Water activity and dielectric properties of gels in frequency range 200MHz to 6GHz

Transcription

1 Water activity and dielectric properties of gels in frequency range 200MHz to 6GHz Sylvie CLERJON, Jean-Dominique DAUDIN, Jean-Louis DAMEZ Centre INRA de Clermont-Ferrand-Theix 1

2 About gels Water Activity measurements Microwave measurements Results Conclusions 2

3 Meat and fermented/dehydrated meat products, cheese products 0.5 (33%) < X eau < 3 (75%) 0 < X sel < < ph < 6 Animal gelatine 1 (50%) < X eau < 10 (90%) 0 < X sel < 0.12 ph = 5.2 ± Homogeneous Isotropic Viscosity compatible with dielectric measurements Easy to prepare

4 Gelatine PS 60 Bloom SKW Biosystems SAS, Angoulême, France ClNa 99.5% ACROS 45 gels samples Gelatine powder Distilled water salt 40 to 80 C 24 h at 18 C 4

5 About gels Water Activity measurements Microwave measurements Results Conclusions 5

6 Water Activity Measurements humid air flow v 10 m.s -1 saturation column HR 100 % Heating resistance T water1 T R 10 cells Novosina AW SPRINT HR = 97 % HR = 98 % HR = 96 % Hygrometer T R From 6 to 100%RH From 0 to 50 C Sample 6 Pump Diaphragms P=1000 Pa T water2 =T isot herm

7 7 Novosina measurements Sorption isotherms Samples 45 not dehydrated samples 5 series of 10 samples. Each series is obtained from a native sample (Xeau =3), and contains 10 final samples with different dehydration rates. Water content range 1 to 10 (50 to 90%) 0.5 to 3 (33 to 75%) Advantages Samples size allows dielectric coaxial measurements Continuous relationship on a wide range of water activity Drawbacks Limited low moisture Final samples size not compatible with coaxial dielectric measurements. Need of cavity measurements (not yet performed) For this study, we only compare dielectric measurements with Novosina data Sorption isotherm allows comparison with Novosina in order to: Wide moisture range a w and dielectric measurements Using sorption isotherm with dielectric [salt] and [water] measurements

8 Water content (kg/kg dry basis) Sodium Chloride 8% Sodium Chloride 4% Without Sodium Chloride Ripoche (1995) Water Activity

9 Water content (kg / kg dry basis) Sodium Chloride 8% Sodium Chloride 6% Sodium Chloride 4% Sodium Chloride 2% Without S odium Chloride Pseudomonas Salmonella Escherichia coli Water Activity

10 About gels Water Activity measurements Microwave measurements Results Conclusions 10

11 Dielectric E r Relaxation ε' Frequency (GHz) ε"

12 Dielectric Relaxation E r Water Activity decrease proteins ε' ε' ε" ε" 12 frequency

13 Hewlett Packard Coaxial Probe Geometry Maxwell s relations ε*=f(γ*) Contact : air pressure homogeneity 1 cm Accuracy is about 5% All measurements are done at 18 C This contact probe could be replace with an antenna to obtain Γ* 13

14 ε' ε ' ε ' Debye relation ( ) ' s s ( ω) = ε + and ε" ( ω) = ω τ ε ' ε ' 1+ ω τ ωτ Water Activity decrease 30 ε' ε" Our experimental range Relaxation Frequency f 14

15 About gels Water Activity measurements Microwave measurements Results Conclusions 15

16 Results ε 10 ε ' " Imaginary part of permittivity ε" w a = E+09 5.E+09 5.E+09 6.E+09 6.E+09 Frequency (Hz) ε " [ 30;70] frequency (Hz) Real part of permittivity ε ε ε' ε" to high frequencies to low frequencies 16

17 ε'/ε" (30-70) Water content (kg / kg dry basis) Sodium Chloride 8% Sodium Chloride 6% Sodium Chloride 4% Sodium Chloride 2% Without Sodium Chloride Pseudomonas Salmonella Escherichia coli Water Activity exponential fit Dielectric properties a w Water content Salt content experimental data Water activity

18 Relaxation frequency ε' Frequency (GHz) ε" 9 8 r 2 =0.93 Relaxation Frequency (GHz) % Water Activity

19 About gels Water Activity measurements Microwave measurements Results Conclusions 19

20 Some conclusions frequency ε' ε' ε" ε" Water Bounding? Dielectric properties In salted animal gelatine material ε /ε (ε [30-70]) Relaxation Frequency Another a w depressor Measuring water and state content with a microwave sensor and use sorption isotherm to control a w Extend moisture range by using sorption isotherm and cavity dielectric measurements 20 Real food products