Abstract

Spectra of direct and scattered light that passed through a tank of water mixed with up to 25ml of homogenized skim milk were measured with a spectroradiometer in a classic experiment used to illustrate why the sky is blue and why the Sun turns red near the horizon. The direct light penetrating the tank was reddened by preferential scattering of short waves by the milk particles (protein casein micelles and fat globules). Scattered light was blue near the light source when the optical thickness was small and red far from the source when the optical thickness was large. The measured radiance spectra and Mie theory were used to estimate that the optically effective mean diameters of protein casein micelles and fat globules were 170 and 610nm.

© 2008 Optical Society of America

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References

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  1. C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Wiley, 1987), pp. 104-108.
  2. G. Seneca Jones, “Studies in light VIII: color,” Self Culture 5, 89-95 (1897), http://books.google.com/books?id=1gFQAAAAMAAJ&pg=PA89&dq=milk+water+light++blue+red&lr=#PPA88,M1.
  3. M. Kerker, “Founding fathers of light scattering and surface-enhanced Raman scattering,” Appl. Opt. 30, 4699-4705, (1991).
    [CrossRef] [PubMed]
  4. G.Bylund, ed., Dairy Processing Handbook (Tetra Pak Processing Systems, 1995).
  5. P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).
  6. S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
    [CrossRef]
  7. C. L. Crofcheck, F. A. Payne, and M. P. Mengüç, “Characterization of milk properties with a radiative transfer model,” Appl. Opt. 41, 2028-2037, (2002).
    [CrossRef] [PubMed]
  8. M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
    [CrossRef]
  9. D. S. Horne, “Casein structure, self-assembly and gelation,” Curr. Opin. Colloid Interface Sci. 7, 456-461 (2002).
    [CrossRef]
  10. Malvern Instruments, “Measurement of dairy and food emulsions using laser diffraction,” publication MRK 497-01, http://www.malvern.co.uk/common/downloads/campaign/mrk497-01.pdf.
  11. C. H. I. McCrae and A. Lepoetre, “Characterization of dairy emulsions by forward lobe laser light scattering-application to milk and cream,” Int. Dairy J. 6, 247-256 (1996).
    [CrossRef]
  12. Photo Research, Inc., 9731 Topanga Canyon Place, Chatsworth, Calif. 91311, USA.

2002 (3)

C. L. Crofcheck, F. A. Payne, and M. P. Mengüç, “Characterization of milk properties with a radiative transfer model,” Appl. Opt. 41, 2028-2037, (2002).
[CrossRef] [PubMed]

M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
[CrossRef]

D. S. Horne, “Casein structure, self-assembly and gelation,” Curr. Opin. Colloid Interface Sci. 7, 456-461 (2002).
[CrossRef]

1996 (1)

C. H. I. McCrae and A. Lepoetre, “Characterization of dairy emulsions by forward lobe laser light scattering-application to milk and cream,” Int. Dairy J. 6, 247-256 (1996).
[CrossRef]

1991 (2)

M. Kerker, “Founding fathers of light scattering and surface-enhanced Raman scattering,” Appl. Opt. 30, 4699-4705, (1991).
[CrossRef] [PubMed]

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

1897 (1)

G. Seneca Jones, “Studies in light VIII: color,” Self Culture 5, 89-95 (1897), http://books.google.com/books?id=1gFQAAAAMAAJ&pg=PA89&dq=milk+water+light++blue+red&lr=#PPA88,M1.

Alexandera, M.

M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
[CrossRef]

Bohren, C. F.

C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Wiley, 1987), pp. 104-108.

Crofcheck, C. L.

Geurts, T. J.

P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).

Hadziyev, O.

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

Horne, D. S.

D. S. Horne, “Casein structure, self-assembly and gelation,” Curr. Opin. Colloid Interface Sci. 7, 456-461 (2002).
[CrossRef]

Jellema, A.

P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).

Jones, G. Seneca

G. Seneca Jones, “Studies in light VIII: color,” Self Culture 5, 89-95 (1897), http://books.google.com/books?id=1gFQAAAAMAAJ&pg=PA89&dq=milk+water+light++blue+red&lr=#PPA88,M1.

Kerker, M.

Lepoetre, A.

C. H. I. McCrae and A. Lepoetre, “Characterization of dairy emulsions by forward lobe laser light scattering-application to milk and cream,” Int. Dairy J. 6, 247-256 (1996).
[CrossRef]

Leserb, M.

M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
[CrossRef]

McCrae, C. H. I.

C. H. I. McCrae and A. Lepoetre, “Characterization of dairy emulsions by forward lobe laser light scattering-application to milk and cream,” Int. Dairy J. 6, 247-256 (1996).
[CrossRef]

Mengüç, M. P.

Noomen, A.

P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).

Ooraikul, B.

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

Ozimek, L.

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

Payne, F. A.

Rojas-Ochoaa, L. F.

M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
[CrossRef]

Schurtenbergera, P.

M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
[CrossRef]

Srilaorkul, S.

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

van Boekel, M. A. J. S.

P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).

Walstra, P.

P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).

Wolf, F.

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

Appl. Opt. (2)

Curr. Opin. Colloid Interface Sci. (1)

D. S. Horne, “Casein structure, self-assembly and gelation,” Curr. Opin. Colloid Interface Sci. 7, 456-461 (2002).
[CrossRef]

Int. Dairy J. (1)

C. H. I. McCrae and A. Lepoetre, “Characterization of dairy emulsions by forward lobe laser light scattering-application to milk and cream,” Int. Dairy J. 6, 247-256 (1996).
[CrossRef]

J. Colloid Interface Sci. (1)

M. Alexandera, L. F. Rojas-Ochoaa, M. Leserb, and P. Schurtenbergera, “Structure, dynamics, and optical properties of concentrated milk suspensions: an analogy to hard-sphere liquids,” J. Colloid Interface Sci. 253, 35-46 (2002).
[CrossRef]

J. Dairy Sci. (1)

S. Srilaorkul, L. Ozimek, B. Ooraikul, O. Hadziyev, and F. Wolf, “Effect of ultrafiltration of skim milk on casein micelle size distribution in retentate,” J. Dairy Sci. 74, 52-57 (1991).
[CrossRef]

Self Culture (1)

G. Seneca Jones, “Studies in light VIII: color,” Self Culture 5, 89-95 (1897), http://books.google.com/books?id=1gFQAAAAMAAJ&pg=PA89&dq=milk+water+light++blue+red&lr=#PPA88,M1.

Other (5)

C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Wiley, 1987), pp. 104-108.

G.Bylund, ed., Dairy Processing Handbook (Tetra Pak Processing Systems, 1995).

P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel, Dairy Technology (Marcel Dekker, 1999).

Malvern Instruments, “Measurement of dairy and food emulsions using laser diffraction,” publication MRK 497-01, http://www.malvern.co.uk/common/downloads/campaign/mrk497-01.pdf.

Photo Research, Inc., 9731 Topanga Canyon Place, Chatsworth, Calif. 91311, USA.

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Figures (8)

Fig. 1
Fig. 1

Design of the experiment, showing positions of the spectroradiometer relative to the light source for scattered and direct penetrating light. All dimensions are given in centimeters.

Fig. 2
Fig. 2

Spectra of direct light penetrating a tank of 2950 ml of water with 0, 10, and 20 ml of Puleva_AD skim milk.

Fig. 3
Fig. 3

Spectra of the ratios of direct light radiance I 20 I 10 and I 10 I 0 for Puleva_AD milk, confirming the validity of Bouguer’s law. Spectra indicate that the second increment contained 7.5% less milk than the first.

Fig. 4
Fig. 4

Measured values of optical depth, τ (solid line) and Ångstrom coefficient, α (dotted line) for direct light passing through the tank with 20 ml of Puleva_AD milk. The small, irregular variations for α result from the small wavelength increment ( Δ λ = 4 nm ).

Fig. 5
Fig. 5

Mie scattering model values of τ (thin contours) and α (thick contours) as a function of r FAT and r PRT for milk with fat content 0.3% and protein content 3.2%. Filled circles are measured values of τ and α for Puleva_AD and Covap milk.

Fig. 6
Fig. 6

Electron micrograph of a dilute mixture of MAS skim milk in distilled water. There are two distinct sizes of particles. These may be desiccated remnants of the smaller protein casein micelles and the larger fat globules. The scale is shown at lower left, and measured diameters of two particles are shown.

Fig. 7
Fig. 7

Spectra of radiance of direct light with no milk (thick curve), and scattered light deflected by 90 ° both 3 cm from the near end of the tank with 10 ml (thin curve) and 3 cm from the far end of the tank with 20 ml (dashed curve) of Puleva_Calcio milk.

Fig. 8
Fig. 8

Ratio of minimum to maximum radiances of polarized light scattered by 90 ° from the direct beam on near and far sides of the tank and with 15 and 20 ml of Puleva_Calcio milk.

Tables (1)

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Table 1 Mie Scattering Theory Calculations of r FAT r PRT a

Equations (2)

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I ( λ ) = I 0 ( λ ) e τ .
α = Δ ln ( I ) / Δ ln ( λ ) .

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