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Design and setting up of a water vapour pressure capacitance manometer for measurement of water activity
Journal of Food Engineering 38 (1999) 407-423 0 1999 Elsevier Science Limited. All rights reserved Printed in &eat Britain O%O-8774M/$ - see front matter PII:
50260.8774(98)00128-9
Design and Setting up of a Water Vapour Pressure Capacitance Manometer for Measurement of Water Activity B. Zanoni,* C.’Peri, G. Giovanelli & E. Pagliarini diSTAM, Sezione Tecnologie Alimentari, Universita di Milano, Via Celoria, 2 20133 MiIano, Italy (Received 7 January 1998; accepted 24 September 1998)
ABSTRACT Water activity (a,,,) plays an essential role in food science and technology in optimizing processes (i.e. drying) and evaluating the microbiological, chemical and physical stabilityof foods. Accurate and precise measurement of a, is also required in view of the new trend to minimal processing of foods. A thermostated vapour pressure capacitance manometer (WM) apparatus, based on the theory of Benado and Rizvi, was designed and built. It consisted of two flasks, one for sample and another for desiccant (CaSO,), connected by means of two solenoid valves to a tubing manifold, connected to a pressure transducer; and to a vacuum pump by means of a solenoid valve. Reference samples (i.e. pure wateq saturated salt solutions and microcrystalline cellulose at different moisture contents) were used to validate the VT’M at 25°C. Results showed an accuracy of kO.01 a,,, and a standard deviation of +O.Ol a, Based on this study the evacuation time and the time of sample equilibrium after evacuation were identified as the two criticalpoints of the VPiUfor accurate measurements. 0 1999 Ehevier Science Limited. All rights reserved
INTRODUCT’ION Water activity (a,) plays an essential role in food science and technology in optimizing processes (i.e. drying) and evaluating the microbiological, chemical and physical stability of foods. Accurate and precise measurement is required in the whole range of a,. Due to increasing interest of food industry in minimal processing of foods the *To whom correspondence should be addressed. 7063-8625; E-mail: [email protected] 407
Tel.:
0039-2-7060-2063;
Fax: 0039-2-
408
B. Zanoni
et al.
accuracy and repeatability at high a, range (i.e. a,20.80), in relation to microbial spoilage and growth of pathogens is of major importance. Silverman et al. (1983) reported that a 0.01 difference in a, results in growth or absence of growth of Staphylococcus aureus. Emodi & Lechowich (1969) reported that even a 0.001 difference in a,., can result in growth or absence of growth of Clostridium botulinurn. Are there sufficiently accurate and precise methods for a,., measurement to have an experimental error of 0.01 to 0.001 a,? Routine methods are often not accurate and precise in the whole range of a,, especially at high a, values. In the framework of the COST 90 project (Spiess & Wolf, 1987) the method of sorption isotherm measurement, which allows a, of foods to be measured indirectly, was standardized. Repeatability of fO.O1 a, can be calculated from the precision of the microcrystalline cellulose sorption isotherm at 25°C studied in this project. This method has two disadvantages: (i) slowness of the equilibrium (i.e. an equilibration time of at least 14 days might be recommended) and (ii) potential mould and bacterial growth on samples at a, > 0.90. Dew point hygrometers have an accuracy of 0.003 a, in the range of 0.75 to 0.99 a, (Prior, 1979). Stamp et al. (1984) reported a lower accuracy than 0.01-0.02 a, and a repeatability of kO.003 a, for saturated salt solutions. The main disadvantages of this method are that, at low water activity range (i.e. a, <0.70), there is not sufficient vapour in the headspace to cover the reflecting surface, and accuracy is diminished (Rahman, 1995). Electric hygrometers are accurate and precise at a, 10.90. Troller (1983a) and Stamp et al. (1984) reported an accuracy of -0.01 a, and a repeatability of +O.Ol a, for saturated salt solutions. Disadvantages of this method are: (i) loss of accuracy and precision at a, > 0.90 and (ii) high frequency of calibration. The most accurate and precise method to measure a, in the whole range is considered to be the direct measurement of the actual water vapour pressure exerted by the sample. This method has been proposed by Makower & Myers (1943) and is still used as a reference method to measure a,.,. It requires a Vapour Pressure Manometer (VPM) apparatus, a schematic diagram of which is shown in Fig. 1. A sample is placed in the sample flask while the desiccant flask is filled with a desiccant material. Keeping the sample flask closed, the system is evacuated. The desiccant flask is then excluded from the system and the sample flask is evacuated rapidly. The system is separated from the vacuum pump and, after equilibration, the pressure (Pr) exerted by the sample is measured. The sample flask is subsequently separated from the system, and the desiccant flask is opened. Water vapour is removed by sorption onto the desiccant, and, after equilibration, the pressure (Pz) exerted by nonadsorbing gases is measured. The a, of the sample can be calculated by the following general equation: a_,,=-
p1 -p2 (1) PW
where P, is the vapour pressure of pure water at the sample temperature. Although the VPM has been improved (Labuza et al., 1976; Troller, 1983b; Stamp et al., 1984; Nunes et al., 1985; Benado & Rizvi, 1987; Lewicki, 1987, 1989), relevant
409
Measuring a,,, with a capacitance manometer
results about both design and setting up of the apparatus and calculation of a, are still partial and inconsistent. A comparison of some VPMs given in Table 1 indicates that the manometric method is an accurate and precise method, though not standardized in terms of operating conditions (i.e. sample evacuation time, equilibrium time). Few applications of VPMs to foods are reported. Stamp et al. (1984) tested VPMs and some hygrometers for their ability to measure water activity of foods over the 0.11-0.85 a, range. VPM provided values differing by an average of -0.05 a,, as compared to hygrometer readings.
TEMP. CONTROL
VAC. GAUGE
VOLl-MflOR
WFFLE
Fig. 1. Schematic
diagram
of a thermostated
vapour 1995).
pressure
manometer
apparatus
(Rizvi,
410
B. Zunoniet al.
m 8
6 +I
22 8 d +I
z 0
6
fl
i2 d +I
Measuring a,,, with a capacitance manometer
411
The objective of this study was to design and set up a VPM using reference materials to identify critical points of apparatus design and operating conditions affecting the accuracy and precision of a, measurement.
MATERIALS AND METHODS Apparatus
A schematic diagram of the thermostated VPM apparatus used is shown in Fig. 2. The VPM was designed according to Benado & Rizvi (1987). It consisted of two flasks (44.12 cm3 capacity), one for sample and another for desiccant (Drier&e? CaSO& connected to a pressure transducer and a vacuum pump by means of a tubing manifold (1 cm internal diameter) and valves. Manual valves (mod. SPlOK) for both breaking vacuum, V,, and desiccant flask, V,, were used. Automatic solenoid valves (mod. PVlOEK) for both desiccant flask, Vi, and tubing manifold, V3, were closed when turned off. The automatic solenoid valve (mod. PVAlOEK) for the same flask, Vz, was open when turned off. The valve V, was connected to a Drierite drying column (Generalcontrol SpA, Milan, Italy) able to adsorb air water vapour. The capacitance manometer (Barocel mod. 655) had a linear range of O-100 mbar and was able to measure pressure with a precision of kO.01 mbar. Vacuum (0.03 mbar) was established by a vacuum pump (Edwards ESlOO). All joints between components were sealed using aluminium
! ! k? Tr
vo
a cl PC
G
Fig. 2.
Diagram for VPM: C, sample flask; D, desiccant flask; G, vacuum pump; M, capaci-
tance manometer; VI, Vz, V,, solenoid valves; V,, V,, manual valves; T,, T,, Td, TO, thermocouples to measure temperature of sample, chamber, desiccant and manometer, respectively; Tr, air humidity trap; PC, personal computer.
B. Zanoni et al.
412
centring rings equipped with O-shaped rings of nitrile rubber. All apparatus components were provided by Edwards Alto Vuoto SpA, Milan, Italy. The measuring temperature, T,,,, and the sample, T,, desiccant, Td, and pressure transducer temperatures, TO,were determined by self-adhesive thermocouples type J (Tersid Srl, Milan, Italy) with a precision of kO.l”C. Values for pressure and temperature were monitored by a data acquisition and recording system Datascan 7220 (Tecnoel S.r.1, Milan, Italy) interfaced by RS232 to a PC. The apparatus was thermostated in a forced-convection Heraeus BK 6160 cell (Heraeus S.p.A., Milan, Italy) equipped with a synoptic panel to open and close automatic valves from outside. Operating procedure
The procedure set-up included the following steps:
(1) When the instrument was turned off, the valve V2 was open and valves V,,
V1, V3 and V4 were closed. (10 g, 4 mesh, Aldrich, Milwaukee, WI, USA) either fresh or DrieriteTM (2) regenerated (i.e. at 200°C for 1 h in an oven) was placed into a glass dish (14.52 cm3 capacity), which was then placed into the desiccant flask. The dish was covered with a thick cloth to avoid system clogging by desiccant dust. (3) Both desiccant flask and tubing manifold were evacuated for approximately 10 min with valves V,, V2 and V3 open to reach 0.03 mbar pressure in the system. (4) The sample flask valve V2 was closed, vacuum in the sample flask was broken bl opening the flask manually. A 3-5-g sample was placed into a 18.85 cm glass dish, which was then placed into the sample flask. The dish was covered with a thick cloth to avoid system clogging by the sample. (5) The desiccant flask valve V1 was closed and the valve Vz was opened for 5 s to evacuate the sample flask; after that the valve V3 was closed. When using either liquid samples or saturated solutions, steps 6-8 were omitted. Valve V2 was closed and the sample was re-equilibrated for 18 h. Due to leakage both desiccant flask and tubing manifold were evacuated for approximately 10 min with valves V1 and V3 open to reach 0.03 mbar pressure in the system. (8) Valves Vi and V3 were closed and the sample flask valve V2 was opened. (9) Pressure was allowed to develop to steady pressure PI in both the tubing manifold and the sample flask at constant temperature for 2-3 h. Pressure equilibrium was reached when four successive 60-s readings differed by fO.O1 mbar. Temperature equilibrium was reached when sample temperature, T,, and pressure transducer temperature, T,,,, differed by fO.l”C. During this phase the desiccant flask was evacuated through valve Vq connected to the vacuum pump. (10) Valves V2 and V4 were closed, and the desiccant flask valve V1 was opened to steady pressure P2 (see step 9) for approximately 15 min. (11) Valve V1 was closed, valve V3 was opened, and both desiccant and sample were removed from the relevant flasks. The system was then evacuated to prepare the apparatus for a new trial or to allow the apparatus to rest.
I’;]
Measuring a, with a capacitance manometer
413
a, calculation
a, was calculated by the following equation:
P %=p w
where P the vapour pressure of water in the sample was calculated based on VPM measurements by the Benado & Rizvi (1987) equation (Table 1): P = P, - (P* - P*)( 1+R)+P”&
(3)
where PI is the pressure exerted by vapour consisting of both water and nonadsorbing gases, P2 is the pressure exerted by nonadsorbing gases, P* is the vapour pressure of the desiccant, POd is the pressure exerted by nonadsorbing gases in the desiccant flask, and R is the apparatus volume ratio, which was calculated as follows:
where V, is the experimental volume of the tubing between the desiccant flask and the desiccant flask valve, and Vi is the experimental volume of the tubing between the desiccant flask valve, the sample flask valve and the vacuum pump valve. During the trial the desiccant flask was evacuated continuously. As a result, the terms PO,,and P* of eqn (3) are negligible, and eqn (3) simplifies to: P=P,
-P,(l+R)
(5)
Operating procedure to measure the apparatus volume ratio
The procedure set-up to evaluate the volume ratio, R, was as follows:
(1) See steps l-3 of the operating procedure to measure water vapour pressure. (2) The desiccant flask valve VI was closed, and dry air was introduced into the
system through valve V,,. A pressure value PI, corresponding to pressure values obtainable for a, measurements (i.e. 10 to 60 mbar), was reached. Valves V3 and V0 were successively closed. (3) Pressure equilibrium was reached within 15-20 min, as described above, pressure PI was read and the sample flask valve V2 was closed. During this phase the desiccant was kept under vacuum by opening valve V,. (4) Valve V, was closed and valve VI was opened. Pressure equilibrium was reached within 5 min, and pressure P2 was read. (5) See step 11 of the operating procedure to measure water vapour pressure.
The volume ratio, R, was calculated by the ideal gas law as follows: (Pl
-P2) p2
(6)
B. Zunoni et al.
414
where V,, is the volume between the valve VI and the desiccant flask, and Vi is the volume between valves VI, V2 and V3. Reference materials
The following reference materials were used to set up and validate the VPM at 25°C: pure water; saturated salt solutions (i.e. LiCl, CH,COOK, KZC03, NaCl, KCl, BaC12, KzNG3, K#&); microcrystalline cellulose MCC Avicel@ PH-101 (Fluka Chemie AG, Buchs, Switzerland) samples at different moisture contents. Salt solutions were prepared following the method described by Spiess & Wolf (1987) within the European COST 90 project on water activity. Table 2 shows the quantity of salt and water used to prepare saturated salt solutions and the relevant a, values at 25”C, as reported by Greenspan (1977). MCC samples at different moisture contents were prepared according to the following procedure. A thin layer of MCC (120 g) was dried at 105°C to constant weight, and residual moisture was determined. The dehydrated sample (100 g) was placed into a hermetically sealed 500 ml glass bottle. The sample was then stored in a glass jar containing silica gel. The jar was kept at room temperature for 24 h to stabilize the dehydrated microcellulose. The bottled sample was rehydrated with distilled water. The quantity of water to be added was calculated according to the following equation:
w*=
i
;u,i -w,
(7)
2
TABLE 2 Preparation of Saturated Salt Solutions at 25°C (aw Values Are Taken From Greenspan, 1977) Salt
LiCl CH&OOK KzCO3 NaCl KC1 BaC&,* KzN03 KS04
Quantityof salt (g)
Quantityof water (ml)
Reference a,
75
42.5 32.5 45
0.1130+0.0027 0.2251 f 0.0032 0.4316 f 0.0039 0.7529 + 0.0012 0.8434 + 0.0026 0.9020 0.9358 f 0.0055 0.9730 f 0.0045
100 100 100 100 125 100 125
*Not reported by Greenspan (1977).
:z :: 35
Measuring a,,, with a capacitance manometer
415
where: tW2 = water (g) to be added to the sample; W1= water (g) present in the sample; U, = final sample moisture (%); D = sample dry matter (g); & = final sample dry matter (%). In order to homogenize the rehydrated sample, it was subjected to successive mechanical stirring in a roller stirrer at 25°C for 5 days. The sample was stored at 25°C until used. VPM tests
Trials for a, measurement were carried out in triplicate on pure water samples (5 g) at different evacuation times (1, 2, 5, 10, 30 and 60 s) and at different equilibrium times of pressure P1 (60,90, 120, 150 and 180 min). Trials for a, measurement were carried out in triplicate on saturated salt solution samples (5 g) at 5 s evacuation time and 120-180 min equilibrium time of pressure PI. Trials for a, measurement were carried out in quadruplicate on MCC samples (3 g) at different moisture contents, evacuation times (2 and 5 s) and equilibrium times of sample after evacuation (0,2 and 18 h). Eight trials were carried out at different times to calculate volume ratio. Measurement of MCC moisture
The moisture of MCC samples was measured in duplicate by oven-drying at 105°C to constant weight both before and after the VPM test.
RESULTS AND DISCUSSION Table 3 shows moistures of MCC samples. a, values were taken from the adsorption isotherm of MCC adsorption at 25”C, calculated by Spiess & Wolf (1987) according to the GAB model:
TABLE 3 Moisture Values and Corresponding a, Values of the MCC Reference Samples MCC samples
Rehydrated 1 Rehydrated 2 Rehydrated 3
Water content (g H20/IoOg dm)
6.41+ 0.07 s.20+0.05 13.64+0.07
Reference a,
0.58 + 0.01 0.71+0.01 0.93 &-0.01
B. Zunoni
416
et al.
TABLE4 Effect of Temperature on a, Value Sample
P (mbar)
Pw
Water K2SO4 K2CO3
31.7 30.9 13.7
25.2T
250°C a,
31.7 -
1.00 0.97 0.43
Pw
32.1 -
a,
0.99 0.96 0.42
nsmCkaw %=
25.6”C
25.4”C
(1 -ka,.,)(l -k+CkQ
P,
32.4 -
a,
0.98 0.95 0.42
P,
32.8 -
a,
0.97 0.94 0.41
(8)
where n,, water content on 100 g dry basis; nsm = 4.064, C = 8.776 and k = 0.772, GAB constants. VPM design The VPM designed by Benado & Rizvi (1987) was modified to improve both system temperature control and desiccant performance, resulting in simplified a, evaluation. Table 4 shows the effect of temperature on a, values for pure water and some saturated salt solutions. Considering pure water, the vapour pressure measured, P, is 31.7 mbar at 25°C (Weast, 1985). If the system temperature measurement is correct, P,,, is 31.7 mbar and the resulting water activity is correct, i.e. 1.00. If the temperature measured differs slightly from the real one, the value for water activity changes. For instance, as a result of a 0.4”C shift, P, is 32.4 mbar and water activity is 0.98. A slight deviation in temperature results in a considerable deviation in a,. This also holds for saturated salt solutions. Consequently, particular attention was paid to system temperature control. For example, automatic valves were selected to avoid heating up of the system. Generally speaking, this can be obtained by opening valves for a limited time. In our case, because of the long valve opening time (2-3 h) during PI reading, a valve V, was chosen, which was on when it was closed. In this way, during the opening step, valve V2 was off and, consequently, unable to perturb the system, thus allowing temperature-pressure equilibrium to be achieved. Four thermocouples were used to monitor the trends of sample, desiccant and pressure transducer temperatures and the temperature inside the thermostated cell. Monitoring of temperature allowed pressures to be measured under identical temperature conditions (see procedure to measure water vapour pressure). Using the desiccant was most important when the VPM was applied to measure foods containing not only water vapour but also other volatile components. In order to perform a correct evaluation of water vapour pressure, pressure PI should be rectified for the pressure exerted by nonadsorbing gas. This can be obtained by
417
Measuring a, with a capacitance manometer
using the desiccant, which adsorbs sample water vapour, resulting in a decrease in pressure to a residual value, Pz, exerted by the nonadsorbing gas. In order to improve the efficiency of the desiccant, a valve, V4, was included to connect the desiccant flask to the vacuum pump. As a result, during PI reading, continuous evacuation of the desiccant flask was obtained. Consequently, both nonadsorbing gas remained in the desiccant flask and a mixture containing water vapour and nonadsorbing gas coming from desiccant particles were removed. The use of valve V4, in addition to the use of either fresh or regenerated desiccant for each trial, allowed us to simplify equations to calculate a,, as described in Section 2. Volume ratio, R, correlated to the use of the desiccant, was an important design element for accuracy and repeatability of vapour pressure measurements. Opening of the desiccant flask resulted in pressure decrease, due to both water vapour adsorption by the desiccant and increase in system volume. Hence, pressure P2 must be corrected by a factor expressing volume variation. Volume ratio measurements should be accurate, and volume variation should not cause a decrease in pressure P2 to too low values, which would not be included in the noise of the pressure transducer. Volume ratio was evaluated using dry air as a sample. This procedure ensured that the pressure decrease occurring when opening the desiccant flask valve was caused by volume variation rather than water vapour adsorption by the desiccant. Table 5 shows experimental values for volume ratios, R, obtained using the same amount of desiccant as that used for a, measurements at different dry air pressures to evaluate the invariability of volume ratio under the various operating conditions applied to a, measurement of real samples. The R values obtained showed that using the desiccant flask caused a threefold volume increase and, consequently, a similar decrease in pressure. Hence, the VPM showed a higher decrease in pressure than the sensitivity of the pressure transducer. If the worst case, i.e. a sample such as LiCl with a low water activity, is taken into account, a value for PI of 3.5 mbar would theoretically be obtained. Since a threefold decrease in pressure PI occurs when opening the desiccant flask, a value for pressure Pz of 1.2 mbar would be obtained, which is by two orders of magnitude higher than the precision of the pressure transducer (50.01 mbar).
Experimentally PI (mbar) 96.6 57.2 49.8 37.7 21.1 17.8 14.6 12.1
Determined
Values
TABLE 5 for Volume 1.77 + 0.03)
Ratio
at
25°C
(Mean
Value+sd
=
P2 (mbar)
R = V,lv,
35.0 20.8 17.9 13.6 7.5
1.76 1.75 1.78 1.77 1.81 1.74 1.75 1.81
!z 413
418
B. Zunoni et al. TABLE 6
Influence of Evacuation and Equilibrium Times on ~1,of Pure Water (aw = 1.00) Equilibrium time (min)
Water vapour pressure (mbar)
a,
: 5* 5*
180 120 60 90
31.1 30.8 30.7 31.1
0.96 0.98 0.97 0.98
2 5
150 120 180
31.7 31.3 31.4
0.99 1.00 0.99
;: 60
120 150 120
31.8 32.1 32.1
1.00 1.01 1.01
Evacuation time (s)
Operating conditions: sample amount = 5 g; fresh desiccant = 10 g; R = 1.77; T, = 25°C. *Samples not in equilibrium.
Setting up of the operating procedure for a, measurement and validation of the apparatus Trials with pure water and saturated salt solutions Trials were carried out to define operating conditions to measure vapour pressure and to determine both the evacuation time of the sample and the procedure to achieve system pressure equilibrium. Table 6 shows results from trials carried out on pure water samples at 25°C at different evacuation times (1, 2, 5, 10, 30 and 60 s) and, for the trial at 5 s evacuation time, at different equilibrium times (60, 90, 120, 150 and 180 min). It should be noted that 1 or 2 s evacuation time did not allow us to measure pure water activity accurately: the error was -0.04 for 1 s and -0.02 for 2 s. A longer evacuation time, i.e. 5 to 60 s under pressure equilibrium conditions, resulted in a pure water activity value similar to the real one with a maximum error of fO.O1. Since the aim of the evacuation step was to remove air from the headspace of the sample, it should last long enough to remove all the air without perturbing the sample considerably. If evacuation time was inadequate, such as for 1 and 2 s evacuation, residual air caused an error in accuracy for measurement of vapour pressure, P. Because of residual air, the water vapour diffusion and subsequent adsorption in the desiccant was slower, resulting in overestimation of pressure P2 and underestimation of vapour pressure P (Benado & Rizvi, 1987). A too long evacuation time caused flash-evaporation of water in the sample, resulting in system temperature perturbation, due to sample cooling. Figure 3 shows the effect of perturbation on sample temperature using 5 s (graph a) and 60 s evacuation (graph b). In the first graph evacuation caused a 2°C decrease in temperature, as compared to the cell temperature; in the second graph a higher decrease (7°C) was observed. Therefore, 5 s evacuation time was chosen to maximize air removal and minimize system perturbation. Table 6 shows the effect of the equilibrium time required to obtain constant reading of pressure PI for trials carried out applying 5 s evacuation. For trials at 60
Measuring a,,, with a capacitance manometer
419
45
32
40
30
35
28
a
26 o^ 24 L
c 30 Q g 25
$
e!
22 6 E 20 :
3 20 o* h 15 10
18
5
16 14
0 0
20
40
80
60
100
120
140
160
The (min)
35
25
I I I
I
5
I
0 -+0
I I ..11
I I 20
40
60
60
100
120
16 14 140
Time (min)
Fig. 3. Trend of sample pressure (. . .) and temperature (-) tion time (a) and one at 60 s (b).
for one trial at 5 s evacua-
B. Zanoni
420
et al.
TABLE7 Comparison Between Experimental
a, Values for Pure Water and Saturated Salt Solutions by VPM and Corresponding Reference Values (Greenspan, 1977) at 25°C
Sample
P (mbar)
a,
Reference a,
Water KS% KzNG3 BaCl, KC1 NaCl
31.7kO.3 30.7 &-0.4 29.2kO.3 28.9 + 0.4 27.lkO.2 24.1 _t 0.3 13.5 kO.2 6.7+0.3 3.2kO.3
1.00+0.01 0.97 * 0.01 0.92 + 0.01 0.91 f 0.01 0.85 +0.01 0.76kO.01 0.43 * 0.01 0.21 f 0.01 O.lOfO.O1
1.00 0.97 0.93 0.90 0.84 0.75 0.43 0.22 0.11
&CO3
CH,COOK LiCl Operating
conditions:
sample amount = 5 g; fresh desiccant = 10 g; R = 1.77.
and 90 min equilibration, step 9 of the operating procedure was stopped when four successive 60-s readings differed by + 1 and fO.l mbar, respectively. For the other trials step 9 was stopped when four successive 60-s readings differed by +O.Ol mbar. The latter procedure allowed us to obtain accurate a, measurement and, consequently, was chosen to define the operating procedure to measure water vapour pressure. Figure 3 shows the trend of pressure during a complete trial: equilibrium of both pressure PI and pressure P2 was achieved. The pressure values were reported from the end of step 3 of the operating procedure and then, pressure values approximately equal to zero corresponded at steps 4 and 5. Table 7 shows accuracy and repeatability of experimental a, values for pure water and saturated salt solutions measured by the optimized procedure. Experimental data showed an accuracy of +O.Ol to -0.01 a, and a standard deviation of kO.01 a,. Experimental and reference data were not significantly different at PcO.01. Trials with MCC Trials were carried out with MCC to validate the VPM for water activity measurement of solid products. Results on activity of foods from the COST 90 project (Spiess & Wolf, 1987) were used as a reference. Table 8 shows results from a series of trials carried out on the rehydrated MCC sample 3. Percentage values for absolute humidity of samples both before, nos, and after the trial, nsf, measured a, values and reference u, values, extrapolated from the MCC isotherm (see eqn (8) according to the Itsf value, are shown. Results from trials carried out using the procedure optimized by trials with water and saturated salt solutions are reported in the first row. As can be seen, the experimental a, value was subject to a significant error of -0.04. This suggested that the shift could be caused by sample perturbation during evacuation, as shown by a considerable decrease, i.e. about 15%, in absolute humidity of the sample within the end of the trial. As a result of perturbation, a, measurement was inaccurate because it had been carried out on a sample not in equilibrium, showing nonhomogeneities in moisture and water activity.
Measuring a, with a capacitance manometer
421
TABLE 8 Comparison Between Experimental a, Values for Rehydrated MCC Sample 3 by VPM and Corresponding Reference Values (Spiess & Wolf, 1987) at 25°C Sample operating conditions 5 s
evacuation 2 s evacuation 5 s evacuation equilibration
time time time and 18 h time
n, (W
P (mbar)
13.64+0.07 13.64+0.07 13.64+0.07
26.1kO.3 26.1kO.3 27.250.2
a,
nSf VW
0.83”+0.01 11.6kO.3 0.82”_tO.O1 12.2kO.3 0.86b+0.01 11.7f0.3
Ref:
a,
0.87b+0.01 0.88b$0.01 0.87b+0.01
rzsoand nsf are sample absolute moistures (g/100 g dry matter) before and after the VPM test,
respectively. a and b show a significant difference at PcO.01. Operating conditions: sample amount = 3 g; fresh desiccant = 10 g; R = 1.77.
In order to minimize sample perturbation, the following two elements were taken into account: reduction of evacuation time and sample equilibrium after evacuation. Regarding the first element, results at 2 s evacuation are shown in the second row in Table 8. Evacuation time reduction did not improve the apparatus performance: a, was subject to the same significant error as that identified for trials at 5 s, and a slightly lower, i.e. about ll%, variation in absolute humidity of the sample was observed. The second element, used to minimize the effect of perturbation, was sample equilibrium after evacuation to obtain moisture homogeneity. Equilibrium of the sample, evacuated into the sample flask, which had been isolated by the system by closing valve Vz, was achieved after leaving overnight (18 h). The efficiency of equilibrium on VPM performance is shown in the third row in Table 8. The behaviour of rehydrated MCC samples 1 and 2 was similar to that of the rehydrated sample 3. Table 9 shows the accuracy and repeatability of experimental a, values for MCC samples measured by the optimized procedure. Moisture data show that sample evacuation resulted in sample dehydration from 8 to 14% as a function of the sample initial moisture. Experimental data showed an accuracy of - 0.01 a, and a standard deviation of kO.01 a,. Experimental and reference data were not significantly different at PcO.01. TABLE 9 Comparison Between Experimental a, Values for MCC Samples by VPM and Corresponding Reference Values (Spiess & Wolf, 1987) at 25°C Sample
Rehydrated 1 Rehydrated 2 Rehydrated 3
nsO(W
6.41+ 0.07 8.20 + 0.05 13.64 + 0.07
P (mbar)
16.5 + 0.4 19.7 + 0.5 27.2 + 0.2
a,
0.52 & 0.01” 0.62 + O.Olb 0.86 + 0.01’
nsf PW
5.9kO.l 7.1 f0.2 11.7f0.3
ReJ a,
0.53 + 0.01” 0.63 f O.Olb 0.87 f 0.01”
nsOand nsf are sample absolute moistures (g/100 g dry matter) before and after the VPM test, respectively. a, b and c show a significant difference at PxO.01. Operating conditions: sample amount = 3 g; fresh desiccant = 10 g; evacuation time = 5 s; equilibrium time of the evacuated sample = 18 h; R = 1.77.
422
B. Zunoni et al.
CONCLUSIONS The VPM apparatus designed was able to provide accurate and precise measurements in the whole range of a, using pure water and saturated salt solutions as reference standards. The VPM was also accurate and precise in measuring a, of MCC samples. This demonstrates that both the VPM design and the methodology by Benado & Rizvi (1987) are valid in a wide range of a,. This study allowed us to identify the following two critical points of the VPM for accurate and precise measurements: (i) evacuation time and (ii) time of sample equilibrium after evacuation. The evacuation time controls the presence of non-water vapours in the apparatus, which hinders water vapour diffusion and subsequent adsorption in the desiccant (Benado & Rizvi, 1987). The optimized evacuation time was 5 s. The time of sample equilibration was critical in solid samples, where evacuation resulted in sample dehydration. The optimal time was 18 h. Based on this work, the following considerations on applicability of the VPM to measure water activity of foods can be drawn: The VPM is accurate and reproducible throughout the a, interval. The VPM requires difficult, accurate setting up of design and operating procedures. If products show dehydration during evacuation, the VPM can be used to build adsorption and desorption isotherms. In this case, sample moisture should be measured after the trial (Lewicki, 1989). The VPM cannot be used to measure the water activity of the sample because the a, value is modified during the measurement as a function of the sample initial moisture. This may explain the differences between VPMs and some hygrometers found by Stamp et al. (1984). It may be concluded that research should be carried out to validate the VPM methodology on various solid foods (e.g. real foods from the COST 90 project) at different moisture contents and to show the significance of dehydration. Research should also be carried out on the VPM design to remove or minimize dehydration during evacuation. REFERENCES Benado, A. L. & Rizvi, S. S. H. (1987). Water activity calculation by direct measurement of vapor pressure. J. Food Sci., 2, 429-432. Emodi, A. S. & Lechowich, R. V. (1969). Low temperature growth of type E Clostridium botulinum spores. 2. Effect of solutes and temperature. J. Food Sci., 34, 82-87. Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. J. Res. Nat. Bureau St&., 81A, 89-96. Iabuza, T. P., Acott, K., Tatini, S. R. & Lee, R. Y. (1976). Water activity determination: a collaborative study of different methods. J. Food Sci., 41, 910-917. Lewicki, P. P. (1987). Design of water vapour pressure manometer. J. Food Eng., 6,405-422. Lewicki, P. P. (1989). Measurement of water activity of saturated salt solutions with the Vapour Pressure Manometer. .I. Food Eng., 10, 39-55. Makower, B. & Myers, S. (1943). A new method for the determination of moisture in dehydrated vegetables. In Proc. Inst. Food Technologists4th Conf., pp. 156-164. Nunes, R. V., Urbican, M. J. & Rotstein, E. (1985). Improving accuracy and precision of water activity measurement with a water pressure manometer. J. Food Sci., 50, 148-149.
Measuring a, with a capacitance manometer
423
Prior, B. A. (1979). Measurement of water activity in foods: a review. J. Food Prot., 42, 668-674. Rahman, S. (1995). Food Properties Handbook. CRC Press, New York, pp. l-86. Rizvi, S. S. H. (1995). Thermodynamic properties of foods in dehydration. In: Engineeting g3~$F of Foods, M. A. Rao and S. S. H. Rizvi (Eds.), Marcel Dekker, New York, pp. Silverman, G. J., Munsey, D. T., Lee, C. & Ebert, E. (1983). Interrelationship between water activity, temperature and 5% oxygen on growth and enterotoxin A secretion by Staphylococcus aureus in precooked bacon. J. Food Sci., 48,1783-1786. Spiess, W. E. L. & Wolf, W. (1987). Critical evaluation of methods to determine moisture sorption isotherms. In: WaterActivity: 7heov and Applications to Food, L. B. Rockland and L. R. Beuchat (Eds.), Marcel Dekker, New York, pp. 215-233. Stamp, J. A., Linscott, S., Lomauro, C. & Labuza, T. P. (1984). Measurement of water activity of salt solutions and foods by several electronic method as compared to direct vapor pressure measurement. J. Food Sci., 49, 1139-l 142. Troller, J. (1983). Methods to measure water activity. J. Food Prot., 46, 129-134. Troller, J. (1983). Water activity measurement with a capacitance manometer. J. Food Sci., 48,739-741. Weast, R. C. (1985). Handbook ofChemistry and Physics, CRC Press, New York, pp. D-190.
50260.8774(98)00128-9
Design and Setting up of a Water Vapour Pressure Capacitance Manometer for Measurement of Water Activity B. Zanoni,* C.’Peri, G. Giovanelli & E. Pagliarini diSTAM, Sezione Tecnologie Alimentari, Universita di Milano, Via Celoria, 2 20133 MiIano, Italy (Received 7 January 1998; accepted 24 September 1998)
ABSTRACT Water activity (a,,,) plays an essential role in food science and technology in optimizing processes (i.e. drying) and evaluating the microbiological, chemical and physical stabilityof foods. Accurate and precise measurement of a, is also required in view of the new trend to minimal processing of foods. A thermostated vapour pressure capacitance manometer (WM) apparatus, based on the theory of Benado and Rizvi, was designed and built. It consisted of two flasks, one for sample and another for desiccant (CaSO,), connected by means of two solenoid valves to a tubing manifold, connected to a pressure transducer; and to a vacuum pump by means of a solenoid valve. Reference samples (i.e. pure wateq saturated salt solutions and microcrystalline cellulose at different moisture contents) were used to validate the VT’M at 25°C. Results showed an accuracy of kO.01 a,,, and a standard deviation of +O.Ol a, Based on this study the evacuation time and the time of sample equilibrium after evacuation were identified as the two criticalpoints of the VPiUfor accurate measurements. 0 1999 Ehevier Science Limited. All rights reserved
INTRODUCT’ION Water activity (a,) plays an essential role in food science and technology in optimizing processes (i.e. drying) and evaluating the microbiological, chemical and physical stability of foods. Accurate and precise measurement is required in the whole range of a,. Due to increasing interest of food industry in minimal processing of foods the *To whom correspondence should be addressed. 7063-8625; E-mail: [email protected] 407
Tel.:
0039-2-7060-2063;
Fax: 0039-2-
408
B. Zanoni
et al.
accuracy and repeatability at high a, range (i.e. a,20.80), in relation to microbial spoilage and growth of pathogens is of major importance. Silverman et al. (1983) reported that a 0.01 difference in a, results in growth or absence of growth of Staphylococcus aureus. Emodi & Lechowich (1969) reported that even a 0.001 difference in a,., can result in growth or absence of growth of Clostridium botulinurn. Are there sufficiently accurate and precise methods for a,., measurement to have an experimental error of 0.01 to 0.001 a,? Routine methods are often not accurate and precise in the whole range of a,, especially at high a, values. In the framework of the COST 90 project (Spiess & Wolf, 1987) the method of sorption isotherm measurement, which allows a, of foods to be measured indirectly, was standardized. Repeatability of fO.O1 a, can be calculated from the precision of the microcrystalline cellulose sorption isotherm at 25°C studied in this project. This method has two disadvantages: (i) slowness of the equilibrium (i.e. an equilibration time of at least 14 days might be recommended) and (ii) potential mould and bacterial growth on samples at a, > 0.90. Dew point hygrometers have an accuracy of 0.003 a, in the range of 0.75 to 0.99 a, (Prior, 1979). Stamp et al. (1984) reported a lower accuracy than 0.01-0.02 a, and a repeatability of kO.003 a, for saturated salt solutions. The main disadvantages of this method are that, at low water activity range (i.e. a, <0.70), there is not sufficient vapour in the headspace to cover the reflecting surface, and accuracy is diminished (Rahman, 1995). Electric hygrometers are accurate and precise at a, 10.90. Troller (1983a) and Stamp et al. (1984) reported an accuracy of -0.01 a, and a repeatability of +O.Ol a, for saturated salt solutions. Disadvantages of this method are: (i) loss of accuracy and precision at a, > 0.90 and (ii) high frequency of calibration. The most accurate and precise method to measure a, in the whole range is considered to be the direct measurement of the actual water vapour pressure exerted by the sample. This method has been proposed by Makower & Myers (1943) and is still used as a reference method to measure a,.,. It requires a Vapour Pressure Manometer (VPM) apparatus, a schematic diagram of which is shown in Fig. 1. A sample is placed in the sample flask while the desiccant flask is filled with a desiccant material. Keeping the sample flask closed, the system is evacuated. The desiccant flask is then excluded from the system and the sample flask is evacuated rapidly. The system is separated from the vacuum pump and, after equilibration, the pressure (Pr) exerted by the sample is measured. The sample flask is subsequently separated from the system, and the desiccant flask is opened. Water vapour is removed by sorption onto the desiccant, and, after equilibration, the pressure (Pz) exerted by nonadsorbing gases is measured. The a, of the sample can be calculated by the following general equation: a_,,=-
p1 -p2 (1) PW
where P, is the vapour pressure of pure water at the sample temperature. Although the VPM has been improved (Labuza et al., 1976; Troller, 1983b; Stamp et al., 1984; Nunes et al., 1985; Benado & Rizvi, 1987; Lewicki, 1987, 1989), relevant
409
Measuring a,,, with a capacitance manometer
results about both design and setting up of the apparatus and calculation of a, are still partial and inconsistent. A comparison of some VPMs given in Table 1 indicates that the manometric method is an accurate and precise method, though not standardized in terms of operating conditions (i.e. sample evacuation time, equilibrium time). Few applications of VPMs to foods are reported. Stamp et al. (1984) tested VPMs and some hygrometers for their ability to measure water activity of foods over the 0.11-0.85 a, range. VPM provided values differing by an average of -0.05 a,, as compared to hygrometer readings.
TEMP. CONTROL
VAC. GAUGE
VOLl-MflOR
WFFLE
Fig. 1. Schematic
diagram
of a thermostated
vapour 1995).
pressure
manometer
apparatus
(Rizvi,
410
B. Zunoniet al.
m 8
6 +I
22 8 d +I
z 0
6
fl
i2 d +I
Measuring a,,, with a capacitance manometer
411
The objective of this study was to design and set up a VPM using reference materials to identify critical points of apparatus design and operating conditions affecting the accuracy and precision of a, measurement.
MATERIALS AND METHODS Apparatus
A schematic diagram of the thermostated VPM apparatus used is shown in Fig. 2. The VPM was designed according to Benado & Rizvi (1987). It consisted of two flasks (44.12 cm3 capacity), one for sample and another for desiccant (Drier&e? CaSO& connected to a pressure transducer and a vacuum pump by means of a tubing manifold (1 cm internal diameter) and valves. Manual valves (mod. SPlOK) for both breaking vacuum, V,, and desiccant flask, V,, were used. Automatic solenoid valves (mod. PVlOEK) for both desiccant flask, Vi, and tubing manifold, V3, were closed when turned off. The automatic solenoid valve (mod. PVAlOEK) for the same flask, Vz, was open when turned off. The valve V, was connected to a Drierite drying column (Generalcontrol SpA, Milan, Italy) able to adsorb air water vapour. The capacitance manometer (Barocel mod. 655) had a linear range of O-100 mbar and was able to measure pressure with a precision of kO.01 mbar. Vacuum (0.03 mbar) was established by a vacuum pump (Edwards ESlOO). All joints between components were sealed using aluminium
! ! k? Tr
vo
a cl PC
G
Fig. 2.
Diagram for VPM: C, sample flask; D, desiccant flask; G, vacuum pump; M, capaci-
tance manometer; VI, Vz, V,, solenoid valves; V,, V,, manual valves; T,, T,, Td, TO, thermocouples to measure temperature of sample, chamber, desiccant and manometer, respectively; Tr, air humidity trap; PC, personal computer.
B. Zanoni et al.
412
centring rings equipped with O-shaped rings of nitrile rubber. All apparatus components were provided by Edwards Alto Vuoto SpA, Milan, Italy. The measuring temperature, T,,,, and the sample, T,, desiccant, Td, and pressure transducer temperatures, TO,were determined by self-adhesive thermocouples type J (Tersid Srl, Milan, Italy) with a precision of kO.l”C. Values for pressure and temperature were monitored by a data acquisition and recording system Datascan 7220 (Tecnoel S.r.1, Milan, Italy) interfaced by RS232 to a PC. The apparatus was thermostated in a forced-convection Heraeus BK 6160 cell (Heraeus S.p.A., Milan, Italy) equipped with a synoptic panel to open and close automatic valves from outside. Operating procedure
The procedure set-up included the following steps:
(1) When the instrument was turned off, the valve V2 was open and valves V,,
V1, V3 and V4 were closed. (10 g, 4 mesh, Aldrich, Milwaukee, WI, USA) either fresh or DrieriteTM (2) regenerated (i.e. at 200°C for 1 h in an oven) was placed into a glass dish (14.52 cm3 capacity), which was then placed into the desiccant flask. The dish was covered with a thick cloth to avoid system clogging by desiccant dust. (3) Both desiccant flask and tubing manifold were evacuated for approximately 10 min with valves V,, V2 and V3 open to reach 0.03 mbar pressure in the system. (4) The sample flask valve V2 was closed, vacuum in the sample flask was broken bl opening the flask manually. A 3-5-g sample was placed into a 18.85 cm glass dish, which was then placed into the sample flask. The dish was covered with a thick cloth to avoid system clogging by the sample. (5) The desiccant flask valve V1 was closed and the valve Vz was opened for 5 s to evacuate the sample flask; after that the valve V3 was closed. When using either liquid samples or saturated solutions, steps 6-8 were omitted. Valve V2 was closed and the sample was re-equilibrated for 18 h. Due to leakage both desiccant flask and tubing manifold were evacuated for approximately 10 min with valves V1 and V3 open to reach 0.03 mbar pressure in the system. (8) Valves Vi and V3 were closed and the sample flask valve V2 was opened. (9) Pressure was allowed to develop to steady pressure PI in both the tubing manifold and the sample flask at constant temperature for 2-3 h. Pressure equilibrium was reached when four successive 60-s readings differed by fO.O1 mbar. Temperature equilibrium was reached when sample temperature, T,, and pressure transducer temperature, T,,,, differed by fO.l”C. During this phase the desiccant flask was evacuated through valve Vq connected to the vacuum pump. (10) Valves V2 and V4 were closed, and the desiccant flask valve V1 was opened to steady pressure P2 (see step 9) for approximately 15 min. (11) Valve V1 was closed, valve V3 was opened, and both desiccant and sample were removed from the relevant flasks. The system was then evacuated to prepare the apparatus for a new trial or to allow the apparatus to rest.
I’;]
Measuring a, with a capacitance manometer
413
a, calculation
a, was calculated by the following equation:
P %=p w
where P the vapour pressure of water in the sample was calculated based on VPM measurements by the Benado & Rizvi (1987) equation (Table 1): P = P, - (P* - P*)( 1+R)+P”&
(3)
where PI is the pressure exerted by vapour consisting of both water and nonadsorbing gases, P2 is the pressure exerted by nonadsorbing gases, P* is the vapour pressure of the desiccant, POd is the pressure exerted by nonadsorbing gases in the desiccant flask, and R is the apparatus volume ratio, which was calculated as follows:
where V, is the experimental volume of the tubing between the desiccant flask and the desiccant flask valve, and Vi is the experimental volume of the tubing between the desiccant flask valve, the sample flask valve and the vacuum pump valve. During the trial the desiccant flask was evacuated continuously. As a result, the terms PO,,and P* of eqn (3) are negligible, and eqn (3) simplifies to: P=P,
-P,(l+R)
(5)
Operating procedure to measure the apparatus volume ratio
The procedure set-up to evaluate the volume ratio, R, was as follows:
(1) See steps l-3 of the operating procedure to measure water vapour pressure. (2) The desiccant flask valve VI was closed, and dry air was introduced into the
system through valve V,,. A pressure value PI, corresponding to pressure values obtainable for a, measurements (i.e. 10 to 60 mbar), was reached. Valves V3 and V0 were successively closed. (3) Pressure equilibrium was reached within 15-20 min, as described above, pressure PI was read and the sample flask valve V2 was closed. During this phase the desiccant was kept under vacuum by opening valve V,. (4) Valve V, was closed and valve VI was opened. Pressure equilibrium was reached within 5 min, and pressure P2 was read. (5) See step 11 of the operating procedure to measure water vapour pressure.
The volume ratio, R, was calculated by the ideal gas law as follows: (Pl
-P2) p2
(6)
B. Zunoni et al.
414
where V,, is the volume between the valve VI and the desiccant flask, and Vi is the volume between valves VI, V2 and V3. Reference materials
The following reference materials were used to set up and validate the VPM at 25°C: pure water; saturated salt solutions (i.e. LiCl, CH,COOK, KZC03, NaCl, KCl, BaC12, KzNG3, K#&); microcrystalline cellulose MCC Avicel@ PH-101 (Fluka Chemie AG, Buchs, Switzerland) samples at different moisture contents. Salt solutions were prepared following the method described by Spiess & Wolf (1987) within the European COST 90 project on water activity. Table 2 shows the quantity of salt and water used to prepare saturated salt solutions and the relevant a, values at 25”C, as reported by Greenspan (1977). MCC samples at different moisture contents were prepared according to the following procedure. A thin layer of MCC (120 g) was dried at 105°C to constant weight, and residual moisture was determined. The dehydrated sample (100 g) was placed into a hermetically sealed 500 ml glass bottle. The sample was then stored in a glass jar containing silica gel. The jar was kept at room temperature for 24 h to stabilize the dehydrated microcellulose. The bottled sample was rehydrated with distilled water. The quantity of water to be added was calculated according to the following equation:
w*=
i
;u,i -w,
(7)
2
TABLE 2 Preparation of Saturated Salt Solutions at 25°C (aw Values Are Taken From Greenspan, 1977) Salt
LiCl CH&OOK KzCO3 NaCl KC1 BaC&,* KzN03 KS04
Quantityof salt (g)
Quantityof water (ml)
Reference a,
75
42.5 32.5 45
0.1130+0.0027 0.2251 f 0.0032 0.4316 f 0.0039 0.7529 + 0.0012 0.8434 + 0.0026 0.9020 0.9358 f 0.0055 0.9730 f 0.0045
100 100 100 100 125 100 125
*Not reported by Greenspan (1977).
:z :: 35
Measuring a,,, with a capacitance manometer
415
where: tW2 = water (g) to be added to the sample; W1= water (g) present in the sample; U, = final sample moisture (%); D = sample dry matter (g); & = final sample dry matter (%). In order to homogenize the rehydrated sample, it was subjected to successive mechanical stirring in a roller stirrer at 25°C for 5 days. The sample was stored at 25°C until used. VPM tests
Trials for a, measurement were carried out in triplicate on pure water samples (5 g) at different evacuation times (1, 2, 5, 10, 30 and 60 s) and at different equilibrium times of pressure P1 (60,90, 120, 150 and 180 min). Trials for a, measurement were carried out in triplicate on saturated salt solution samples (5 g) at 5 s evacuation time and 120-180 min equilibrium time of pressure PI. Trials for a, measurement were carried out in quadruplicate on MCC samples (3 g) at different moisture contents, evacuation times (2 and 5 s) and equilibrium times of sample after evacuation (0,2 and 18 h). Eight trials were carried out at different times to calculate volume ratio. Measurement of MCC moisture
The moisture of MCC samples was measured in duplicate by oven-drying at 105°C to constant weight both before and after the VPM test.
RESULTS AND DISCUSSION Table 3 shows moistures of MCC samples. a, values were taken from the adsorption isotherm of MCC adsorption at 25”C, calculated by Spiess & Wolf (1987) according to the GAB model:
TABLE 3 Moisture Values and Corresponding a, Values of the MCC Reference Samples MCC samples
Rehydrated 1 Rehydrated 2 Rehydrated 3
Water content (g H20/IoOg dm)
6.41+ 0.07 s.20+0.05 13.64+0.07
Reference a,
0.58 + 0.01 0.71+0.01 0.93 &-0.01
B. Zunoni
416
et al.
TABLE4 Effect of Temperature on a, Value Sample
P (mbar)
Pw
Water K2SO4 K2CO3
31.7 30.9 13.7
25.2T
250°C a,
31.7 -
1.00 0.97 0.43
Pw
32.1 -
a,
0.99 0.96 0.42
nsmCkaw %=
25.6”C
25.4”C
(1 -ka,.,)(l -k+CkQ
P,
32.4 -
a,
0.98 0.95 0.42
P,
32.8 -
a,
0.97 0.94 0.41
(8)
where n,, water content on 100 g dry basis; nsm = 4.064, C = 8.776 and k = 0.772, GAB constants. VPM design The VPM designed by Benado & Rizvi (1987) was modified to improve both system temperature control and desiccant performance, resulting in simplified a, evaluation. Table 4 shows the effect of temperature on a, values for pure water and some saturated salt solutions. Considering pure water, the vapour pressure measured, P, is 31.7 mbar at 25°C (Weast, 1985). If the system temperature measurement is correct, P,,, is 31.7 mbar and the resulting water activity is correct, i.e. 1.00. If the temperature measured differs slightly from the real one, the value for water activity changes. For instance, as a result of a 0.4”C shift, P, is 32.4 mbar and water activity is 0.98. A slight deviation in temperature results in a considerable deviation in a,. This also holds for saturated salt solutions. Consequently, particular attention was paid to system temperature control. For example, automatic valves were selected to avoid heating up of the system. Generally speaking, this can be obtained by opening valves for a limited time. In our case, because of the long valve opening time (2-3 h) during PI reading, a valve V, was chosen, which was on when it was closed. In this way, during the opening step, valve V2 was off and, consequently, unable to perturb the system, thus allowing temperature-pressure equilibrium to be achieved. Four thermocouples were used to monitor the trends of sample, desiccant and pressure transducer temperatures and the temperature inside the thermostated cell. Monitoring of temperature allowed pressures to be measured under identical temperature conditions (see procedure to measure water vapour pressure). Using the desiccant was most important when the VPM was applied to measure foods containing not only water vapour but also other volatile components. In order to perform a correct evaluation of water vapour pressure, pressure PI should be rectified for the pressure exerted by nonadsorbing gas. This can be obtained by
417
Measuring a, with a capacitance manometer
using the desiccant, which adsorbs sample water vapour, resulting in a decrease in pressure to a residual value, Pz, exerted by the nonadsorbing gas. In order to improve the efficiency of the desiccant, a valve, V4, was included to connect the desiccant flask to the vacuum pump. As a result, during PI reading, continuous evacuation of the desiccant flask was obtained. Consequently, both nonadsorbing gas remained in the desiccant flask and a mixture containing water vapour and nonadsorbing gas coming from desiccant particles were removed. The use of valve V4, in addition to the use of either fresh or regenerated desiccant for each trial, allowed us to simplify equations to calculate a,, as described in Section 2. Volume ratio, R, correlated to the use of the desiccant, was an important design element for accuracy and repeatability of vapour pressure measurements. Opening of the desiccant flask resulted in pressure decrease, due to both water vapour adsorption by the desiccant and increase in system volume. Hence, pressure P2 must be corrected by a factor expressing volume variation. Volume ratio measurements should be accurate, and volume variation should not cause a decrease in pressure P2 to too low values, which would not be included in the noise of the pressure transducer. Volume ratio was evaluated using dry air as a sample. This procedure ensured that the pressure decrease occurring when opening the desiccant flask valve was caused by volume variation rather than water vapour adsorption by the desiccant. Table 5 shows experimental values for volume ratios, R, obtained using the same amount of desiccant as that used for a, measurements at different dry air pressures to evaluate the invariability of volume ratio under the various operating conditions applied to a, measurement of real samples. The R values obtained showed that using the desiccant flask caused a threefold volume increase and, consequently, a similar decrease in pressure. Hence, the VPM showed a higher decrease in pressure than the sensitivity of the pressure transducer. If the worst case, i.e. a sample such as LiCl with a low water activity, is taken into account, a value for PI of 3.5 mbar would theoretically be obtained. Since a threefold decrease in pressure PI occurs when opening the desiccant flask, a value for pressure Pz of 1.2 mbar would be obtained, which is by two orders of magnitude higher than the precision of the pressure transducer (50.01 mbar).
Experimentally PI (mbar) 96.6 57.2 49.8 37.7 21.1 17.8 14.6 12.1
Determined
Values
TABLE 5 for Volume 1.77 + 0.03)
Ratio
at
25°C
(Mean
Value+sd
=
P2 (mbar)
R = V,lv,
35.0 20.8 17.9 13.6 7.5
1.76 1.75 1.78 1.77 1.81 1.74 1.75 1.81
!z 413
418
B. Zunoni et al. TABLE 6
Influence of Evacuation and Equilibrium Times on ~1,of Pure Water (aw = 1.00) Equilibrium time (min)
Water vapour pressure (mbar)
a,
: 5* 5*
180 120 60 90
31.1 30.8 30.7 31.1
0.96 0.98 0.97 0.98
2 5
150 120 180
31.7 31.3 31.4
0.99 1.00 0.99
;: 60
120 150 120
31.8 32.1 32.1
1.00 1.01 1.01
Evacuation time (s)
Operating conditions: sample amount = 5 g; fresh desiccant = 10 g; R = 1.77; T, = 25°C. *Samples not in equilibrium.
Setting up of the operating procedure for a, measurement and validation of the apparatus Trials with pure water and saturated salt solutions Trials were carried out to define operating conditions to measure vapour pressure and to determine both the evacuation time of the sample and the procedure to achieve system pressure equilibrium. Table 6 shows results from trials carried out on pure water samples at 25°C at different evacuation times (1, 2, 5, 10, 30 and 60 s) and, for the trial at 5 s evacuation time, at different equilibrium times (60, 90, 120, 150 and 180 min). It should be noted that 1 or 2 s evacuation time did not allow us to measure pure water activity accurately: the error was -0.04 for 1 s and -0.02 for 2 s. A longer evacuation time, i.e. 5 to 60 s under pressure equilibrium conditions, resulted in a pure water activity value similar to the real one with a maximum error of fO.O1. Since the aim of the evacuation step was to remove air from the headspace of the sample, it should last long enough to remove all the air without perturbing the sample considerably. If evacuation time was inadequate, such as for 1 and 2 s evacuation, residual air caused an error in accuracy for measurement of vapour pressure, P. Because of residual air, the water vapour diffusion and subsequent adsorption in the desiccant was slower, resulting in overestimation of pressure P2 and underestimation of vapour pressure P (Benado & Rizvi, 1987). A too long evacuation time caused flash-evaporation of water in the sample, resulting in system temperature perturbation, due to sample cooling. Figure 3 shows the effect of perturbation on sample temperature using 5 s (graph a) and 60 s evacuation (graph b). In the first graph evacuation caused a 2°C decrease in temperature, as compared to the cell temperature; in the second graph a higher decrease (7°C) was observed. Therefore, 5 s evacuation time was chosen to maximize air removal and minimize system perturbation. Table 6 shows the effect of the equilibrium time required to obtain constant reading of pressure PI for trials carried out applying 5 s evacuation. For trials at 60
Measuring a,,, with a capacitance manometer
419
45
32
40
30
35
28
a
26 o^ 24 L
c 30 Q g 25
$
e!
22 6 E 20 :
3 20 o* h 15 10
18
5
16 14
0 0
20
40
80
60
100
120
140
160
The (min)
35
25
I I I
I
5
I
0 -+0
I I ..11
I I 20
40
60
60
100
120
16 14 140
Time (min)
Fig. 3. Trend of sample pressure (. . .) and temperature (-) tion time (a) and one at 60 s (b).
for one trial at 5 s evacua-
B. Zanoni
420
et al.
TABLE7 Comparison Between Experimental
a, Values for Pure Water and Saturated Salt Solutions by VPM and Corresponding Reference Values (Greenspan, 1977) at 25°C
Sample
P (mbar)
a,
Reference a,
Water KS% KzNG3 BaCl, KC1 NaCl
31.7kO.3 30.7 &-0.4 29.2kO.3 28.9 + 0.4 27.lkO.2 24.1 _t 0.3 13.5 kO.2 6.7+0.3 3.2kO.3
1.00+0.01 0.97 * 0.01 0.92 + 0.01 0.91 f 0.01 0.85 +0.01 0.76kO.01 0.43 * 0.01 0.21 f 0.01 O.lOfO.O1
1.00 0.97 0.93 0.90 0.84 0.75 0.43 0.22 0.11
&CO3
CH,COOK LiCl Operating
conditions:
sample amount = 5 g; fresh desiccant = 10 g; R = 1.77.
and 90 min equilibration, step 9 of the operating procedure was stopped when four successive 60-s readings differed by + 1 and fO.l mbar, respectively. For the other trials step 9 was stopped when four successive 60-s readings differed by +O.Ol mbar. The latter procedure allowed us to obtain accurate a, measurement and, consequently, was chosen to define the operating procedure to measure water vapour pressure. Figure 3 shows the trend of pressure during a complete trial: equilibrium of both pressure PI and pressure P2 was achieved. The pressure values were reported from the end of step 3 of the operating procedure and then, pressure values approximately equal to zero corresponded at steps 4 and 5. Table 7 shows accuracy and repeatability of experimental a, values for pure water and saturated salt solutions measured by the optimized procedure. Experimental data showed an accuracy of +O.Ol to -0.01 a, and a standard deviation of kO.01 a,. Experimental and reference data were not significantly different at PcO.01. Trials with MCC Trials were carried out with MCC to validate the VPM for water activity measurement of solid products. Results on activity of foods from the COST 90 project (Spiess & Wolf, 1987) were used as a reference. Table 8 shows results from a series of trials carried out on the rehydrated MCC sample 3. Percentage values for absolute humidity of samples both before, nos, and after the trial, nsf, measured a, values and reference u, values, extrapolated from the MCC isotherm (see eqn (8) according to the Itsf value, are shown. Results from trials carried out using the procedure optimized by trials with water and saturated salt solutions are reported in the first row. As can be seen, the experimental a, value was subject to a significant error of -0.04. This suggested that the shift could be caused by sample perturbation during evacuation, as shown by a considerable decrease, i.e. about 15%, in absolute humidity of the sample within the end of the trial. As a result of perturbation, a, measurement was inaccurate because it had been carried out on a sample not in equilibrium, showing nonhomogeneities in moisture and water activity.
Measuring a, with a capacitance manometer
421
TABLE 8 Comparison Between Experimental a, Values for Rehydrated MCC Sample 3 by VPM and Corresponding Reference Values (Spiess & Wolf, 1987) at 25°C Sample operating conditions 5 s
evacuation 2 s evacuation 5 s evacuation equilibration
time time time and 18 h time
n, (W
P (mbar)
13.64+0.07 13.64+0.07 13.64+0.07
26.1kO.3 26.1kO.3 27.250.2
a,
nSf VW
0.83”+0.01 11.6kO.3 0.82”_tO.O1 12.2kO.3 0.86b+0.01 11.7f0.3
Ref:
a,
0.87b+0.01 0.88b$0.01 0.87b+0.01
rzsoand nsf are sample absolute moistures (g/100 g dry matter) before and after the VPM test,
respectively. a and b show a significant difference at PcO.01. Operating conditions: sample amount = 3 g; fresh desiccant = 10 g; R = 1.77.
In order to minimize sample perturbation, the following two elements were taken into account: reduction of evacuation time and sample equilibrium after evacuation. Regarding the first element, results at 2 s evacuation are shown in the second row in Table 8. Evacuation time reduction did not improve the apparatus performance: a, was subject to the same significant error as that identified for trials at 5 s, and a slightly lower, i.e. about ll%, variation in absolute humidity of the sample was observed. The second element, used to minimize the effect of perturbation, was sample equilibrium after evacuation to obtain moisture homogeneity. Equilibrium of the sample, evacuated into the sample flask, which had been isolated by the system by closing valve Vz, was achieved after leaving overnight (18 h). The efficiency of equilibrium on VPM performance is shown in the third row in Table 8. The behaviour of rehydrated MCC samples 1 and 2 was similar to that of the rehydrated sample 3. Table 9 shows the accuracy and repeatability of experimental a, values for MCC samples measured by the optimized procedure. Moisture data show that sample evacuation resulted in sample dehydration from 8 to 14% as a function of the sample initial moisture. Experimental data showed an accuracy of - 0.01 a, and a standard deviation of kO.01 a,. Experimental and reference data were not significantly different at PcO.01. TABLE 9 Comparison Between Experimental a, Values for MCC Samples by VPM and Corresponding Reference Values (Spiess & Wolf, 1987) at 25°C Sample
Rehydrated 1 Rehydrated 2 Rehydrated 3
nsO(W
6.41+ 0.07 8.20 + 0.05 13.64 + 0.07
P (mbar)
16.5 + 0.4 19.7 + 0.5 27.2 + 0.2
a,
0.52 & 0.01” 0.62 + O.Olb 0.86 + 0.01’
nsf PW
5.9kO.l 7.1 f0.2 11.7f0.3
ReJ a,
0.53 + 0.01” 0.63 f O.Olb 0.87 f 0.01”
nsOand nsf are sample absolute moistures (g/100 g dry matter) before and after the VPM test, respectively. a, b and c show a significant difference at PxO.01. Operating conditions: sample amount = 3 g; fresh desiccant = 10 g; evacuation time = 5 s; equilibrium time of the evacuated sample = 18 h; R = 1.77.
422
B. Zunoni et al.
CONCLUSIONS The VPM apparatus designed was able to provide accurate and precise measurements in the whole range of a, using pure water and saturated salt solutions as reference standards. The VPM was also accurate and precise in measuring a, of MCC samples. This demonstrates that both the VPM design and the methodology by Benado & Rizvi (1987) are valid in a wide range of a,. This study allowed us to identify the following two critical points of the VPM for accurate and precise measurements: (i) evacuation time and (ii) time of sample equilibrium after evacuation. The evacuation time controls the presence of non-water vapours in the apparatus, which hinders water vapour diffusion and subsequent adsorption in the desiccant (Benado & Rizvi, 1987). The optimized evacuation time was 5 s. The time of sample equilibration was critical in solid samples, where evacuation resulted in sample dehydration. The optimal time was 18 h. Based on this work, the following considerations on applicability of the VPM to measure water activity of foods can be drawn: The VPM is accurate and reproducible throughout the a, interval. The VPM requires difficult, accurate setting up of design and operating procedures. If products show dehydration during evacuation, the VPM can be used to build adsorption and desorption isotherms. In this case, sample moisture should be measured after the trial (Lewicki, 1989). The VPM cannot be used to measure the water activity of the sample because the a, value is modified during the measurement as a function of the sample initial moisture. This may explain the differences between VPMs and some hygrometers found by Stamp et al. (1984). It may be concluded that research should be carried out to validate the VPM methodology on various solid foods (e.g. real foods from the COST 90 project) at different moisture contents and to show the significance of dehydration. Research should also be carried out on the VPM design to remove or minimize dehydration during evacuation. REFERENCES Benado, A. L. & Rizvi, S. S. H. (1987). Water activity calculation by direct measurement of vapor pressure. J. Food Sci., 2, 429-432. Emodi, A. S. & Lechowich, R. V. (1969). Low temperature growth of type E Clostridium botulinum spores. 2. Effect of solutes and temperature. J. Food Sci., 34, 82-87. Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. J. Res. Nat. Bureau St&., 81A, 89-96. Iabuza, T. P., Acott, K., Tatini, S. R. & Lee, R. Y. (1976). Water activity determination: a collaborative study of different methods. J. Food Sci., 41, 910-917. Lewicki, P. P. (1987). Design of water vapour pressure manometer. J. Food Eng., 6,405-422. Lewicki, P. P. (1989). Measurement of water activity of saturated salt solutions with the Vapour Pressure Manometer. .I. Food Eng., 10, 39-55. Makower, B. & Myers, S. (1943). A new method for the determination of moisture in dehydrated vegetables. In Proc. Inst. Food Technologists4th Conf., pp. 156-164. Nunes, R. V., Urbican, M. J. & Rotstein, E. (1985). Improving accuracy and precision of water activity measurement with a water pressure manometer. J. Food Sci., 50, 148-149.
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Prior, B. A. (1979). Measurement of water activity in foods: a review. J. Food Prot., 42, 668-674. Rahman, S. (1995). Food Properties Handbook. CRC Press, New York, pp. l-86. Rizvi, S. S. H. (1995). Thermodynamic properties of foods in dehydration. In: Engineeting g3~$F of Foods, M. A. Rao and S. S. H. Rizvi (Eds.), Marcel Dekker, New York, pp. Silverman, G. J., Munsey, D. T., Lee, C. & Ebert, E. (1983). Interrelationship between water activity, temperature and 5% oxygen on growth and enterotoxin A secretion by Staphylococcus aureus in precooked bacon. J. Food Sci., 48,1783-1786. Spiess, W. E. L. & Wolf, W. (1987). Critical evaluation of methods to determine moisture sorption isotherms. In: WaterActivity: 7heov and Applications to Food, L. B. Rockland and L. R. Beuchat (Eds.), Marcel Dekker, New York, pp. 215-233. Stamp, J. A., Linscott, S., Lomauro, C. & Labuza, T. P. (1984). Measurement of water activity of salt solutions and foods by several electronic method as compared to direct vapor pressure measurement. J. Food Sci., 49, 1139-l 142. Troller, J. (1983). Methods to measure water activity. J. Food Prot., 46, 129-134. Troller, J. (1983). Water activity measurement with a capacitance manometer. J. Food Sci., 48,739-741. Weast, R. C. (1985). Handbook ofChemistry and Physics, CRC Press, New York, pp. D-190.