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Survey of weight loss from lamb in frozen storage
Survey of weight loss from lamb in frozen storage Q. T. Pham, J. R. Durbin and J. W i l l i x Key words: refrigeration, lamb, cold storage, weight loss
Recherches sur les pertes de masse de I'agneau entrepos6 1'6tat congel6 On rend compte de mesures des pertes de masse de lots de carcasses d'agneau p/ac#s dans trois entrepSts frigorifiques p/us ou moins anciens. Les carcasses #taient de tallies et de teneurs en graisse diff~rentes mais el/es ~taient toutes enve/opp~es clans une stockinette. On a aussi proc#d# ~ des v#rifications en p/a~ant des assiettes de g/ace recouvertes de stockinette a divers endroits des entrepSts frigor/fiques. On a trouv# que /es carcasses d'agneau entrepos#es I~tat conge/# perdaient de/'eau b u n taux constant j u s q u 9 au moins 10% de pertes de masse. Les carcasses maigres pr#sentaient des pertes p/us rapides que /es carcasses grasses, m#me en va/eurs abso/ues, bien que pouvant #tre p/us/#g#res.
Le facteur de loin/e p/us important ayant une influence sur /es pertes de masse est ce/ui des variations de/a temperature locale b/'int#rieur d'un entrep(~t (points chauds) provoqu#es par I'#c/airage, une isolation inappropri#e et le manque de circulation d'air. Les vieux entrep(~ts ~ plusieurs niveaux sent particuli#rement sensibles b ce dernier prob/#me et i / p e u t se produire de tres grandes variations entre/es pertes de masse dans ces entrepSts. Le renouve/lement d'air ne favorise pas forc#ment /a dessiccation ~ condition qu'il ne s'#tab/isse pas de forts courants d'air. La temp#rature d'entreposage a une influence importante sur /es pertes de masse, mais dans la pratique cette influence tend a #tre d#pass#e par /'influence des h#t~rog#n#it#s de temp#rature. La courbe de temperature-masse de Cutting et Malton est un assez ben guide pour/'estimation de cette influence en /'absence d'autres facteurs d'aggra ration.
M e a s u r e m e n t s are reported of w e i g h t loss f r o m batches of lamb carcasses, s t o c k i n e t t e covered, in t h r e e d i f f e r e n t types of cold stores. It w a s found t h a t lamb carcasses lose m o i s t u r e at a c o n s t a n t rate for up to at least
10% w e i g h t loss. N e w e r cold stores have m o r e u n i f o r m behaviour. S t o r a g e t e m p e r a t u r e is i m p o r t a n t but t e m p e r a t u r e n o n - u n i f o r m i t y is m o r e significant.
Meat consists mainly of water. During refrigerated processing some of this water has a tendency to evaporate or sublime into vapour. The problem is particularly acute for lambs, which spend considerable periods in a relatively unprotected state, wrapped only in stockinet.
years ago. Since then processing methods have changed greatly, and although valuable work has been done elsewhere, in particular by Cutting and Malton 2, it was felt that the situation needed to be re-evaluated. In consequence, a programme of survey and experimentation has been carried out.
Major investigations into this phenomenon were carried out in New Zealand by Griffiths et al. 1 50
We concentrated on stockinet-wrapped carcasses since it has been previously found 23 that plasticwrapped carcasses lose negligible amounts of moisture during storage. This finding was confirmed by our own results which showed that plasticwrapped carcasses lost only 0.05% of their weight per month at -17.5°C, compared with 0.81% per month for stockinet-wrapped lamb in the same conditions.
This paper is a revised version of that given at the IIR conference in Hamilton, New Zealand, 25-29 January 1982 but with new results and conclusions concerning the effect of lamb grade on weight loss. The results on temperature effects (Fig. 4) have been slightly modified. The authors are from the Meat Industry Research Institute of New Zealand, PO Box 617, Hamilton, New Zealand.
VoLume 5 Number 6 November 1982
0140-7007/82/060337-06S3.00 © 1982 Butterworth & Co (Publishers) Ltd and IIR
337
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All weighings were carried out inside the cold stores with steelyards graduated to 28 g. Readings were estimated to 4-10 g. I
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In this work all three main types of cold stores in New Zealand were covered: one, modern stores, single-storey structures, forced-draft cooled, in which products are stacked on pallets; two, semimodern stores, single-storey structures, pipegridcooled. Product can be palletized or bulk-stowed; three, old stores, multi-storey structures, pipegridcooled with products being bulk-stowed, ie arranged in large stacks.
Experimental In all, 1 220 carcasses were surveyed in twenty different stores. The carcasses were divided into 37 batches, each consisting of 20-40 carcasses of the same grade, stacked at the same location. Eight batches (320 carcasses) in four different stores were weighed every one or two months for up to ten
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months and various temperatures were recorded. For these long experiments two grades of lamb, 'YL' (8 to 12.5 kg with light fat covers) and 'PM' (13 to 16 kg with medium to heavy fat covers) were used and every care was taken to stack the two grades in an identical manner. Also, trays of ice weighing about 5 kg wrapped in stockinet were placed at various locations in the stores to measure the relative variation in weight losses in different positions. The rest of the carcasses were weighed twice only at an interval of about one month, except for batches 35 to 37 which were weighed three times.
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Air, carcass, wall, ceiling and floor temperatures were measured with fine (28 gauge) copperconstantan thermocouples. For carcasses, the thermocouples were inserted into a l m m by 10 mm hole drilled into the loin area, and sealed with ice.
Store temperatures From the temperature history of the product and its environment one could answer such questions as whether there was any appreciable evaporativecooling or radiation effect that would cause a large temperature difference between air and product, whether there were large fluctuations in the environment conditions which would tend to aggravate the problem, whether there was a large difference between enclosure (wall and floor) temperatures and air temperature which would tend to increase the heat load and hence the rate of desiccation.
Revue Internationale du Froid
Typical temperature records for three stores (one modern, one semi-modern, and one old) during the test period are s h o w n in Fig. 1. Each plot s h o w s wall, floor, air and carcass temperatures. When sufficient k n o w l e d g e of the b e h a v i o u r of a temperature has been obtained, recording of that temperature ceased. Thus the carcass temperatures were recorded for only one to t w o months, by w h i c h time they c o u l d clearly be seen to f o l l o w closely the air temperature changes. There w a s a temperature difference of up to 3°C b e t w e e n the f l o o r and the air in the old store, as compared w i t h 1.2°C m a x i m u m for the other stores. This caused carcasses lying on the f l o o r of the old store to be a b o u t 1°C w a r m e r than the others. In the semimodern store, the air near the ceiling was up to 2°C w a r m e r than average store temperature, and the carcasses on the top pallet (near the ceiling) were 1°C w a r m e r than the carcasses on the bottom pallet (on the floor). Our observations s h o w e d that the most likely cause of this warm air layer was infiltration from a nearby door.
As far as the air temperature is concerned, there did not seem to be any s i g n i f i c a n t difference in the long term performance of the temperature control system b e t w e e n the three stores s h o w n , w i t h the m o n t h l y averages f l u c t u a t i n g by a b o u t 3 or 4°C. However, the range of daily f l u c t u a t i o n s for the old store was s i g n i f i c a n t l y larger than for the other stores (5°C vs 1.5 to 2"C).
Weight
loss
W e i g h t loss data are s h o w n in Table 1 together w i t h the experimental c o n d i t i o n s .
Shape of weight/oss curves. Fig. 2 s h o w s plots of percentage w e i g h t loss against time for the long experiments (batches 1 - 8 ) . The ordinate is the average percentage w e i g h t loss for the w h o l e batch, and the abcissa is the time after freezing. If one disregards the p o i n t (0, 0), all data fall on straight lines (the o n l y exception being the last
T a b l e 1. E x p e r i m e n t a l c o n d i t i o n s and results
Tab/eau 1. Batch no
Conditions et r#sultats des experiences Store name
A A
B B 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
C C D D E F G H
J K L M N N N O P Q
R S S T
I I I
Store type a Temperature, °C % store loading
M M SM SM O O O O SM O O O M M M M M M M M O O O M M M M O O O M M M M M M M
-17.4 - 17.4 -16.6 -16.6 - 16.4 - 1 6.4 - 18.8 - 18.8 -13.6 - 1 3.3 -14.6 - 14.1 -10.0 -10.0 -10.0 -10.0 - 1 5.1 - 1 5.1 - 1 5.1 -15.1 - 11.1 - 1 2.5 - 1 5.4 - 1 5.2 - 20.8 -20.8 - 20.8 -19.2 -22.5 - 1 6.3 - 16.4 - 17.5 - 1 7.5 - 15.4 -18.0 - 1 8.O -18.0
60 60 60 60 60 60 60 60 100 1O0 100 1 O0 75 75 75 75 75 75 75 75 80 65 60 75 60 60 60 75 75 60 50 --75 ----
Volume, m3
Lamb grade
Average Weight loss, weight, kg %/month
24000 24 000 14000 14000 ----15000 ---40000 40000 40000 40000 40000 40000 40 000 40000 ---7500 5700 5700 5700 ---28 000 6000 6000 -40000 40000 40000
YL PM YL PM YL PM YL PM PM PM PM PM YL YM PL PM PL YL YM YL PM PM PM PM PM YM YL PM PM PM PM YL PM PM YL PM PL
11.67 14.50 11.48 14.26 10.1 9 14.36 11.07 14.30 14.79 14.34 14.88 14.41 11.67 13.46 12.17 14.22 1 2.00 11.04 13.67 11.24 14.40 14.48 14.17 14.02 14.1 5 13.42 11.92 14.28 14.19 14.58 14.35 11.37 14.03 14.1 6 11.53 1 5.31 12.55
0.45 0.33 0.53 0.36 0.69 0.44 0.44 0.31 0.57 0.06 0.01 0.16 1.26 1.11 0.99 1.03 0.32 0.37 0.68 0.56 0.80 0.57 1.21 0.29 0.50 0.58 0.49 0.10 0.56 0.63 0.71 0.81 0.05 b 0.56 0.79 0.45 0.48
aM: modern store, SM: semi-modern store, O: old store; bBatch 33 consisted of plastic-wrapped lambs.
V o l u m e 5 Num6ro 6 November 1982
339
point of batch 1, which was therefore discarded. The store temperature rose about 3°C above average just before the last weighing then fell again, and this fluctuation could have caused an increase in desiccation rate). The linear regression correlation coefficients of all lines were 1.00 except for batch 2 for which it was 0.99. Similar results are obtained if one splits each batch into top, side, inside and bottom of stacks. Typical results are shown in Fig. 3 for batch 3. All correlation coefficients were in the range 0.99 to 1.00. In one case where carcasses were lying directly on the ground level floor of an old store (batch 5) the linearity persisted to 10%, when weighing was stopped. This indicates that the rate of desiccation was constant up to at least 1 0% weight loss, a surprising result since one would expect some fall-off as the carcasses become drier. Could this fall-off have existed but have been counteracted by seasonal changes in the external conditions? This is unlikely as the experiments occupied different periods of the year (Fig. 1), yet they all yielded linear weight loss vs time plots. 8[ 7
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The most likely explanation of this linearity is that the main resistance to mass transfer is in the air/carcass interface and/or in a relatively impervious layer near the meat surface (eg fat and connective tissue, or peritoneal membrane), while the rest of the carcass is either relatively porous or sustains an effective diffusion mechanism of moisture to the meat surface. The straight lines representing weight loss after the first month do not converge to the origin at time 0, but intersect the v-axis at some positive value. For the carcasses tested, this extra weight loss amounted to an average of 0,29% with a standard deviation of 0.1 2%. This indicates that within the first month the recently frozen carcasses were still equilibrating with the store temperature.
There were two factors that could explain this difference: carcass weight and carcass fat cover (see experiment section). To distinguish between these two effects an experiment was carried out in which 20 YL, 20 PM and 20 PL carcasses (batches 35, 36 and 37 respectively) were stacked together in a modern store at -18°C. Again every care was taken to give the three grades the same treatment in terms of position in the stack. The carcasses were weighed three times at intervals of two months. PL carcasses are in the 8 to 12.5 kg range, with a medium to heavy fat cover; ie they weight the same as YL but have the same fat cover as PM.
Fig. 2 Courbes de/a perte de masse en foncdon du temps (mo/enne des lots)
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Batch 3, pallet near ceiling
Average Average weight Standard deviation, weight, loss,% per %permonth kg month
experiments the weight loss in YL lamb was higher than that in PM lamb both as a percentage of carcass weight (the ratio being 1.45:1) and in absolute terms, even though the YL lambs were much lighter.
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Tableau 2. Influence d e / a cat#gone sur /es pertes de masse
Fig. 3 also shows that carcasses lying on the outside of the stack (side, bottom or top) lost weight much more quickly than those on the inside. This effect was found for all batches tested.
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The results are shown in Table 2. There was no significant difference between PM and PL, and a large difference between YL and the other two grades. Thus carcass weight in itself was not a significant factor in weight loss, and the major difference was in the fat cover. Effect o f temperature. Fig. 4 shows the rate of weight loss plotted against store temperature. Each point means one batch and is expressed in terms of
International Journal of Refrigeration
Weight losses from the trays varied over the range 0.003 to 0.061 kg/month. For each pair of trays at the same location the weight losses were (with one exception which was discarded) less than 25% apart. The results indicate that the modern store surveyed (store A) had a fairly narrow range of variation (max/min=l .8:1 ), with the highest values near the alleyway. Similar results were obtained for a semi-modern store (store B), except for a high value near an external door, which was inadequately insulated. Old stores showed a much wider range of variation, 12:1 for store C and 20:1 for store D, with the worst values occurring on the ground-level floor or near external walls or doors, and the best values near the ceiling (ie near the cooling coils).
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PM carcass loss. Weight loss values for Y-grade (leaner) carcasses were divided by 1.45 to give equivalent PM-carcass weight loss. The data are very scattered due to other factors, especially carcass location. This was particularly serious for old stores. A linear regression line of log (weight loss) vs store temperature for modern and semi-modern store is plotted. The correlation coefficient for this line is 0.641 which is rather low, due to the large scatter. Also shown is a line based on past data by Cutting and Malton. 2 In view of the scatter of the data there is not much to choose between the two lines. All data points lying close to or above the lines are for carcasses which lie at critical locations: near a door to a warm area, directly on the ground level floor or next to an external wall. On the other hand all data lying well below the lines are for carcasses stacked away from these critical spots. On the whole, modern and semi-modern single-storey stores whether forced-draft or pipegrid-cooled, seem much more predictable than old pipegrid-cooled stores. In conclusion, Cutting and Malton's line can be taken as a good estimate of expected weight loss in modern single-storey stores in New Zealand. For old multistorey stores this line can also be used provided there is no aggravating factor, especially with regard to location. Correction must be made for carcass grade, the given line giving a good estimate for PM carcasses only. Effect of location in store. In the long experiments
ice trays were put in pairs at various locations in the store. Assuming that the sublimation rate from ice is proportional to that from meat, the variations in the rates of weight loss from the ice trays give an indication of the effect of location in each store.
Volume 5 Number 6 November 1982
Actual carcass weight loss results confirmed these trends: while modern and semi-modern stores had fairly predictable weight losses, the data for old stores were very scattered: at similar temperatures batch 10 in store "F' had 0.06% per month weight loss while batch 23 in store 'L" had 1.21% per month. The large variations within old stores were probably caused both by inadequate or deteriorating insulation and lack of air movement, which created local temperature variations, the worst thing that could happen in relation to the desiccation process. From standard diffusion theory we can expect the rate of desiccation to be proportional to (Ps-Pw) where Ps is the saturation vapour pressure at the meat temperature andPw the water vapour pressure in the air. Since ps varies exponentially with meat temperature, while Pw is a function of the average store temperature, a small difference between meat temperature and average store temperature can have enormous effects. However, our results also showed that a warm region created by infiltrating hot air does not necessarily increase the weight loss: as the air becomes saturated upon being cooled down, it could even reduce it. Thus in batch 3 (tested in the semi-modern store referred to in Fig. 1 ), although the top pallet (near the ceiling) was 1°C warmer than the bottom pallet (near the floor), its rate of weight loss was 10% less (Fig. 3). In section 3 we pointed out that the high ceiling temperature in this case was caused by warm air infiltration. Some frosting could be observed on structures near the ceiling, indicating that the air was close to satu ration. In modern palletized stores, temperature unevenness is probably not a major problem, except near the lights: measurements indicated that carcasses lying at a distance of 1.1 m under a 275 W mercury vapour bulb were 5°C hotter than non-lit carcasses. At 1.7 m away the temperature was still 3°C higher. Under such circumstances the rate of desiccation can be expected to increase considerably. In one aspect our results contradict those of Cutting and Malton 2 who reported that pipegrid-cooled
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caused by the large random effect of carcass location as well as the uncertainty in estimating store loading. However, the results suggest the following trends which may be industrially important. For modern or semi-modern stores there does not appear to be any large effect of loading on weight loss; for old stores there appears to be a strong effect, with fully-laden stores showing smaller weight losses.
20 Regression line (old stores) ~ . ~
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Conclusions
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Lamb carcasses in frozen storage lose moisture at a constant rate for up to at least 10% weight loss. Lean carcasses lose weight faster than fatter carcasses, even in absolute terms, although they may be lighter.
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stores (in England) tended to cause smaller weight losses than forced-draft stores because of the lower air velocity encountered. In the New Zealand situation this is not necessarily true, as can be seen from Fig. 4. E f f e c t o f s t o r e l o a d i n g . Fig. 5 shows the normalized
weight loss with respect to loading, where normalization is done by dividing the actual weight loss from that which can be expected from Cutting and Malton's correlation. No firm conclusions can be drawn from this graph because of the large scatter of data, which is
342
By far the most important factor affecting weight loss is local temperature variations within a store (hot spots) caused by lighting, inadequate insulation and lack of air circulation. Old multistorey stores are particularly susceptible to this last problem, and there can be a very large variation in weight loss from these stores. Infiltration does not necessarily promote desiccation, provided strong air currents are not set up. Storage temperature has an important effect on weight loss, but in practice this effect tends to be swamped by the effect of temperature nonuniformity. Cutting and Malton's temperatureweight loss curve is a fair guide to the extent of this effect in the absence of other aggravating factors.
References 1 Griffith, E., Vickery, J. R., Holmes, N. E. The freezing.
storage and transport of New Zealand lamb. HMSO. London (1932) 2 Cutting, C. L, Malton, R. Evaporative losses in the commercial freezing and storage of meat. Meat Res Inst Syrup, 3, Bristol, UK (1974) 3 Keeley, G. M. Canterbury Frozen Meat Co, NZ. unpublished (1978)
Revue Internationale du Froid
Recherches sur les pertes de masse de I'agneau entrepos6 1'6tat congel6 On rend compte de mesures des pertes de masse de lots de carcasses d'agneau p/ac#s dans trois entrepSts frigorifiques p/us ou moins anciens. Les carcasses #taient de tallies et de teneurs en graisse diff~rentes mais el/es ~taient toutes enve/opp~es clans une stockinette. On a aussi proc#d# ~ des v#rifications en p/a~ant des assiettes de g/ace recouvertes de stockinette a divers endroits des entrepSts frigor/fiques. On a trouv# que /es carcasses d'agneau entrepos#es I~tat conge/# perdaient de/'eau b u n taux constant j u s q u 9 au moins 10% de pertes de masse. Les carcasses maigres pr#sentaient des pertes p/us rapides que /es carcasses grasses, m#me en va/eurs abso/ues, bien que pouvant #tre p/us/#g#res.
Le facteur de loin/e p/us important ayant une influence sur /es pertes de masse est ce/ui des variations de/a temperature locale b/'int#rieur d'un entrep(~t (points chauds) provoqu#es par I'#c/airage, une isolation inappropri#e et le manque de circulation d'air. Les vieux entrep(~ts ~ plusieurs niveaux sent particuli#rement sensibles b ce dernier prob/#me et i / p e u t se produire de tres grandes variations entre/es pertes de masse dans ces entrepSts. Le renouve/lement d'air ne favorise pas forc#ment /a dessiccation ~ condition qu'il ne s'#tab/isse pas de forts courants d'air. La temp#rature d'entreposage a une influence importante sur /es pertes de masse, mais dans la pratique cette influence tend a #tre d#pass#e par /'influence des h#t~rog#n#it#s de temp#rature. La courbe de temperature-masse de Cutting et Malton est un assez ben guide pour/'estimation de cette influence en /'absence d'autres facteurs d'aggra ration.
M e a s u r e m e n t s are reported of w e i g h t loss f r o m batches of lamb carcasses, s t o c k i n e t t e covered, in t h r e e d i f f e r e n t types of cold stores. It w a s found t h a t lamb carcasses lose m o i s t u r e at a c o n s t a n t rate for up to at least
10% w e i g h t loss. N e w e r cold stores have m o r e u n i f o r m behaviour. S t o r a g e t e m p e r a t u r e is i m p o r t a n t but t e m p e r a t u r e n o n - u n i f o r m i t y is m o r e significant.
Meat consists mainly of water. During refrigerated processing some of this water has a tendency to evaporate or sublime into vapour. The problem is particularly acute for lambs, which spend considerable periods in a relatively unprotected state, wrapped only in stockinet.
years ago. Since then processing methods have changed greatly, and although valuable work has been done elsewhere, in particular by Cutting and Malton 2, it was felt that the situation needed to be re-evaluated. In consequence, a programme of survey and experimentation has been carried out.
Major investigations into this phenomenon were carried out in New Zealand by Griffiths et al. 1 50
We concentrated on stockinet-wrapped carcasses since it has been previously found 23 that plasticwrapped carcasses lose negligible amounts of moisture during storage. This finding was confirmed by our own results which showed that plasticwrapped carcasses lost only 0.05% of their weight per month at -17.5°C, compared with 0.81% per month for stockinet-wrapped lamb in the same conditions.
This paper is a revised version of that given at the IIR conference in Hamilton, New Zealand, 25-29 January 1982 but with new results and conclusions concerning the effect of lamb grade on weight loss. The results on temperature effects (Fig. 4) have been slightly modified. The authors are from the Meat Industry Research Institute of New Zealand, PO Box 617, Hamilton, New Zealand.
VoLume 5 Number 6 November 1982
0140-7007/82/060337-06S3.00 © 1982 Butterworth & Co (Publishers) Ltd and IIR
337
-I0
Store A (modern)
-81 Store C (old) -IO
-12 -14 \\\
xWoil (east)
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-14'
-16
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-I0 Store B (semimodern)
-12
-14
-26 Fig. 1
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M o n t h l y average temperatures for three stores
All weighings were carried out inside the cold stores with steelyards graduated to 28 g. Readings were estimated to 4-10 g. I
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Oct
In this work all three main types of cold stores in New Zealand were covered: one, modern stores, single-storey structures, forced-draft cooled, in which products are stacked on pallets; two, semimodern stores, single-storey structures, pipegridcooled. Product can be palletized or bulk-stowed; three, old stores, multi-storey structures, pipegridcooled with products being bulk-stowed, ie arranged in large stacks.
Experimental In all, 1 220 carcasses were surveyed in twenty different stores. The carcasses were divided into 37 batches, each consisting of 20-40 carcasses of the same grade, stacked at the same location. Eight batches (320 carcasses) in four different stores were weighed every one or two months for up to ten
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months and various temperatures were recorded. For these long experiments two grades of lamb, 'YL' (8 to 12.5 kg with light fat covers) and 'PM' (13 to 16 kg with medium to heavy fat covers) were used and every care was taken to stack the two grades in an identical manner. Also, trays of ice weighing about 5 kg wrapped in stockinet were placed at various locations in the stores to measure the relative variation in weight losses in different positions. The rest of the carcasses were weighed twice only at an interval of about one month, except for batches 35 to 37 which were weighed three times.
'
~Jr averaqe
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Fig. I Moyennes mensueltes des temperatures pour/es trois en trep 5 ts
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Air, carcass, wall, ceiling and floor temperatures were measured with fine (28 gauge) copperconstantan thermocouples. For carcasses, the thermocouples were inserted into a l m m by 10 mm hole drilled into the loin area, and sealed with ice.
Store temperatures From the temperature history of the product and its environment one could answer such questions as whether there was any appreciable evaporativecooling or radiation effect that would cause a large temperature difference between air and product, whether there were large fluctuations in the environment conditions which would tend to aggravate the problem, whether there was a large difference between enclosure (wall and floor) temperatures and air temperature which would tend to increase the heat load and hence the rate of desiccation.
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Typical temperature records for three stores (one modern, one semi-modern, and one old) during the test period are s h o w n in Fig. 1. Each plot s h o w s wall, floor, air and carcass temperatures. When sufficient k n o w l e d g e of the b e h a v i o u r of a temperature has been obtained, recording of that temperature ceased. Thus the carcass temperatures were recorded for only one to t w o months, by w h i c h time they c o u l d clearly be seen to f o l l o w closely the air temperature changes. There w a s a temperature difference of up to 3°C b e t w e e n the f l o o r and the air in the old store, as compared w i t h 1.2°C m a x i m u m for the other stores. This caused carcasses lying on the f l o o r of the old store to be a b o u t 1°C w a r m e r than the others. In the semimodern store, the air near the ceiling was up to 2°C w a r m e r than average store temperature, and the carcasses on the top pallet (near the ceiling) were 1°C w a r m e r than the carcasses on the bottom pallet (on the floor). Our observations s h o w e d that the most likely cause of this warm air layer was infiltration from a nearby door.
As far as the air temperature is concerned, there did not seem to be any s i g n i f i c a n t difference in the long term performance of the temperature control system b e t w e e n the three stores s h o w n , w i t h the m o n t h l y averages f l u c t u a t i n g by a b o u t 3 or 4°C. However, the range of daily f l u c t u a t i o n s for the old store was s i g n i f i c a n t l y larger than for the other stores (5°C vs 1.5 to 2"C).
Weight
loss
W e i g h t loss data are s h o w n in Table 1 together w i t h the experimental c o n d i t i o n s .
Shape of weight/oss curves. Fig. 2 s h o w s plots of percentage w e i g h t loss against time for the long experiments (batches 1 - 8 ) . The ordinate is the average percentage w e i g h t loss for the w h o l e batch, and the abcissa is the time after freezing. If one disregards the p o i n t (0, 0), all data fall on straight lines (the o n l y exception being the last
T a b l e 1. E x p e r i m e n t a l c o n d i t i o n s and results
Tab/eau 1. Batch no
Conditions et r#sultats des experiences Store name
A A
B B 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
C C D D E F G H
J K L M N N N O P Q
R S S T
I I I
Store type a Temperature, °C % store loading
M M SM SM O O O O SM O O O M M M M M M M M O O O M M M M O O O M M M M M M M
-17.4 - 17.4 -16.6 -16.6 - 16.4 - 1 6.4 - 18.8 - 18.8 -13.6 - 1 3.3 -14.6 - 14.1 -10.0 -10.0 -10.0 -10.0 - 1 5.1 - 1 5.1 - 1 5.1 -15.1 - 11.1 - 1 2.5 - 1 5.4 - 1 5.2 - 20.8 -20.8 - 20.8 -19.2 -22.5 - 1 6.3 - 16.4 - 17.5 - 1 7.5 - 15.4 -18.0 - 1 8.O -18.0
60 60 60 60 60 60 60 60 100 1O0 100 1 O0 75 75 75 75 75 75 75 75 80 65 60 75 60 60 60 75 75 60 50 --75 ----
Volume, m3
Lamb grade
Average Weight loss, weight, kg %/month
24000 24 000 14000 14000 ----15000 ---40000 40000 40000 40000 40000 40000 40 000 40000 ---7500 5700 5700 5700 ---28 000 6000 6000 -40000 40000 40000
YL PM YL PM YL PM YL PM PM PM PM PM YL YM PL PM PL YL YM YL PM PM PM PM PM YM YL PM PM PM PM YL PM PM YL PM PL
11.67 14.50 11.48 14.26 10.1 9 14.36 11.07 14.30 14.79 14.34 14.88 14.41 11.67 13.46 12.17 14.22 1 2.00 11.04 13.67 11.24 14.40 14.48 14.17 14.02 14.1 5 13.42 11.92 14.28 14.19 14.58 14.35 11.37 14.03 14.1 6 11.53 1 5.31 12.55
0.45 0.33 0.53 0.36 0.69 0.44 0.44 0.31 0.57 0.06 0.01 0.16 1.26 1.11 0.99 1.03 0.32 0.37 0.68 0.56 0.80 0.57 1.21 0.29 0.50 0.58 0.49 0.10 0.56 0.63 0.71 0.81 0.05 b 0.56 0.79 0.45 0.48
aM: modern store, SM: semi-modern store, O: old store; bBatch 33 consisted of plastic-wrapped lambs.
V o l u m e 5 Num6ro 6 November 1982
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point of batch 1, which was therefore discarded. The store temperature rose about 3°C above average just before the last weighing then fell again, and this fluctuation could have caused an increase in desiccation rate). The linear regression correlation coefficients of all lines were 1.00 except for batch 2 for which it was 0.99. Similar results are obtained if one splits each batch into top, side, inside and bottom of stacks. Typical results are shown in Fig. 3 for batch 3. All correlation coefficients were in the range 0.99 to 1.00. In one case where carcasses were lying directly on the ground level floor of an old store (batch 5) the linearity persisted to 10%, when weighing was stopped. This indicates that the rate of desiccation was constant up to at least 1 0% weight loss, a surprising result since one would expect some fall-off as the carcasses become drier. Could this fall-off have existed but have been counteracted by seasonal changes in the external conditions? This is unlikely as the experiments occupied different periods of the year (Fig. 1), yet they all yielded linear weight loss vs time plots. 8[ 7
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11.53 15.31 12.55
0.79 0.45 0.48
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The most likely explanation of this linearity is that the main resistance to mass transfer is in the air/carcass interface and/or in a relatively impervious layer near the meat surface (eg fat and connective tissue, or peritoneal membrane), while the rest of the carcass is either relatively porous or sustains an effective diffusion mechanism of moisture to the meat surface. The straight lines representing weight loss after the first month do not converge to the origin at time 0, but intersect the v-axis at some positive value. For the carcasses tested, this extra weight loss amounted to an average of 0,29% with a standard deviation of 0.1 2%. This indicates that within the first month the recently frozen carcasses were still equilibrating with the store temperature.
There were two factors that could explain this difference: carcass weight and carcass fat cover (see experiment section). To distinguish between these two effects an experiment was carried out in which 20 YL, 20 PM and 20 PL carcasses (batches 35, 36 and 37 respectively) were stacked together in a modern store at -18°C. Again every care was taken to give the three grades the same treatment in terms of position in the stack. The carcasses were weighed three times at intervals of two months. PL carcasses are in the 8 to 12.5 kg range, with a medium to heavy fat cover; ie they weight the same as YL but have the same fat cover as PM.
Fig. 2 Courbes de/a perte de masse en foncdon du temps (mo/enne des lots)
Bt -' ,S's1
YL PM PL
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Fig. 2 Weight Ioss- - time curves (batch average)
Batch 3, pallet near ceiling
Average Average weight Standard deviation, weight, loss,% per %permonth kg month
experiments the weight loss in YL lamb was higher than that in PM lamb both as a percentage of carcass weight (the ratio being 1.45:1) and in absolute terms, even though the YL lambs were much lighter.
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Influence o f size a n d fatness o f lamb. In the long
~3 2
4 6 8 Time, months
Tableau 2. Influence d e / a cat#gone sur /es pertes de masse
Fig. 3 also shows that carcasses lying on the outside of the stack (side, bottom or top) lost weight much more quickly than those on the inside. This effect was found for all batches tested.
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Table 2. Effect of grade on weight loss
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The results are shown in Table 2. There was no significant difference between PM and PL, and a large difference between YL and the other two grades. Thus carcass weight in itself was not a significant factor in weight loss, and the major difference was in the fat cover. Effect o f temperature. Fig. 4 shows the rate of weight loss plotted against store temperature. Each point means one batch and is expressed in terms of
International Journal of Refrigeration
Weight losses from the trays varied over the range 0.003 to 0.061 kg/month. For each pair of trays at the same location the weight losses were (with one exception which was discarded) less than 25% apart. The results indicate that the modern store surveyed (store A) had a fairly narrow range of variation (max/min=l .8:1 ), with the highest values near the alleyway. Similar results were obtained for a semi-modern store (store B), except for a high value near an external door, which was inadequately insulated. Old stores showed a much wider range of variation, 12:1 for store C and 20:1 for store D, with the worst values occurring on the ground-level floor or near external walls or doors, and the best values near the ceiling (ie near the cooling coils).
1.5 • Modern stores • Semi-modern stores • Old stores Log lineor regression - - - - - - C u t t i n g ond Molton
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/nfluence de la temp#rature sur /es pertes de masse
-8
loss
PM carcass loss. Weight loss values for Y-grade (leaner) carcasses were divided by 1.45 to give equivalent PM-carcass weight loss. The data are very scattered due to other factors, especially carcass location. This was particularly serious for old stores. A linear regression line of log (weight loss) vs store temperature for modern and semi-modern store is plotted. The correlation coefficient for this line is 0.641 which is rather low, due to the large scatter. Also shown is a line based on past data by Cutting and Malton. 2 In view of the scatter of the data there is not much to choose between the two lines. All data points lying close to or above the lines are for carcasses which lie at critical locations: near a door to a warm area, directly on the ground level floor or next to an external wall. On the other hand all data lying well below the lines are for carcasses stacked away from these critical spots. On the whole, modern and semi-modern single-storey stores whether forced-draft or pipegrid-cooled, seem much more predictable than old pipegrid-cooled stores. In conclusion, Cutting and Malton's line can be taken as a good estimate of expected weight loss in modern single-storey stores in New Zealand. For old multistorey stores this line can also be used provided there is no aggravating factor, especially with regard to location. Correction must be made for carcass grade, the given line giving a good estimate for PM carcasses only. Effect of location in store. In the long experiments
ice trays were put in pairs at various locations in the store. Assuming that the sublimation rate from ice is proportional to that from meat, the variations in the rates of weight loss from the ice trays give an indication of the effect of location in each store.
Volume 5 Number 6 November 1982
Actual carcass weight loss results confirmed these trends: while modern and semi-modern stores had fairly predictable weight losses, the data for old stores were very scattered: at similar temperatures batch 10 in store "F' had 0.06% per month weight loss while batch 23 in store 'L" had 1.21% per month. The large variations within old stores were probably caused both by inadequate or deteriorating insulation and lack of air movement, which created local temperature variations, the worst thing that could happen in relation to the desiccation process. From standard diffusion theory we can expect the rate of desiccation to be proportional to (Ps-Pw) where Ps is the saturation vapour pressure at the meat temperature andPw the water vapour pressure in the air. Since ps varies exponentially with meat temperature, while Pw is a function of the average store temperature, a small difference between meat temperature and average store temperature can have enormous effects. However, our results also showed that a warm region created by infiltrating hot air does not necessarily increase the weight loss: as the air becomes saturated upon being cooled down, it could even reduce it. Thus in batch 3 (tested in the semi-modern store referred to in Fig. 1 ), although the top pallet (near the ceiling) was 1°C warmer than the bottom pallet (near the floor), its rate of weight loss was 10% less (Fig. 3). In section 3 we pointed out that the high ceiling temperature in this case was caused by warm air infiltration. Some frosting could be observed on structures near the ceiling, indicating that the air was close to satu ration. In modern palletized stores, temperature unevenness is probably not a major problem, except near the lights: measurements indicated that carcasses lying at a distance of 1.1 m under a 275 W mercury vapour bulb were 5°C hotter than non-lit carcasses. At 1.7 m away the temperature was still 3°C higher. Under such circumstances the rate of desiccation can be expected to increase considerably. In one aspect our results contradict those of Cutting and Malton 2 who reported that pipegrid-cooled
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caused by the large random effect of carcass location as well as the uncertainty in estimating store loading. However, the results suggest the following trends which may be industrially important. For modern or semi-modern stores there does not appear to be any large effect of loading on weight loss; for old stores there appears to be a strong effect, with fully-laden stores showing smaller weight losses.
20 Regression line (old stores) ~ . ~
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Conclusions
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Lamb carcasses in frozen storage lose moisture at a constant rate for up to at least 10% weight loss. Lean carcasses lose weight faster than fatter carcasses, even in absolute terms, although they may be lighter.
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sM 'o°'es,tores :'
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Old stores
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stores (in England) tended to cause smaller weight losses than forced-draft stores because of the lower air velocity encountered. In the New Zealand situation this is not necessarily true, as can be seen from Fig. 4. E f f e c t o f s t o r e l o a d i n g . Fig. 5 shows the normalized
weight loss with respect to loading, where normalization is done by dividing the actual weight loss from that which can be expected from Cutting and Malton's correlation. No firm conclusions can be drawn from this graph because of the large scatter of data, which is
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By far the most important factor affecting weight loss is local temperature variations within a store (hot spots) caused by lighting, inadequate insulation and lack of air circulation. Old multistorey stores are particularly susceptible to this last problem, and there can be a very large variation in weight loss from these stores. Infiltration does not necessarily promote desiccation, provided strong air currents are not set up. Storage temperature has an important effect on weight loss, but in practice this effect tends to be swamped by the effect of temperature nonuniformity. Cutting and Malton's temperatureweight loss curve is a fair guide to the extent of this effect in the absence of other aggravating factors.
References 1 Griffith, E., Vickery, J. R., Holmes, N. E. The freezing.
storage and transport of New Zealand lamb. HMSO. London (1932) 2 Cutting, C. L, Malton, R. Evaporative losses in the commercial freezing and storage of meat. Meat Res Inst Syrup, 3, Bristol, UK (1974) 3 Keeley, G. M. Canterbury Frozen Meat Co, NZ. unpublished (1978)
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