace Corp., Los Angeles, 1971.
24, 58-64 (1987)
Freezing
Characteristics of Fresh Water Fish (Labeo Liquid Nitrogen Vapor
SUVENDU BHATTACHARYA,’ Department
of Food Technology
rohita)
by
SILA BHATTACHARYA,* AND D. R. CHOWDHURY
and BioChemical
Engineering,
Jadavpur
University,
Calcutta,
700032, India
The lean fish Laheo rohita was subjected to liquid nitrogen vapor in a batch freezer to determine its freezing characteristics. The moisture content of the fish muscle tissue was about 75%. The initial and average freezing points were found to be - 1.34 and -5.25”C, respectively. The fish slab sample showed a decreased rate of temperature change in that region. The suggested cut-off freezing temperature was - 3O”C, corresponding to about 90% of total water frozen. Continued freezing below ~ 30°C resulted in negligible freezing of residual liquid water in the sample. The effects of low-density polyethylene (LDPE) bags and aluminum foil on the freezing times and the overall heat transfer coefficients were sufficiently small that the authors recommended thicknesses of up to 0.1 and 0.5 mm for LDPE bags and aluminum foil, respectively. In these cases there was only 5% increase in freezing time compared to unpackaged fish samples. o 1987 Academic press, Inc.
Fillets of rohu fish (Labeo rohita) were frozen in liquid nitrogen vapor both with and without common packaging materials including low-density polyethylene (LDPE) bags and aluminum foil of different thicknesses. The effect of these packaging materials on freezing time and on the heat transfer coefficient were studied. For unpackaged fish fillets, the initial and average freezing points, the amount and fraction of frozen and unfrozen water, and the freezing times were determined theoretically. Theoretical freezing times were found to be comparable to the experimental values. Freezing is considered to be the simplest and safest method for long-term preservation of fish. Radiation, antibiotics, and canning all have advantages, but freezing is regarded as the most effective option. The quality of the preserved food depends on the rate of freezing. Rapid freezing in freon- 12, propylene glycol, liquid nitrogen (LN,), liquid carbon dioxide, etc., give the best quality product, and the current trend
is to use liquid nitrogen. The use of liquid nitrogen in food freezing has been summarized by Chattopadhyay and Debling (4) and advantages of cryofrozen products were discussed by Wagner (13). The most frequently adopted techniques are immersion, spraying, or freezing in LN, vapor. The present communication deals with the vapor phase freezing characteristics of the common freshwater lean fish Rohu (La&o vohita) using liquid nitrogen. The objectives of the present work are to determine (1) the initial and average freezing points and total freezing time of fish muscle tissue, and (2) the effects of common packaging materials like aluminum foil and low-density polyethylene bags on quick freezing of individual samples. MATERIALS
Preparation
58 Copyright All riehts
0 1987 by Academic Press. Inc. of reoroduction in anv form reserved.
of j?sh sample.
Live rohu fish, about 4 kg in weight, were collected from a local market and brought to the laboratory at the Jadavpur University. Fillets from the dorsum of the fish (including red meat) with known dimensions and temperature were placed in a well-insulated evaporating LN, vapor chamber designed by Advanced Centre of Cryogenic Research, Jadavpur, Calcutta. The samples
Received December 12, 1985; accepted September 5, 1986. ’ Present address: Department of Agricultural Engineering, I.J.T., Kharagpur Pin 721302, India. 2 Present address: Post Harvest Technology Centre, I.I.T., Kharagpur Pin 721302, India.
001l-2240/87 $3.00
AND METHODS
and freezing
FREEZING CHARACTERISTICS
were wrapped in a minimum of packaging material of appropriate thickness. Temperature measurement. All temperatures, including the temperature of the fish slab at its geometric center, were measured with copper-constantan thermocouples. Density of fish slab. Density was measured at - 17°C by the liquid (toluene, sp gr 0.871) displacement technique after coating the fish slab with thin layer of wax. Composition determination. Moisture, ash, carbohydrate, fat, and protein content (N x 6.25) were determined according to AOAC methods (1). Initial freezing point. The equilibrium phase change temperature for the muscle was calculated using the Clausius-Clapeyron equation (Eq. [ 11)for freezing point depression as described by Heldman (6).
l,X,=$
0
[f-i 1 0
m&G m,lM,
f m,lM,
’
PI
Average freezing point. This parameter was calculated using the method described by Charm (3). Overall
heat transfer
coefficient,
U. U
was calculated by Eq. [3], neglecting smaller terms (8): 1 1 -z-+U h
d K,,,’
Z = C, (T, - T,) + L, + Ci (T, - T*) [5] RESULTS
(a) Composition
The approximate composition of Rohu fish muscle tissue is given in Table 1. (b) Time-Temperature Relationship Fish Muscle during Freezing
of
The time-temperature relationships of fish slab during freezing for determination of the initial freezing point are given in Table 2. Assuming 100 kg of fish slab as the basis of calculation, the values of nonfat solids (NFS), bound water [25% of NFS (12)], and free water are 20.44 kg, 5.11 kg, and 70.1 kg, respectively. Using Eq. [2], the value of X, is calculated to be 0.987 which, when used in Eq. [l], gives the initial freezing temperature as - 1.34”C. In this calcultion the ash content of fish muscle tissue was modeled as NaCl. A factor of 2 was used to account for the two ions of NaCl in aqueous solution. In calculating the mole fraction of water, X,, the carbohydrates, fats, and proteins were neglected
times,
TABLE 1 Approximate Composition of Rohu Fish Muscle Tissue Water Protein Fat Ash Carbohydrate
-30°C.
The theoretical freezing time was calculated by using Levy’s modification (11) of the Nagaoka model for freezing of fish (Eq. [41):
of Fish Muscle
[31
8,. The experimental freezing time was determined from Table 2, corresponding to the time required for bringing the center of slab from 16 to Freezing
where
111 (c) Initial Freezing Point
where A, is the latent heat of fusion and the mole fraction of water in the fish slab, X,, is given by Eq. [2].
x, =
59
OF ROHU IN LN,
75.2 k 0.1 18.0 f 0.3 8.79 k 0.18 1.48 k 0.09 0.96 k 0.26
Note. The values are means k standard deviation
(n = 4).
60
BHATTACHARYA,
BHATTACHARYA.
TABLE 2 Time-temperature Relationship of Fish Slab during Freezing by LN, Vapor for Determination of the Initial Freezing Point Time (min) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2s 26 27 28 29 30 31 32
Center temperature (“Cl 16.0 k 0.1 11.3 2 0.2 8.4 -c 0.3 6.1 k 0.2 5.6 k 0.3 5.2 k 0.2 5.0 k 0.4 1.2 2 0.2 -1.9 t 0.4 -3.6 !I 0.5 -6.2 k 0.3 -9.7 * 0.3 -13.6 2 0.4 -17.7 * 0.5 -22.6 t 1.0 -27.9 ” 0.6 -34.2 k 1.7 -39.8 k 2.01 -47.2 ” 1.27 -55.0 2 1.59 -63.2 i 2.0 -70.5 2 0.7 -78.2 2 2.0 -88.0 +- 1.8 -93.1 2 2.3 -105.2 t 3.2 -118.2 ‘- 2.5 -129.6 -+ 3.7 -140.1 ” 3.7 -152.6 2 3.1 -163.0 + 5.8 - 173.4 +- 3.4 -180.3 2 6.1
AND CHOWDHURY
the Appendix. From Fig. 1, the value of average freezing temperature is - 5.2X. (e) Effect of Common Packaging Materials on 8, and U The effects of packaging materials on freezing time and overall heat transfer coefficient during freezing by LN, vapor are shown in Table 3. (f) Calculation of Total Freezing Time, 0, (Theoretical) Dimension of fish slab: 9.5 cm x 5 cm x 4 cm; average temperature of freezing chamber = - 110.7”C; density of fish slab at - 17°C = 966 kg/m3; initial center temperature of fish slab = 16°C; final center temperature of fish slab = -30°C. Using Eq. [4], 0r = 860 set (detailed calculation is given in Appendix 2). DISCUSSION
The fat content of fish muscle (Table 1) was slightly high due to its fatty red meat portion. During freezing of the fish slab (Table 2), the center temperature fell sharply except in the region of the initial and average freezing points. The theoretical initial phase change temperature of the fish muscle came out to be - 1.34”C. It is
Note. Dimensions of fish slab: 9.5 cm x 5 cm X 4 cm; average temperature of freezer = - 190°C; initial center temperature of fish slab = 16°C; height of fish slab above LN, liquid surface = 2 cm. The center temperature values are the means 4 standard deviation (n = 5).
due to their low solubility in water and their high molecular weight. (d) Determination of Average Freezing Point of Fish Slab Details of the calculations for fraction of frozen and of unfrozen water are shown in
0.51
0
’
-51 125’C
-10
-15
TEMPERATURE
-20
-25
t-C )
FIG. 1. Determination of average freezing point of
fish slab.
FREEZING CHARACTERISTICS
61
OF ROHU IN LNZ
TABLE 3 Effect of Packaging Materials on Freezing Time and Overall Heat Transfer Coefficient during Freezing by LN, Vapor
Thickness of packaging material (mm)
Observed freezing time,= 8, (xc)
Calculated overall heat transfer coefficient, U (W/m2 “K)
Changes in the values for packaged fish slab compared to unpackaged slab 8, m
u mo)
919
70.4b
LDPE bag
0.041 0.050 0.062 0.072 0.085 0.090 0.099 0.112 0.140 0.148 0.183 0.207 0.248 0.310 0.354 0.401 0.489
932 933 934 937 940 948 960 974 987 1005 1010 1016 1021 1037 1047 1068 1117
69.8 69.7 69.5 69.3 69.1 69.0 68.9 68.7 68.3 68.2 67.7 67.4 66.8 65.9 65.3 64.7 63.6
1.34 1.53 1.61 1.89 2.20 3.09 4.38 5.91 7.36 9.31 9.88 10.5 11.1 12.8 13.9 16.2 21.5
-0.85 -0.99 - 1.28 - 1.56 - 1.84 - 1.94 -2.13 -2.41 -3.99 -3.15 -3.87 -4.36 -5.18 - 6.39 -7.23 -8.11 -9.72
Aluminum foil
0.012 0.035 0.090 0.103 0.209 0.296 0.412 0.508
920 926 932 934 841 949 952 969
70.4 70.4 70.4 70.4 70.4 70.4 70.4 70.3
0.12 0.69 1.34 1.54 2.39 3.25 3.52 5.39
- 0.000 -0.001 - 0.003 - 0.004 - 0.007 - 0.009 -0.014 -0.018
Unpackaged
Note. Dimension of fish slab: 9.5 cm x 5 cm x 4 cm; initial temperature of slab: 16°C; height of slab above LN, liquid surface: 7 cm. The observed 0r values are the means ? standard deviation (n = 5). a Freezing time was the time noted to lower the center temperature of fish slab from 16 to - 30°C. b The heat transfer coefficient is the value for gas recirculation zone of a cryotransfer liquid nitrogen freezer. Converted from British unit of source (13).
worth mentioning that many food freezing ture thus differed from the theoretically processes begin between -0.5 and -2°C calculated initial freezing temperature by depending on the product composition. For about 4°C. This difference suggested that fruits, vegetables, and meat, the reported maximum ice formation did not occur at value is about - 1°C (7). that initial freezing temperature but at From Fig. 1, the average freezing tem- somewhat lower temperatures. perature was found to be -5.25”C, correFigure 2 shows an additional factor for sponding to the state for which the rate of fish freezing; the fraction of frozen water change of fraction of frozen water was changes little below the temperature of maximum. The average freezing tempera- -30°C where 90.1% of total water was
62
BHATTACHARYA,
BHATTACHARYA,
AND CHOWDHURY
times, 8,, from the theoretical values was about 7% for the modified Nagaoka equation (Levy modification), indicating agreement between this model and the experimental results. APPENDIX
Thickness of fish fillet, m Specific heat of fish below freezing ci point, J/kg “K Specific heat of fish above freezing G point, J/kg “K d Thickness of packaging material, m Fraction of frozen water Fraction of unfrozen water 2 h Heat transfer coefficient for LN2 vapor, W/m2 “K K Thermal conductivity of fish fillet below freezing point, W/m “K KlIl Thermal conductivity of packaging material, W/m “K Latent heat of fusion of water Lf multiplied by fraction of freezable water, J/kg m’ Fraction of freezable water frozen at - 30°C dimensionless mA Mass fraction of water in fish slab, dimensionless MA Molecular weight of water mB Mass fraction of fish solute, dimensionless Molecular weight of fish solute MB P, R Geometric constants for rectangular slab, dimensionless Universal gas constant, Cal/g-m01 RO Initial freezing temperature of fish T muscle, “K Freezing point of water, “K To Average temperature of freezer, “K T’ Initial temperature of fish muscle, “K Tl Final temperature of frozen product, T2 “K Average freezing point of fish Tf muscle, “K Overall heat transfer coefficient, u W/m2 “K Mole fraction of water in fish slab, xA dimensionless a
Q Q
1” ‘t
FIG. 2. Change in fraction of unfrozen and frozen water (f” and fr) with temperature during LN, vapor freezing.
frozen, indicating that it might not be economical to freeze rohu fish beyond a center temperature of - 30°C. Persson (10) has shown that the storage life of lean fish is about 2 years at that temperature; thus, a cut-off freezing temperature of -30°C for lean fish like rohu is recommended. Table 3 shows the change in observed freezing time Gr (for lowering center temperature from 16 to -30°C) and the calculated overall heat transfer coefficient, U, during the process. It is noted that experimental 0r values are higher for both the LDPE bag and aluminum foil than for unpackaged specimens. The variation in 8, and U values due to packaging thickness was low for aluminum foil packed samples due to the relatively high conductivity of aluminum compared to LDPE. If we allow an increase in 0, values of 5% over those of unpackaged fish, the use of LDPE bags and aluminum foil up to the thickness of 0.1 and 0.5 mm, respectively, is acceptable. These data might also be useful in the individual quick freezing technique of fish freezing. The deviation of experimental freezing
1: NOMENCLATURE
FREEZING
CHARACTERISTICS
Heat to be removed from fish in lowering it from T, to T2, J/kg Total freezing time (16 to - 3O”C), set Density of fish slab, below freezing point, kg/m3
Z % P
APPENDIX
Calculation Freezing
2
of Initial Temperature
OF ROHU
IN LN2
63
mA = 0.305.
So, at -3°C mass of frozen water = mass of total water - mass of unfrozen water mass of bound water = 0.396 kg. Fraction of frozen water, mass of frozen water ” .tf = mass of total water = o’396 kg = 0 526 0.752 kg ’ ’
Fraction of unfrozen water, f,, = 1 - ff Basis of calculation: 100 kg of fish slab. Nonfat solids (NFS) = (0.96 + 1.48 + = 0.474. Figures 1 and 2 were obtained similarly. 18.0) kg = 20.44 kg. Bound water = 25% of NFS = 5.11 kg. Free water = total water - bound water Calculation of U Sample calculation: Packaging with = (75.2 - 5.11) kg = 70.1 LDPE bag of thickness 0.041 mm. kgPutting K, = 0.329 W/m “K (9), d = 4.1 Using Eq. [l], x lop5 m and h = 70.4 W/m* “K in Eq. [3], x, = U = 69.8 W/m* “K. 0.701/18 Changes in eT values = 0.987. 0.70108 + 0.0148 x2135.5 + 23 = or, packaged - 8,, unpackaged x loo 8,, unpackaged Using Eq. [2], -
In 0.987 =
--
or T = 271.66”K = - 1.34”C. Calculation Freezing
of Average Temperature
Basis of calculation: 1 kg of fish slab. Sample calculation: at -3°C using Eq.
PI, lnX,
1 273 - 3
=
or x,
= 0.971.
Now, using Eq. [l], 0.971 =
or
X
;19.8 x 100 = 1.34%
Changes in U values = U, packaged - U, unpackaged x loo U, unpackaged 69.8 70.4 = x 100 = -0.85%. 70.4 K, for aluminum foil = 202 W/m “K (5). Calculation
ofe,
(Theoretical)
Putting C, = 3766 J/kg “K (2), Ci = 1674 J/kg “K (2), T, = 16”C, T, = -5.25”C, T2 = -3O”C, and Lf = 3.015 x IO5 J/kg, where m’ = 0.9 in Eq. [5], the value of Z becomes 4.23 x lo5 J/kg. From Eq. 141,or becomes 860 set when Z = 4.23 x lo5 J/kg, p = 966 kg/m3, P = 0.225, R = 0.063, a = 0.04 m, h = 70.4 W/m* “K, and K = 1.628 W/m “K (2). ACKNOWLEDGMENT
m,/18
m,/18 + 0.0148
= ““.‘,,,
2/(35.5 + 23)
The authors are grateful to the Advanced Centre of Cryogenic Research (Jadavpur University, Calcutta) for use of their liquid nitrogen freezing facilities.
64
BHATTACHARYA,
BHATTACHARYA,
REFERENCES 1. Association of Official Analytical Chemists (AOAC) “Official Methods of Analysis,” 1lth ed. AOAC, Washington, D.C., 1970. 2. American Society of Heating, Refrigeration and Air Conditioning Engineers “ASHRAE Handbook and Product Directory.” ASHRAE Inc. New York, 1977. 3. Charm, S. E. “The Fundamentals of Food Engineering,” p. 622. AVI Publ., Westport, Conn., 1971. 4. Chattopadhyay, P., and Debling, G. B. Effect of operating variables on the performance of a J. liquid nitrogen freezing tunnel. Indian ctyog. 2, 93-100 (1977). 5. Geankoplis, C. J. “Transport Processes and Unit Operations,” p. 650. Allyn & Bacon, Boston, 1978. 6. Heldman, D. R. Prediction of food product freezing rates. In “Food Process Engineering” (P. Linko, Y. Malkki, J. Olkku, and J. Larinkari, Eds.), Vol. 1, pp. 58-64. Applied Science Publ., London, 1980. 7. Kessler, H. G. “Food Engineering and Dairy Technology,” p. 335. Kessler, Freising, 1981.
AND CHOWDHURY
8. McCabe, W. L., and Smith, J. C. “Unit Operations of Chemical Engineering,” 2nd ed., p. 315. McGraw-Hill, New York, 1967. 9. Perry, J. H. “Chemical Engineers Handbook,” 4th ed. McGraw-Hill, New York, 1969. 10. Persson, P. 0. Frozen storage, refrigeration equipment and freezing systems. In “Advances in Technology in the Chilling, Freezing Processing, Storage and Transport of Fish, Especially Underutilized Species,” pp. 49-60. International Institute of Refrigeration, Paris, 1981. 11. Ramaswamy, H. S., and Tung, M. A. A review on predicting freezing times of foods. J. Food Puocess. Eng. 7, 169-203 (1984). 12. Schwartzberg, H. G. Effective heat capacities for the freezing and thawing of foods. In “Proceedings of the Commissions Cl, C2,” pp. 303-309. International Institute of Refrigeration, Karlsruhe, 1977. 13. Wagner, R. C. Engineering considerations for the design of a cryogenic food freezer. In “Applications of Cryogenic Technology” (R. W. Vance, Ed.), Vol. 4, pp. 202-214. The Aerospace Corp., Los Angeles, 1971.