- Email: [email protected]
Processes that Contribute to Radiocesium Decontamination of Feta Cheese
DAIRY
FOODS
RESEARCH
PAPERS
P r o c e a a e s t h a t C o n t r i b u t e to R a d i o c e s i u m Decontamination of F e t a C h e e s e
C. P. PAPPAS Dairy Research Institute of Ioannina 454 44 Ioannina, Greece
P. A. ASSIMAKOPOULOS, K. G. IOANNIDES, and A. A. PAKOU Nuclear Physics Laboratory The University of Ioannina, 451 10 Ioannina, Greece
A. S. MAN'I-ZIOS Agricultural Research StatJon of Ioannina PO Box 1103 451 10 Ioannina, Greece
ABffl'RAG~r In a series of experiments, the transfer of radiocesium from ovine milk to feta cheese was invesdgated through modifications of the standard cheese making procedure. All variations explored showed nø significant change in the percentage of radiocesium transfer and the milk-to-cheese transfer coefficient was determined as f = .79 + .04 L.kg -1. It is concluded that cesium, like the rest of the alkali metals, remains in the water phase and thus follows very closely the distribunon of moisture into the products of cheese making. The possibility of radiocesium decontaminafion of mature feta during the customary storage of the product in brine was also explored in a second series of experiments. The theoretical model employed in the analysis of cesium transport from feta to brine is presented in the Appendix to this paper. Predictions of the model were validated by experiments. A procedure is thus proposed for decontaminating mature feta during storage through successive replacements of the storage medium. Nomograms are presented for the determination of the opfimum time interval between changes of the brine and the radiocesium concentration remaining in the feta. Changes in the Received June 2, 1988. Accepted October 14, 1988. 1989 J Dairy Sei 72:1081-1091
properties of the product induced by the proposed treatment were also investigated with respect to composition, taste, and overall quality. INTRODUCTION
The radioactive plume from the nuclear accident at Chernobyl on April 26, 1986 resulted in serious contaminadon of pastures in northwestern Greece. Radionuclides in the fallout soon penetrated into the milk of lactating animals--especially free-grazing sheep and goats. During May 1986 levels reaching 18,000 Bq.L -1 for 131I and 5000 Bq.L-1 for total radiocesium concentration were measured (1). All ovine milk produced in Greece during May and June 1986 was withheld from immediate consumption and was processed into (prima.rily) feta cheese. This eliminated the problem arising from the short-lived isotope 131I (T1/2 = 8.05 d), because any activity from this source was expected to become negligible during the maturation period of the cheese (2 mo, according to Greek regulations). A serious problem remained, however, with the long-lived isotopes 134Cs (T1/2 = 2.06 y) and 137Cs (T1Æ = 30.1 y). Transport of these isotopes during the cheese making process into the final product has recently been the subject of several studies (2, 3, 4). The object of the research presented here was to investigate possible processes, either during cheese making, or later, which may contribute to the reduction of radiocesium concentration in feta cheese. 1081
1082
PAPPAS ET AI.,. MATERIALS AND METHODS
Milk Radiocontamination
For the purposes of the research presented here, 20 ewes of the Agricultural Research Station of Ioannina were segregated from the herd and held for 2 mo in an isolated pen. The animals were allowed to feed ad libitum on hay (Medicago sativa) that was harvested at Yannitsa (in central Macedonia) during late spring 1986 and was thus strongly affected by the Chernobyl fallout. Total radiocesium concentration in the animals' feed was measured as 9000 Bq.kg -1, and the average daily consumption of hay per animal was determined as 1.2 kg. The daily diet of the animals contained in addition .5 kg of a mixture of corn and barley with average radiocesium concentration of 50 Bq.kg -1. The evolution of radiocesium contamination of the animals' mill( was monitored through daily measurements at the Nuclear Physics Laboratory of the University of Ioannina, which determined that contamination reached an equilibrium level of approximately 700 Bq.L -1 within 2 wk. Following this, the daily production of milk was transported to the laboratories of the Dairy Research Institute of Ioannina, where it was processed into feta cheese. Cheese Msking
Each batch of milk was pasteurized at 63 to 65°C for 20 min. Starter culture (Streptococcus thermophilus and Lactobacillus bulgaricus) at .2% and liquid (Hansens) rennet were added so that coagulation was achieved in 42 to 45 min at 35°C. Following coagulation, the curd was cut into blocks of 2 to 3-cm thickness and was allowed to rest for l0 min. The sliced curd and the whey were then placed in molds for drainage. Weight of the curd and volume of the whey were measured 6 h alter placement in the molds (end of the drainage period). At the same time, samples of both the curd and whey were measured for radiocesium concentration. The curd was then sliced into 11 x 12 x 6-cm blocks and was placed temporarily in open vessels for dry salting with the addition of 3.6% of NaCl by weight of the curd. Three days later, the curd was transported to permaJournal of Dairy Science Vol. 72, Nø. 5, 1989
nent vessels, where it remalned for 40 d in an environment of 15 to 170C. During this period, the feta was allowed to mature in the liquids secreted by the cheese. On the 40th d, samples of the cheese and the secreted liquids were measured again for radiocesium concentration. All liquids were subsequently removed and replaced by a 7% NaC1 solution in a ratio of 1:2 of brine volume to feta weight. The processing procedure detailed herein is the standard method employed by the large processing plants in northwestern Greece for the manufacture of feta until final storage stage. Radioactivlty Measurements
All measurements of radioactivity concentration were performed at the Nuclear Physics Laboratory, University of Ioannina. Samples were measured in a standard geometry of 400 ml with a 1.0 kilo electron volts (keV) resolution (for the 661.65 keV line of 137Cs), 18% efficiency, intrinsic Ge detector. The detector was shielded with 5 cm of lead against background radiation, which in our area is significant due to radon emanation. Standard electronics were used and the spectra were accumulated in 1024 channels. The analysis was performed in the NPL's Hewlett-Packard HP 1000 computer with a modified version of code ANNA (5). The detector was calibrated for efficiency versus energy through a standard 129.5.kBq.L -1 152Eu source. The time required for the accumulation of a spectrum with adequate statistics ranged from 1000 to 3000 s. In all spectra obtained in this experiment, the only peaks arising from radiocontamination in the samples were due to 134Cs and 137Cs. Several peaks arising from background radiatjon (radon emanation in the area) were also present in the spectra. These peaks did not present any background problem, because all radon-daughter peaks are well separated in the spectra from the cesium isotope peaks. Background spectra were taken every few days in 8h overnight runs with a distilled water phantom in the same geometry, and cesium background radiation was determined as 2 Bq.L -1 for 134Cs and 9 Bq.L -1 for 137Cs. The average ratio of activity concentraUon for the two cesium isotopes in the dairy samples was determined as:
RADIOCESIUM DECONTAMINATION OF FETA CHEESE
1083
TABLE 1. Effects of the amount of culture employedon the transport of radiocesium from milk into the curd and whey during feta cheese making. Measurements on the products were performed immediately after end of drainage. Radiocesium concentration
Percent culture
.2 1.0 1.5 2.0
Milk
690 636 775 711
(Bq/L) m SD 19 21 20 11
Transfer coefficient
Curd m (Bq/kg) ~ X SD 544 19 583 30 574 24 556 19
Whey
Milk-to-curd
(Bq/L) SD 30 21 25 30
X 724 731 792 792
(L/kg) SD .04 .04 .04 .08
X .79 .92 .74 .79
Milk-to-whey X 1.05 1.15 1.02 1.11
SD .04 .03 .03 .04
RESULTS
I (I~Cs) R = ~ = .4 + .3
Invøstig,,tion of the ProducUon Phase Chemiœl Anølysis and Sønsory Evaluation of Chøø~ Fat content of cheese was measured by the Gerber method (6). Total nitrogen was determined by the Kjeldahl method as described by Kosikowski (7). Moisture was determined by the method adopted by the International Dairy Federation (8) and ash according to analytical methods of the Association of Official Analytical Chemists (9). The pH of cheese was also measured. Feta cheese made from uncontaminated milk was subjected after the brine replacements to sensory evaluation by a group of three trained assessors for taste and texture. The raring was based on a seven-point hedonic scale (10).
In order to investigate the transport of radiocesium from ovine milk to the final product in the manufacture of feta cheese, 18 different batches of milk were processed by varying some of the conditions in the standard method described. The factors investigated in particular were 1) the percentage of culture employed, 2) the degree of division of the curd, and 3) the coagulation temperature of the milk. The results of this investigation are summarized in Tables 1 to 3. The values contained in these tables represent averages of several measurements, whereas the associated errors denote 1 SD. The transfer coefficients f (L.Bq-1), also contained in the tables, measure the percentage transfer of radiocontamination and are defined through the ratio:
TABLE 2. Effects of degree of division of the curd on the transport of radiocesium from milk into the curd and whey during feta cheese making. Measurements on the products were performed immediately alter end of drainage. Degree of division (thickness) of slices (cm) 2-3 1-1.5
Radiocesium concentration Milk (BqÆ) ~ SD 19 690 734 22
Curd ~ (Bq/kg) ~ X SD 544 19 658 31
Transfer coefficient Whey
Milk-to-curd
(Bq/L) ~ X SD 724 30 846 31
~ X .79 .90
(I/kg) SD .04 .05
Milk-to-whey X 1.054 1.15
SD .04 .04
Journal of Dalry Science Vol. 72, Nø. 5, 1989
1084
PAPPAS ET AL.
TABLE 3. Effect of milk coagulation temperature on the transport of radiocesium from milk into the curd and whey during feta cheese making. Measurements on the products were performed immediately arier end of drainage. Coagulation
Radiocesium concentration
temperature
Milk
(*C) 30 35 39
Ctu'd
Transfer coefficient Whey
Milk-to-whey
(Bq/L) ~
~
~
(Bo]L) ~
~
(L/kg)
SD
X
SD
X
SD
X
SD
X
SD
18 19 31
713 544 556
28 19 26
868 724 736
46 30 36
.97 .79 .84
.04 .04 .05
1.18 1.05 1.11
.05 .04 .05
734 690 662
f=
(Bq/kg)
Milk-to-curd
Cp C~
[1]
in which Cp and C M are radiocesium concentrations in the product and the original milk, respectively. For the standard cheese making procedure, described in the previous secfion, the milk-to-feta transfer coefficient was determined from 11 separate measurements, as: f = .79 + .04 L.kg -1 Comparison of the data in through a t-test (11) revealed significant differentiation with factors considered here. Indeed,
[2]
Tables 1 to 3 nø statisticaUy respect to the the transport of
radiocesium contaminafion from the original milk into curd and whey was determined from the data in these tables with a ratio of (38 + 6)% to (58 + 5)%, respectively. This subdivision follows closely the corresponding subdivision of moisture (40 to 60%). Because the percentage transport of radiocesium during feta cheese making was practically independent of factors entering the production stage, subsequent experiments focused on processes, which may contribute to the decontamination of the product during storage. InvøstigaUon of the Storage Phase
As analyzed in the Appendix, the transport of radiocesium from feta to brine evolves according to the function: k~Co Pkl + k2 t]} C2(t) = la~ ~-k2 {1 - exp [V2 [3]
i Roo
,o%
Ioo
I
zoo
i
aoo
400
Figure 1. Total radiocesium concentration in brine as a function of time, arising from initial feta radiocesium concentration of 384 + 11 Bq.L -1. The ratio of brine volume to feta weight was la = .5. The continuous curve through the data is the prediction of the two-compartrnent transfer model (see Equation [3]) employed here. Journal of Dairy Science Vol. 72, Nø. 5, 1989
in which Co is the original radiocesium concentration in feta, V2 is the volume of brine, and 19 the ratio of volume of brine to weight of feta; k 1 and k 2 are two parameters, which may be determined from the best tit of the function to experimental data. In one experiment, 5.950 kg of feta cheese with initial radiocesium contamination of Co = 438 +_ 19 Bq.kg -1 were placed in (V2=) 2.975 L of 7% brine, and the brine's content of radiocesium was measured at regular intervals. The parameter p, defined in Equation [A6], was in this case equal to .5. Results of the measurements are contained in Figure 1 and display the temporal evolution predicted by Equation [3]. The best tit of Equation [3] to the data, repre-
RADIOC~IUM DECONTAMINATIONOF PB-rA CHEESE
1085
400
800
,(,,) ø
=~
,(.) 1o
æO0
!
*g i~~~
of ch~.ge~ o~ a~ln~
Figure 3. Percentage reduction of total radiocesium concentration in feta arier successive replacements of the brine in five distinct experiments. The ratio of brine volurne to feta weight was la = .5. The horizontal heavy lines are predictions of the two-compartmenttransfer model (see Equation [A13]) employed here.
I00
t
• NUmbet o f
Œa/~geø o f
Brlae
Figure 2. Total radiocesium concentration in brine arier successive replacements in two distinct experiments. The ratio of brine volume to feta weight was p = .5. The continuous curve through the data are prediction of the two-compartment transfer model (see Equation [AIO]) employed here.
sented by the continuous curve in Figure 1, yielded the values of the free parameters: k 1 = .12 + .04 kg.h -1
[4]
k 2 = .08 + .03 L.h -1.
[5]
Through Equation [A13], these values predict a final radiocesium concentrafion level in feta: Cl~q = (.57 + .27)Co = 250 + 120 Bq.kg -~
[6]
which, within experimental error, agrees with the value 245 __. 25 Bq.kg -1 actually measured after removal of the cheese from the brine. The half-life with which the equilibrium state is reached may also be calculated from the experimentally determined values of k 1 and k2 in Equations [4] and [5] as:
Tit2 = 15 +_ 5 h
[7]
Because equilibrium is effectively reached within 3T1/2, this value predicts attainment of this state in approximately 48 h. In view of these results, it was decided to study the decontamination of mature feta cheese during storage by replacing the brine at regular intervals x > 5TIÆ, which allows maximum transfer of the feta's radiocesium into brine. For this purpose, eight different quantities of feta were produced according to the standard method described previously from contaminated milk measured at 700 Bq-L -1 total radiocesium concentration. Following maturafion of the cheese, each quantity was placed in 7% brine at a rafio 2:1 of weight of cheese per volume of brine. The brine in which each quantity was stored was replaced every 5 d, and at that time, radiocesium concentration in the cheese (three experiments) or the removed liquid (two experiments) was measured. Parameters pertaining to each experiment are summarized in Table 4, while the results of the measurements are contained in Figures 2 and 3. Figure 2 contains actual measurements of brine radiocesium concentraUon after several successive changes of the storage liquids in two experiments. The continuous curves in the figure, which represent predictions of the model Journal of Dairy Science Vol. 72, No. 5, 1989
1086
PAPPAS ET AL.
60
50
Cheese/Brlne = 2:1 ( w t / v o l } :<.:<:
o=
:!!:!:
~
~s.
~Cbeese/Brine
,,cx
= 1:1
(wt/yol)
4o
iii~iiii; iiii::!ii ~!!!!:!!
Æ
iiiiiiii
.::
!i!~!i!l
;
20 i:i:!:!: Iiiiiili
10
iiiiiiiill i!!!!!!!!l åS
5O
Tlae
SO
158
{dl
Figure 4. Effects of sodium chloride brme concentration on the rate of radiocesium decontamination of feta during storage.
i:iill !iii!!
50
~
7~ N a C l
!!!iii
Ø 40 «
iiiii: :i:ii
//
~ g 3o
ililil illili
2o
~--
i:~::: i~!~!!i
--
iiiiii
!i!~!z
!i!iill
!!!!ii
ii!i!!i
••~ilili
:fil!li :::
li/. :: .: :. : 2:1:11
:!!!:?!
::::::
•.:. ;:i!ii
!0
:i~i~ii
::ri:
ililil
~!!!ii
=2=2!i ::::: ::::
i::::: ::::: :::
0
~ ~ 4!5
--
5O
55
TIœe
6O Tiåe
60
{dl
Figure 5. Effects of brine composition (sodium chloride versus potassium chloride) on the rate of radiocesium decontamination of feta during storage.
Journal of Dairy Science Vol. 72, Nø. 5, 1989
(d)
Figure 6. Effects of brine volume on the rate of radiocesium decontamination of feta during storage.
through Equation [A10], account fairly well for the data. In Figure 3, the percent reduction of radiocesium concentration in feta (measured or inferred through measurements of brine) is plotted as a funcuon of the number of changes. Again, predictions of the model, represented by the horizontal heavy line segments, account rather well for the percentage drop measured in all experiments. Several parameters pertaining to compositjon and volume of the brine used during storage were also investigated with regard to decontarnination rates. Figure 4 contains the results of one experiment comparing radiocesium levels in feta as a function of number of changes when brine concentration in NaCI is increased from 7 to 15%. Similar results from a different experiment are presented in Figure 5 where the effect of employing a 7% solution of KCI as storage medium was investigated. Finally, effects of doubling the volume of brine during storage of the feta are presented in Figure 6. As is evident from these results, nø significant changes in decontamination rate is observed through variations in the composition or salt concentration of the brine. The increase
RADIOCESIUM DECONTAMINATION OF FETA CHEESE
~
:åt
1087
o~
o
t'N
g.
IX
t--
,~-
0
'9
~~#.~
¢e~
~1~ oo
~,~.~ [Journal of Dalry Science VoL 72, Nø. 5, 1989
1088
PAPPAS ET AL.
of the storage medium volume, however, observed in Figure 6, follows closely the prediction of the model adopted here (see Equation [A10]).
k 1 = .12 + .02 kg-h -a k 2 = .08 -t- .01 L.h -~
[8] [9]
which are adopted in the computation of model predictions throughout this paper.
Transport Pammlers kI and k2
Parameters kt and k 2, which govem the transport of cesium from feta to brine in Equation [AS], were determined from a single experiment (see Figure 1) in which the continuous build-up of radiocesium concentration in the storage medium was monitored. This resulted in the values of Equations [4] and [5], which involve a considerable (almost 30%) experimental error. Subsequent experiments, however, yielded results that permit a more accurate deterrnination of these parameters. As it may be easily shown from Equations [A10] and [A13], the fractional radiocesium contamination expected in feta or brine after N successive changes of the storage medium may be expressed, respectively, through the expressions:
Co
[61
and:
c-S- = ~
[71
These expressions were used to tit the data in Table 5, yielding the values:
Efføcts of Brinø Replacements on the Compo~tion and T u t e of Chøøu
Any process aiming to improve a parUcular parameter of cheese should also examine the final product for alterations in composiUon and organoleptic properties. For these purposes, feta cheese was made as previously described from contamination-free milk. On the 40th d after preparation, the brine (7% in NaC1) was replaeed every 3 d. Arier each replacement, the fat, protein, ash, moisture content, and pH of the resulting product were determined. Finally, cheese wa subjected to sensory evaluation by a group of three assessors. Results of the compositional analysis and sensory evaluation are presented in Table 6. Comparison of the data in Table 6 shows that the fat and ash contents of feta cheese were not affected by the replacements of the brine. Protein content was slightly decreased, probably due to losses of soluble nitrogen products. Moisture was slightly increased, whereas pH was increased, apparently due to the removal of lactic acid. A minor decrease of the organoleptie properties was observed. This was described by the assessors as the loss of the pleasant sour taste, (confirmed by the observed increase of
TABLE 5. Time interval (ha days) bctwecn changcs of the brine for maximum transfer of radiocesium from feta to the storagc medium. The time intervals correspond to 5Tla, as calculated through Equation [A12]. They are given as a function of the brine volume to feta weight ratio (p) and the mass of feta (M0. P Ml
.20
.30
.40
.50
.60
.70
.80
.90
1
2 3 4 5 6 7
.6 .8 1.1 1.4 1.7 1.9 2.2 2.5 2.8 3.1 3.3
.7 1.1 1.5 1.9 2.2 2.6 3.0 3.4 3.7 4.1 4.5
.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 5.0 5.4
1.0 1.5 2.1 2.6 3.1 3.6 4.1 4.6 5.2 5.7 6.2
1.1 1.7 2.3 2.8 3.4 4.0 4.6 5.1 5.7 6.3 6.8
1.2 1.8 2.5 3.1 3.7 4.3 4.9 5.5 6.2 6.8 7.4
1.3 2.0 2.6 3.3 3.9 4.6 5.3 5.9 6.6 7.2 7.9
1.4 2.1 2.8 3.5 4.1 4.8 5.5 6.2 6.8 7.6 8.3
1.4 2.2 2.9 3.6 4.3 5.1 5.8 6.5 7.2 7.9 8.7
8
9 10 11 12
Journal of Dalry Science Vol. 72, Nø. 5, 1989
RADIOCESIUM DECONTAMINATIONOF FETA CHEESE pil) traditionally preferred by consumers in Greece. However, in all cases the product was found acceptable. DISCUSSION AND CONCLUSlONS
With the exception of some isolated studies (12, 13, 14) during the mid-1960's, very little information existed prior to 1986 about the transfer of radiocontamination during processing of milk into products. More recent studies following the nuclear accident at Chernobyl have addressed this problem, and some data have been published conceming transfer coefficients to Gruyère cheese made from sheep milk in Greece (2, 3) or to several commercial dairy products from the processing of bovine milk in the United Kingdom (4). It is concluded in all these studies that cesium, like the rest of the alkali metals, remains in the water phase and thus follows very closely the subåivision of moisture into the various products of cheese making. This is in contrast to strontium, which closely follows the behavior of calcium and in milk is bonded to caseins. It was therefore with nø surprise that we discovered through this research that attempts to influence the transfer of radiocesium into feta cheese through variations in the cheese making procedure were not successful. The problem of removing radiocontaminants from milk and dairy products has been the subject of an early investigation in the Netherlands by de Ruig (14). That research focuses mainly on the removal of radiostrontium from dairy products, and a procedure is given for the manufacture of a low strontium cheese similar to Gouda. Alternatively, ion exchange techniques and electrodialysis are proposed for direct decontamination of milk. However, as is pointed out by de Ruig (9) although these techniques may be well-known in factories making dietary products (e.g., desahed milk), most dairy plants do not possess suitable equipment. The technique explored in the research presented here for the removal of radiocesium from feta cheese during storage involves periodic replacements of the storage medium (brine). This procedure should be easily applicable, although some problems may arise from the disposal of the removed brine. The theory leading to the proposed method is presented in detail in
1089
0 M
¢q
t ' q ee~ ~ "
t-4 cq"cq"~ ¢~ II
@.
å .~~.~
e~
~. ~. ~ oq.~
8. ..-:,
"i::
r~
¢-.i ¢,q t ' q ¢,';
'
Ø
0
g [..
Journal of Dairy Science Vol. 72, Nø. 5, 1989
1090
PAPPAS ET AI+.
~ 1ooI
.g n©
1
8 .~~,
o~ c å a . « + s
lO in
a[lne
Figure 7. Nomograms for percentage radiocesium concentration reduction in feta cheese through successive replacements of brine during storage, for several values on the ratio p of storage medium to cheese.
the Appendix by means of a simple two-compamnent model, simulating the diffusion of an ideal gas through a bidirectional membrane. Predictions of this model are satisfactorily validated by the data obtained in the experiments conducted during this research. They may thus be used as guidelines by processing plants for the treatment of mature feta cheese, prior to distribution, in the event of milk radiocesium contamination. In this context, Table 5 supplies the time intervals (5T1/2), in terms of Equation [A12]), which should be adequate for maximum radiocesium transport from stored feta to brine. As indicated in Equation [A12], this is given as a function of the mass M t of the feta stored and the ratio of p of storage medium to cheese. Similarly, predictions of the model for percentage reduction of radiocesium concentration in feta as a function of the number of changes of brine dm-ing the storage phase, are given in the nomograms of Figure 7. REFERENCES 1 Andritsopoulos, G., P. A. Assimakopoulos, K. G. Ioannides, and A. A. Pakou. 1987. Surface soil fallout measurements in northwestern Greece following the reactor accident at Chernobyl. Proc. Conf. on the Effects of the Chemobyl Accident in Greece. Greek Atomic Energy Commiss., Athens, Greece. 2 Kandarakis, I. G., and E. M. Anyfantakis. 1986. Transport of t31I, ~~Cs and 137Csfrom ovine milk into dairy products. Bull. Hell. Milk. Commiss. 3:20. 3 Assimakopoulos, P. A., K. G. Ioannides, A. A. Pakou, and C. V. Papadopoulou. 1987. Transport of the radioisotopes iodine-131, cesium-134 and cesium-137 from the fallout following the accident at the Chernobyl nuclear Journal of Dairy Science Vol. 72, Nø. 5, 1989
reactor into cheese and other cheesemaking products. J. Dalry Sci. 70:1338. 4 Wilson, L. G., R. C. Bottomley, P. M. Sutton, and C. H. Sisk. 1988. Transfer of radioactive contamination from milk to commercial dairy products. J. Soc. Dairy Technol. 41:10. 5Assimakopoulos, P. A., and S. Kossionides. 1975. ANNA-An interactive program for one-dimensional pulse-height spectra. Comp. Phys. Commun. 11:37. 6 British Standards Institution. 1955. Gerber method for the determination of fat in milk and milk products. B.S. 696, London, Engl. 7 Kosikowski, F. 1966. Cheese and fermented milk products. F. V. Kosikowski and Assoc., Brooktondale, NY. 8 International Dairy Federation. 1958. Determination of dry matter ha cheese and processed cheese. Int. Stand. FIL-IDF 4, IDF, Brussels, Belgium. 9 Association of Official Analytical Chemists. 1984. Official methods of analysis. 4th ed. Arlington, VA. 10 Kramer, A., and B. A. Twigg. 1970. Quality control for the food industry. 3rd ed. Vol. 1. AV1, Westport, CT. 11 Bryant, E. C. 1966. StaUstical analysis. McGraw+Hill, New York, NY. 12 Lengemann, F. W. 1962. Distribution of radioslxontium and radiocesium in milk and milk preducts. J. Dalry Sci. 45:538. 13 Lagoni, H., O. Paakkola, and K. H. Peters. 1963. Untersuchungen uber die quantitative verteihmg radioaktiver falloutprodukte in milch. Milchwissenschaft 18:340. 14 de Ruig, W. G. 1966. Prospects for the manufacture of cheese tit for human consumption from milk contaminated with radioactive nuclear fission products. Neth. Milk Dairy J. 20:283.
APPENDIX Radiocesium Transport from Feta to Brine
The transport of radiocesium contained in the curd to the surrounding brine may be described in the framework of a simple twocompartment model, depicted schematically in Figure A1. We shall denote with N k and Ck, k = 1, 2, the number of radiocesium nuclei, and the radiocesium concentradon in each compartment, respectively. Because C1 is measured in becquerels per kilogram and C2 in becquerels per liter, they are related to the corresponding populations through the equations: Ni = C1MI
[A1]
N 2 = C2V 2
[A2]
in which M 1 and V 2 represent mass and volume
1091
RADIOCESIUM DECONTAMINATION OF FETA CHEESE ....-. . ~..: "..." ....,~,-.o;...~.• 9..o~« .,b~«.l.
~d
æl
~
' œl ' %
Equation [A5] may be written as:
t IM2 ø C 2
C] + pC2 = Co
[A7]
In view of the last relation, Equation [A4] may be written in the final form: ~l
~ --;'.'.. .-. .¢,-...-_ . . . .-:
o.: .
-..k°:@.
»ø., o - .. • , O..å~~fd
ø-, ø-~. ... ø(C..t
~
".o.ri
dC2
V2-'~" + (Pkl + k2)C2 = k l C o
[A8]
, OØ
Figure AI. A simple two-compartment model employed in the analysis of radiocesium transfer from feta to brine.
If we assume that at time t = 0 all the radiocesium nuclei are in compartment 1 (the curd), then by direct integration or other methods Equation [A8] yields the solution:
of the corresponding compartments. Then the rate of transport of radiocesium nuclei from compartment 1 (curd) to compartment 2 (brine) is proportional to the difference of concentrations in the two compartments
klC0 Pkl + k 2 C2(t) = p ~ + k2 {1 - exp [ ~ t]}[A9 ]
dN 2
d--i- = klC1 - k 2 C 2
[A3]
in which kl (kg'h -1) and k2 (L-h-1) are constants of proportionality. This may be seen as analogous to the diffusion of an ideal gas through a membrane separating two vessels, when the impedance of the membrane is different in the two directions of flow; the net rate of flow is then proportional to the pressure difference across the membrane. From the relation in Equation [A2], this differential equation may be written entirely in terms of acfivity conservatjon as: dC 2
V2 - ~
= ksC1 - k2C2
lA4]
Because the total number of radiocesium nuclei No = N1 + N2 in the two compartments remains constant, we may write the conversion relation: M1C1 + V 2 C 2 = M1C0
[A5]
in which Co is the initial activity concentration in the curd. If we further introduce the parameter: V2
13 = M---~
The last expression is a function of two free parameters k 1 and k2, which may be determined through the best tit to experimental data. The function predicts an asymptotic equilibrium value of radiocesium concentration in the brine: klC0 1"2eq ---- Pkl + k2 f~
[A10]
which, as far as geometry is concerned, depends only on the ratio p, defined in Equation [A6]. It is noted in contrast that the characteristic half-life with which this equilibrium state is reached: .693V2 T1/'2 = Pkl + k2
[All]
or, in terms of the mass of the feta involved: •693pM 1 T1/2 = Ok1 +k2
[Ål2]
depends in addition on the quantity of the stored product. Equation [A10] finally permits the calculauon of radiocesium concentration remaining in the feta after equilibrium with the brine has been reached. In terms of fractional reduction it may be easily shown that: Cleq
k2
Co
Pkl + k2
[A13]
[A63
Journal of Dairy Science Vol. 72, Nø. 5, 1989
FOODS
RESEARCH
PAPERS
P r o c e a a e s t h a t C o n t r i b u t e to R a d i o c e s i u m Decontamination of F e t a C h e e s e
C. P. PAPPAS Dairy Research Institute of Ioannina 454 44 Ioannina, Greece
P. A. ASSIMAKOPOULOS, K. G. IOANNIDES, and A. A. PAKOU Nuclear Physics Laboratory The University of Ioannina, 451 10 Ioannina, Greece
A. S. MAN'I-ZIOS Agricultural Research StatJon of Ioannina PO Box 1103 451 10 Ioannina, Greece
ABffl'RAG~r In a series of experiments, the transfer of radiocesium from ovine milk to feta cheese was invesdgated through modifications of the standard cheese making procedure. All variations explored showed nø significant change in the percentage of radiocesium transfer and the milk-to-cheese transfer coefficient was determined as f = .79 + .04 L.kg -1. It is concluded that cesium, like the rest of the alkali metals, remains in the water phase and thus follows very closely the distribunon of moisture into the products of cheese making. The possibility of radiocesium decontaminafion of mature feta during the customary storage of the product in brine was also explored in a second series of experiments. The theoretical model employed in the analysis of cesium transport from feta to brine is presented in the Appendix to this paper. Predictions of the model were validated by experiments. A procedure is thus proposed for decontaminating mature feta during storage through successive replacements of the storage medium. Nomograms are presented for the determination of the opfimum time interval between changes of the brine and the radiocesium concentration remaining in the feta. Changes in the Received June 2, 1988. Accepted October 14, 1988. 1989 J Dairy Sei 72:1081-1091
properties of the product induced by the proposed treatment were also investigated with respect to composition, taste, and overall quality. INTRODUCTION
The radioactive plume from the nuclear accident at Chernobyl on April 26, 1986 resulted in serious contaminadon of pastures in northwestern Greece. Radionuclides in the fallout soon penetrated into the milk of lactating animals--especially free-grazing sheep and goats. During May 1986 levels reaching 18,000 Bq.L -1 for 131I and 5000 Bq.L-1 for total radiocesium concentration were measured (1). All ovine milk produced in Greece during May and June 1986 was withheld from immediate consumption and was processed into (prima.rily) feta cheese. This eliminated the problem arising from the short-lived isotope 131I (T1/2 = 8.05 d), because any activity from this source was expected to become negligible during the maturation period of the cheese (2 mo, according to Greek regulations). A serious problem remained, however, with the long-lived isotopes 134Cs (T1/2 = 2.06 y) and 137Cs (T1Æ = 30.1 y). Transport of these isotopes during the cheese making process into the final product has recently been the subject of several studies (2, 3, 4). The object of the research presented here was to investigate possible processes, either during cheese making, or later, which may contribute to the reduction of radiocesium concentration in feta cheese. 1081
1082
PAPPAS ET AI.,. MATERIALS AND METHODS
Milk Radiocontamination
For the purposes of the research presented here, 20 ewes of the Agricultural Research Station of Ioannina were segregated from the herd and held for 2 mo in an isolated pen. The animals were allowed to feed ad libitum on hay (Medicago sativa) that was harvested at Yannitsa (in central Macedonia) during late spring 1986 and was thus strongly affected by the Chernobyl fallout. Total radiocesium concentration in the animals' feed was measured as 9000 Bq.kg -1, and the average daily consumption of hay per animal was determined as 1.2 kg. The daily diet of the animals contained in addition .5 kg of a mixture of corn and barley with average radiocesium concentration of 50 Bq.kg -1. The evolution of radiocesium contamination of the animals' mill( was monitored through daily measurements at the Nuclear Physics Laboratory of the University of Ioannina, which determined that contamination reached an equilibrium level of approximately 700 Bq.L -1 within 2 wk. Following this, the daily production of milk was transported to the laboratories of the Dairy Research Institute of Ioannina, where it was processed into feta cheese. Cheese Msking
Each batch of milk was pasteurized at 63 to 65°C for 20 min. Starter culture (Streptococcus thermophilus and Lactobacillus bulgaricus) at .2% and liquid (Hansens) rennet were added so that coagulation was achieved in 42 to 45 min at 35°C. Following coagulation, the curd was cut into blocks of 2 to 3-cm thickness and was allowed to rest for l0 min. The sliced curd and the whey were then placed in molds for drainage. Weight of the curd and volume of the whey were measured 6 h alter placement in the molds (end of the drainage period). At the same time, samples of both the curd and whey were measured for radiocesium concentration. The curd was then sliced into 11 x 12 x 6-cm blocks and was placed temporarily in open vessels for dry salting with the addition of 3.6% of NaCl by weight of the curd. Three days later, the curd was transported to permaJournal of Dairy Science Vol. 72, Nø. 5, 1989
nent vessels, where it remalned for 40 d in an environment of 15 to 170C. During this period, the feta was allowed to mature in the liquids secreted by the cheese. On the 40th d, samples of the cheese and the secreted liquids were measured again for radiocesium concentration. All liquids were subsequently removed and replaced by a 7% NaC1 solution in a ratio of 1:2 of brine volume to feta weight. The processing procedure detailed herein is the standard method employed by the large processing plants in northwestern Greece for the manufacture of feta until final storage stage. Radioactivlty Measurements
All measurements of radioactivity concentration were performed at the Nuclear Physics Laboratory, University of Ioannina. Samples were measured in a standard geometry of 400 ml with a 1.0 kilo electron volts (keV) resolution (for the 661.65 keV line of 137Cs), 18% efficiency, intrinsic Ge detector. The detector was shielded with 5 cm of lead against background radiation, which in our area is significant due to radon emanation. Standard electronics were used and the spectra were accumulated in 1024 channels. The analysis was performed in the NPL's Hewlett-Packard HP 1000 computer with a modified version of code ANNA (5). The detector was calibrated for efficiency versus energy through a standard 129.5.kBq.L -1 152Eu source. The time required for the accumulation of a spectrum with adequate statistics ranged from 1000 to 3000 s. In all spectra obtained in this experiment, the only peaks arising from radiocontamination in the samples were due to 134Cs and 137Cs. Several peaks arising from background radiatjon (radon emanation in the area) were also present in the spectra. These peaks did not present any background problem, because all radon-daughter peaks are well separated in the spectra from the cesium isotope peaks. Background spectra were taken every few days in 8h overnight runs with a distilled water phantom in the same geometry, and cesium background radiation was determined as 2 Bq.L -1 for 134Cs and 9 Bq.L -1 for 137Cs. The average ratio of activity concentraUon for the two cesium isotopes in the dairy samples was determined as:
RADIOCESIUM DECONTAMINATION OF FETA CHEESE
1083
TABLE 1. Effects of the amount of culture employedon the transport of radiocesium from milk into the curd and whey during feta cheese making. Measurements on the products were performed immediately after end of drainage. Radiocesium concentration
Percent culture
.2 1.0 1.5 2.0
Milk
690 636 775 711
(Bq/L) m SD 19 21 20 11
Transfer coefficient
Curd m (Bq/kg) ~ X SD 544 19 583 30 574 24 556 19
Whey
Milk-to-curd
(Bq/L) SD 30 21 25 30
X 724 731 792 792
(L/kg) SD .04 .04 .04 .08
X .79 .92 .74 .79
Milk-to-whey X 1.05 1.15 1.02 1.11
SD .04 .03 .03 .04
RESULTS
I (I~Cs) R = ~ = .4 + .3
Invøstig,,tion of the ProducUon Phase Chemiœl Anølysis and Sønsory Evaluation of Chøø~ Fat content of cheese was measured by the Gerber method (6). Total nitrogen was determined by the Kjeldahl method as described by Kosikowski (7). Moisture was determined by the method adopted by the International Dairy Federation (8) and ash according to analytical methods of the Association of Official Analytical Chemists (9). The pH of cheese was also measured. Feta cheese made from uncontaminated milk was subjected after the brine replacements to sensory evaluation by a group of three trained assessors for taste and texture. The raring was based on a seven-point hedonic scale (10).
In order to investigate the transport of radiocesium from ovine milk to the final product in the manufacture of feta cheese, 18 different batches of milk were processed by varying some of the conditions in the standard method described. The factors investigated in particular were 1) the percentage of culture employed, 2) the degree of division of the curd, and 3) the coagulation temperature of the milk. The results of this investigation are summarized in Tables 1 to 3. The values contained in these tables represent averages of several measurements, whereas the associated errors denote 1 SD. The transfer coefficients f (L.Bq-1), also contained in the tables, measure the percentage transfer of radiocontamination and are defined through the ratio:
TABLE 2. Effects of degree of division of the curd on the transport of radiocesium from milk into the curd and whey during feta cheese making. Measurements on the products were performed immediately alter end of drainage. Degree of division (thickness) of slices (cm) 2-3 1-1.5
Radiocesium concentration Milk (BqÆ) ~ SD 19 690 734 22
Curd ~ (Bq/kg) ~ X SD 544 19 658 31
Transfer coefficient Whey
Milk-to-curd
(Bq/L) ~ X SD 724 30 846 31
~ X .79 .90
(I/kg) SD .04 .05
Milk-to-whey X 1.054 1.15
SD .04 .04
Journal of Dalry Science Vol. 72, Nø. 5, 1989
1084
PAPPAS ET AL.
TABLE 3. Effect of milk coagulation temperature on the transport of radiocesium from milk into the curd and whey during feta cheese making. Measurements on the products were performed immediately arier end of drainage. Coagulation
Radiocesium concentration
temperature
Milk
(*C) 30 35 39
Ctu'd
Transfer coefficient Whey
Milk-to-whey
(Bq/L) ~
~
~
(Bo]L) ~
~
(L/kg)
SD
X
SD
X
SD
X
SD
X
SD
18 19 31
713 544 556
28 19 26
868 724 736
46 30 36
.97 .79 .84
.04 .04 .05
1.18 1.05 1.11
.05 .04 .05
734 690 662
f=
(Bq/kg)
Milk-to-curd
Cp C~
[1]
in which Cp and C M are radiocesium concentrations in the product and the original milk, respectively. For the standard cheese making procedure, described in the previous secfion, the milk-to-feta transfer coefficient was determined from 11 separate measurements, as: f = .79 + .04 L.kg -1 Comparison of the data in through a t-test (11) revealed significant differentiation with factors considered here. Indeed,
[2]
Tables 1 to 3 nø statisticaUy respect to the the transport of
radiocesium contaminafion from the original milk into curd and whey was determined from the data in these tables with a ratio of (38 + 6)% to (58 + 5)%, respectively. This subdivision follows closely the corresponding subdivision of moisture (40 to 60%). Because the percentage transport of radiocesium during feta cheese making was practically independent of factors entering the production stage, subsequent experiments focused on processes, which may contribute to the decontamination of the product during storage. InvøstigaUon of the Storage Phase
As analyzed in the Appendix, the transport of radiocesium from feta to brine evolves according to the function: k~Co Pkl + k2 t]} C2(t) = la~ ~-k2 {1 - exp [V2 [3]
i Roo
,o%
Ioo
I
zoo
i
aoo
400
Figure 1. Total radiocesium concentration in brine as a function of time, arising from initial feta radiocesium concentration of 384 + 11 Bq.L -1. The ratio of brine volume to feta weight was la = .5. The continuous curve through the data is the prediction of the two-compartrnent transfer model (see Equation [3]) employed here. Journal of Dairy Science Vol. 72, Nø. 5, 1989
in which Co is the original radiocesium concentration in feta, V2 is the volume of brine, and 19 the ratio of volume of brine to weight of feta; k 1 and k 2 are two parameters, which may be determined from the best tit of the function to experimental data. In one experiment, 5.950 kg of feta cheese with initial radiocesium contamination of Co = 438 +_ 19 Bq.kg -1 were placed in (V2=) 2.975 L of 7% brine, and the brine's content of radiocesium was measured at regular intervals. The parameter p, defined in Equation [A6], was in this case equal to .5. Results of the measurements are contained in Figure 1 and display the temporal evolution predicted by Equation [3]. The best tit of Equation [3] to the data, repre-
RADIOC~IUM DECONTAMINATIONOF PB-rA CHEESE
1085
400
800
,(,,) ø
=~
,(.) 1o
æO0
!
*g i~~~
of ch~.ge~ o~ a~ln~
Figure 3. Percentage reduction of total radiocesium concentration in feta arier successive replacements of the brine in five distinct experiments. The ratio of brine volurne to feta weight was la = .5. The horizontal heavy lines are predictions of the two-compartmenttransfer model (see Equation [A13]) employed here.
I00
t
• NUmbet o f
Œa/~geø o f
Brlae
Figure 2. Total radiocesium concentration in brine arier successive replacements in two distinct experiments. The ratio of brine volume to feta weight was p = .5. The continuous curve through the data are prediction of the two-compartment transfer model (see Equation [AIO]) employed here.
sented by the continuous curve in Figure 1, yielded the values of the free parameters: k 1 = .12 + .04 kg.h -1
[4]
k 2 = .08 + .03 L.h -1.
[5]
Through Equation [A13], these values predict a final radiocesium concentrafion level in feta: Cl~q = (.57 + .27)Co = 250 + 120 Bq.kg -~
[6]
which, within experimental error, agrees with the value 245 __. 25 Bq.kg -1 actually measured after removal of the cheese from the brine. The half-life with which the equilibrium state is reached may also be calculated from the experimentally determined values of k 1 and k2 in Equations [4] and [5] as:
Tit2 = 15 +_ 5 h
[7]
Because equilibrium is effectively reached within 3T1/2, this value predicts attainment of this state in approximately 48 h. In view of these results, it was decided to study the decontamination of mature feta cheese during storage by replacing the brine at regular intervals x > 5TIÆ, which allows maximum transfer of the feta's radiocesium into brine. For this purpose, eight different quantities of feta were produced according to the standard method described previously from contaminated milk measured at 700 Bq-L -1 total radiocesium concentration. Following maturafion of the cheese, each quantity was placed in 7% brine at a rafio 2:1 of weight of cheese per volume of brine. The brine in which each quantity was stored was replaced every 5 d, and at that time, radiocesium concentration in the cheese (three experiments) or the removed liquid (two experiments) was measured. Parameters pertaining to each experiment are summarized in Table 4, while the results of the measurements are contained in Figures 2 and 3. Figure 2 contains actual measurements of brine radiocesium concentraUon after several successive changes of the storage liquids in two experiments. The continuous curves in the figure, which represent predictions of the model Journal of Dairy Science Vol. 72, No. 5, 1989
1086
PAPPAS ET AL.
60
50
Cheese/Brlne = 2:1 ( w t / v o l } :<.:<:
o=
:!!:!:
~
~s.
~Cbeese/Brine
,,cx
= 1:1
(wt/yol)
4o
iii~iiii; iiii::!ii ~!!!!:!!
Æ
iiiiiiii
.::
!i!~!i!l
;
20 i:i:!:!: Iiiiiili
10
iiiiiiiill i!!!!!!!!l åS
5O
Tlae
SO
158
{dl
Figure 4. Effects of sodium chloride brme concentration on the rate of radiocesium decontamination of feta during storage.
i:iill !iii!!
50
~
7~ N a C l
!!!iii
Ø 40 «
iiiii: :i:ii
//
~ g 3o
ililil illili
2o
~--
i:~::: i~!~!!i
--
iiiiii
!i!~!z
!i!iill
!!!!ii
ii!i!!i
••~ilili
:fil!li :::
li/. :: .: :. : 2:1:11
:!!!:?!
::::::
•.:. ;:i!ii
!0
:i~i~ii
::ri:
ililil
~!!!ii
=2=2!i ::::: ::::
i::::: ::::: :::
0
~ ~ 4!5
--
5O
55
TIœe
6O Tiåe
60
{dl
Figure 5. Effects of brine composition (sodium chloride versus potassium chloride) on the rate of radiocesium decontamination of feta during storage.
Journal of Dairy Science Vol. 72, Nø. 5, 1989
(d)
Figure 6. Effects of brine volume on the rate of radiocesium decontamination of feta during storage.
through Equation [A10], account fairly well for the data. In Figure 3, the percent reduction of radiocesium concentration in feta (measured or inferred through measurements of brine) is plotted as a funcuon of the number of changes. Again, predictions of the model, represented by the horizontal heavy line segments, account rather well for the percentage drop measured in all experiments. Several parameters pertaining to compositjon and volume of the brine used during storage were also investigated with regard to decontarnination rates. Figure 4 contains the results of one experiment comparing radiocesium levels in feta as a function of number of changes when brine concentration in NaCI is increased from 7 to 15%. Similar results from a different experiment are presented in Figure 5 where the effect of employing a 7% solution of KCI as storage medium was investigated. Finally, effects of doubling the volume of brine during storage of the feta are presented in Figure 6. As is evident from these results, nø significant changes in decontamination rate is observed through variations in the composition or salt concentration of the brine. The increase
RADIOCESIUM DECONTAMINATION OF FETA CHEESE
~
:åt
1087
o~
o
t'N
g.
IX
t--
,~-
0
'9
~~#.~
¢e~
~1~ oo
~,~.~ [Journal of Dalry Science VoL 72, Nø. 5, 1989
1088
PAPPAS ET AL.
of the storage medium volume, however, observed in Figure 6, follows closely the prediction of the model adopted here (see Equation [A10]).
k 1 = .12 + .02 kg-h -a k 2 = .08 -t- .01 L.h -~
[8] [9]
which are adopted in the computation of model predictions throughout this paper.
Transport Pammlers kI and k2
Parameters kt and k 2, which govem the transport of cesium from feta to brine in Equation [AS], were determined from a single experiment (see Figure 1) in which the continuous build-up of radiocesium concentration in the storage medium was monitored. This resulted in the values of Equations [4] and [5], which involve a considerable (almost 30%) experimental error. Subsequent experiments, however, yielded results that permit a more accurate deterrnination of these parameters. As it may be easily shown from Equations [A10] and [A13], the fractional radiocesium contamination expected in feta or brine after N successive changes of the storage medium may be expressed, respectively, through the expressions:
Co
[61
and:
c-S- = ~
[71
These expressions were used to tit the data in Table 5, yielding the values:
Efføcts of Brinø Replacements on the Compo~tion and T u t e of Chøøu
Any process aiming to improve a parUcular parameter of cheese should also examine the final product for alterations in composiUon and organoleptic properties. For these purposes, feta cheese was made as previously described from contamination-free milk. On the 40th d after preparation, the brine (7% in NaC1) was replaeed every 3 d. Arier each replacement, the fat, protein, ash, moisture content, and pH of the resulting product were determined. Finally, cheese wa subjected to sensory evaluation by a group of three assessors. Results of the compositional analysis and sensory evaluation are presented in Table 6. Comparison of the data in Table 6 shows that the fat and ash contents of feta cheese were not affected by the replacements of the brine. Protein content was slightly decreased, probably due to losses of soluble nitrogen products. Moisture was slightly increased, whereas pH was increased, apparently due to the removal of lactic acid. A minor decrease of the organoleptie properties was observed. This was described by the assessors as the loss of the pleasant sour taste, (confirmed by the observed increase of
TABLE 5. Time interval (ha days) bctwecn changcs of the brine for maximum transfer of radiocesium from feta to the storagc medium. The time intervals correspond to 5Tla, as calculated through Equation [A12]. They are given as a function of the brine volume to feta weight ratio (p) and the mass of feta (M0. P Ml
.20
.30
.40
.50
.60
.70
.80
.90
1
2 3 4 5 6 7
.6 .8 1.1 1.4 1.7 1.9 2.2 2.5 2.8 3.1 3.3
.7 1.1 1.5 1.9 2.2 2.6 3.0 3.4 3.7 4.1 4.5
.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 5.0 5.4
1.0 1.5 2.1 2.6 3.1 3.6 4.1 4.6 5.2 5.7 6.2
1.1 1.7 2.3 2.8 3.4 4.0 4.6 5.1 5.7 6.3 6.8
1.2 1.8 2.5 3.1 3.7 4.3 4.9 5.5 6.2 6.8 7.4
1.3 2.0 2.6 3.3 3.9 4.6 5.3 5.9 6.6 7.2 7.9
1.4 2.1 2.8 3.5 4.1 4.8 5.5 6.2 6.8 7.6 8.3
1.4 2.2 2.9 3.6 4.3 5.1 5.8 6.5 7.2 7.9 8.7
8
9 10 11 12
Journal of Dalry Science Vol. 72, Nø. 5, 1989
RADIOCESIUM DECONTAMINATIONOF FETA CHEESE pil) traditionally preferred by consumers in Greece. However, in all cases the product was found acceptable. DISCUSSION AND CONCLUSlONS
With the exception of some isolated studies (12, 13, 14) during the mid-1960's, very little information existed prior to 1986 about the transfer of radiocontamination during processing of milk into products. More recent studies following the nuclear accident at Chernobyl have addressed this problem, and some data have been published conceming transfer coefficients to Gruyère cheese made from sheep milk in Greece (2, 3) or to several commercial dairy products from the processing of bovine milk in the United Kingdom (4). It is concluded in all these studies that cesium, like the rest of the alkali metals, remains in the water phase and thus follows very closely the subåivision of moisture into the various products of cheese making. This is in contrast to strontium, which closely follows the behavior of calcium and in milk is bonded to caseins. It was therefore with nø surprise that we discovered through this research that attempts to influence the transfer of radiocesium into feta cheese through variations in the cheese making procedure were not successful. The problem of removing radiocontaminants from milk and dairy products has been the subject of an early investigation in the Netherlands by de Ruig (14). That research focuses mainly on the removal of radiostrontium from dairy products, and a procedure is given for the manufacture of a low strontium cheese similar to Gouda. Alternatively, ion exchange techniques and electrodialysis are proposed for direct decontamination of milk. However, as is pointed out by de Ruig (9) although these techniques may be well-known in factories making dietary products (e.g., desahed milk), most dairy plants do not possess suitable equipment. The technique explored in the research presented here for the removal of radiocesium from feta cheese during storage involves periodic replacements of the storage medium (brine). This procedure should be easily applicable, although some problems may arise from the disposal of the removed brine. The theory leading to the proposed method is presented in detail in
1089
0 M
¢q
t ' q ee~ ~ "
t-4 cq"cq"~ ¢~ II
@.
å .~~.~
e~
~. ~. ~ oq.~
8. ..-:,
"i::
r~
¢-.i ¢,q t ' q ¢,';
'
Ø
0
g [..
Journal of Dairy Science Vol. 72, Nø. 5, 1989
1090
PAPPAS ET AI+.
~ 1ooI
.g n©
1
8 .~~,
o~ c å a . « + s
lO in
a[lne
Figure 7. Nomograms for percentage radiocesium concentration reduction in feta cheese through successive replacements of brine during storage, for several values on the ratio p of storage medium to cheese.
the Appendix by means of a simple two-compamnent model, simulating the diffusion of an ideal gas through a bidirectional membrane. Predictions of this model are satisfactorily validated by the data obtained in the experiments conducted during this research. They may thus be used as guidelines by processing plants for the treatment of mature feta cheese, prior to distribution, in the event of milk radiocesium contamination. In this context, Table 5 supplies the time intervals (5T1/2), in terms of Equation [A12]), which should be adequate for maximum radiocesium transport from stored feta to brine. As indicated in Equation [A12], this is given as a function of the mass M t of the feta stored and the ratio of p of storage medium to cheese. Similarly, predictions of the model for percentage reduction of radiocesium concentration in feta as a function of the number of changes of brine dm-ing the storage phase, are given in the nomograms of Figure 7. REFERENCES 1 Andritsopoulos, G., P. A. Assimakopoulos, K. G. Ioannides, and A. A. Pakou. 1987. Surface soil fallout measurements in northwestern Greece following the reactor accident at Chernobyl. Proc. Conf. on the Effects of the Chemobyl Accident in Greece. Greek Atomic Energy Commiss., Athens, Greece. 2 Kandarakis, I. G., and E. M. Anyfantakis. 1986. Transport of t31I, ~~Cs and 137Csfrom ovine milk into dairy products. Bull. Hell. Milk. Commiss. 3:20. 3 Assimakopoulos, P. A., K. G. Ioannides, A. A. Pakou, and C. V. Papadopoulou. 1987. Transport of the radioisotopes iodine-131, cesium-134 and cesium-137 from the fallout following the accident at the Chernobyl nuclear Journal of Dairy Science Vol. 72, Nø. 5, 1989
reactor into cheese and other cheesemaking products. J. Dalry Sci. 70:1338. 4 Wilson, L. G., R. C. Bottomley, P. M. Sutton, and C. H. Sisk. 1988. Transfer of radioactive contamination from milk to commercial dairy products. J. Soc. Dairy Technol. 41:10. 5Assimakopoulos, P. A., and S. Kossionides. 1975. ANNA-An interactive program for one-dimensional pulse-height spectra. Comp. Phys. Commun. 11:37. 6 British Standards Institution. 1955. Gerber method for the determination of fat in milk and milk products. B.S. 696, London, Engl. 7 Kosikowski, F. 1966. Cheese and fermented milk products. F. V. Kosikowski and Assoc., Brooktondale, NY. 8 International Dairy Federation. 1958. Determination of dry matter ha cheese and processed cheese. Int. Stand. FIL-IDF 4, IDF, Brussels, Belgium. 9 Association of Official Analytical Chemists. 1984. Official methods of analysis. 4th ed. Arlington, VA. 10 Kramer, A., and B. A. Twigg. 1970. Quality control for the food industry. 3rd ed. Vol. 1. AV1, Westport, CT. 11 Bryant, E. C. 1966. StaUstical analysis. McGraw+Hill, New York, NY. 12 Lengemann, F. W. 1962. Distribution of radioslxontium and radiocesium in milk and milk preducts. J. Dalry Sci. 45:538. 13 Lagoni, H., O. Paakkola, and K. H. Peters. 1963. Untersuchungen uber die quantitative verteihmg radioaktiver falloutprodukte in milch. Milchwissenschaft 18:340. 14 de Ruig, W. G. 1966. Prospects for the manufacture of cheese tit for human consumption from milk contaminated with radioactive nuclear fission products. Neth. Milk Dairy J. 20:283.
APPENDIX Radiocesium Transport from Feta to Brine
The transport of radiocesium contained in the curd to the surrounding brine may be described in the framework of a simple twocompartment model, depicted schematically in Figure A1. We shall denote with N k and Ck, k = 1, 2, the number of radiocesium nuclei, and the radiocesium concentradon in each compartment, respectively. Because C1 is measured in becquerels per kilogram and C2 in becquerels per liter, they are related to the corresponding populations through the equations: Ni = C1MI
[A1]
N 2 = C2V 2
[A2]
in which M 1 and V 2 represent mass and volume
1091
RADIOCESIUM DECONTAMINATION OF FETA CHEESE ....-. . ~..: "..." ....,~,-.o;...~.• 9..o~« .,b~«.l.
~d
æl
~
' œl ' %
Equation [A5] may be written as:
t IM2 ø C 2
C] + pC2 = Co
[A7]
In view of the last relation, Equation [A4] may be written in the final form: ~l
~ --;'.'.. .-. .¢,-...-_ . . . .-:
o.: .
-..k°:@.
»ø., o - .. • , O..å~~fd
ø-, ø-~. ... ø(C..t
~
".o.ri
dC2
V2-'~" + (Pkl + k2)C2 = k l C o
[A8]
, OØ
Figure AI. A simple two-compartment model employed in the analysis of radiocesium transfer from feta to brine.
If we assume that at time t = 0 all the radiocesium nuclei are in compartment 1 (the curd), then by direct integration or other methods Equation [A8] yields the solution:
of the corresponding compartments. Then the rate of transport of radiocesium nuclei from compartment 1 (curd) to compartment 2 (brine) is proportional to the difference of concentrations in the two compartments
klC0 Pkl + k 2 C2(t) = p ~ + k2 {1 - exp [ ~ t]}[A9 ]
dN 2
d--i- = klC1 - k 2 C 2
[A3]
in which kl (kg'h -1) and k2 (L-h-1) are constants of proportionality. This may be seen as analogous to the diffusion of an ideal gas through a membrane separating two vessels, when the impedance of the membrane is different in the two directions of flow; the net rate of flow is then proportional to the pressure difference across the membrane. From the relation in Equation [A2], this differential equation may be written entirely in terms of acfivity conservatjon as: dC 2
V2 - ~
= ksC1 - k2C2
lA4]
Because the total number of radiocesium nuclei No = N1 + N2 in the two compartments remains constant, we may write the conversion relation: M1C1 + V 2 C 2 = M1C0
[A5]
in which Co is the initial activity concentration in the curd. If we further introduce the parameter: V2
13 = M---~
The last expression is a function of two free parameters k 1 and k2, which may be determined through the best tit to experimental data. The function predicts an asymptotic equilibrium value of radiocesium concentration in the brine: klC0 1"2eq ---- Pkl + k2 f~
[A10]
which, as far as geometry is concerned, depends only on the ratio p, defined in Equation [A6]. It is noted in contrast that the characteristic half-life with which this equilibrium state is reached: .693V2 T1/'2 = Pkl + k2
[All]
or, in terms of the mass of the feta involved: •693pM 1 T1/2 = Ok1 +k2
[Ål2]
depends in addition on the quantity of the stored product. Equation [A10] finally permits the calculauon of radiocesium concentration remaining in the feta after equilibrium with the brine has been reached. In terms of fractional reduction it may be easily shown that: Cleq
k2
Co
Pkl + k2
[A13]
[A63
Journal of Dairy Science Vol. 72, Nø. 5, 1989