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Dipartimento di Fisica - Politecnico di Milano

Photonics for Food (PHOOD)


ACTIVITIES
The laboratory deals with the development, characterization and application of advanced photonic techniques for nondestructive quality assessment of fruit and vegetables.
In particular time-resolved reflectance spectroscopy (TRS) is used to provide a complete optical characterisation with the simultaneous non-invasive measurement of the optical properties (absorption and scattering) of diffusive media like biological tissue. This can be of special interest for most fruits and vegetables, as well as other foods (e.g. meat, fish, cheese), because information derived by TRS refers to the internal properties of the medium, and is not significantly affected by surface features. A short (picosecond) light pulse is injected into the medium to be characterised and either the transmitted or the diffusely reflected pulse is detected. Attenuation, delay and changes in pulse shape are due to the absorption and scattering events occurring into the medium, and an estimate of the optical parameters can be derived from the measurement of the reflected or transmitted pulse, based on a suitable model of light propagation (Diffusion theory or Random Walk model). From the assessment of the absorption and scattering coefficients, information can then be derived on the composition and internal structure of the medium. Main advantages of the technique are the following: absolute non-invasiveness, potentiality for non-contact measurements, and capacity to probe internal properties with no influence from the skin.
The research group was the first worldwide to apply TRS for the nondestructive optical characterisation of fruits and vegetables (apples, kiwis, peaches, melons, tomatoes) and for the correlation of optical properties with conventional quality parameters (firmness, total soluble solids, acidity) and with the noninvasive detection of internal defects in pears, apples, plums and peaches.


INSTRUMENTATION
The laboratory is equipped with two advanced TRS prototypes.

A multiwavelength protable TRS prototype. The light source consists of a supercontinuum fibre laser providing white-light pulses (duration <10 ps, repetition rate 40 MHz, 450-2400 nm bandwidth, total power <6 W). A couple of 8 position filter wheels loaded with band-pass interference filters  is used to sequentially select light at 14 discrete wavelengths in the 540-900 nm spectral range. Multimode graded index fibers are used to deliver light to the sample. Light diffusely transmitted through the sample is detected using 1 mm plastic-glass fibers coupled to a photomultiplier. The signal from the detector is driven to a time-correlated single-photon counting board. The acquisition of the time-resolved curves and the synchronous movement of the interference filter wheels and the variable neutral density filters are automatically controlled by a PC.

A compact prototype for TRS at discrete wavelength. The light source is a pulsed laser diode working at 670 nm, with 80 MHz repetition frequency, 100 ps duration, and 1 mW average power. A compact photomultiplier and an integrated PC board for time-correlated single photon counting are used to detect TRS data. A band pass filter tuned at the laser wavelength is used to cut off the fluorescence signal due to chlorophyll. The system is supplied with a portable electrical power generator to allow for measurements in the field.


PROJECTS

DIFFRUIT, EU FP4, 1996-1999
TRS APPLE, MAFF (UK), 2000
AGROTEC, MIUR (I), 2000-2002
CUSBO, LASERLAB, EU FP5+FP6+FP7, 2004-2014
INSIDEFOOD, EU FP7 2009-2013
TROPICO, Regione Lombardia (I), 2010-2012
3D Mosaic, EU ICT-AGRI, 2011-2013
USER-PA EU ICT-AGRI 2013-2016


COLLABORATIONS

CREA-IAA, Milan (I), Paola Eccher-Zerbini , Anna Rizzolo, Maristella Vanoli, Maurizio Grassi
Laimburg (I), Angelo Zanella
Agricultural Research Organization, Bet Dagan (Israel), Susan Lurie, Victor Alchanitis
Wageningen Universiteit (NL), Pol Tijskens, Olaf Van Kooten, Rob Schouten
Planteforsk, Lofthus  (N),  Eivind Vangdal
Potsdam (D), Manuela Zude-Sasse
Leuven (B), Bart Nicolai, Bert Verlinden, Wouter Sayes, Pieter Verboven, Maarten Hertog
UPM, ETSI Agronomos Madrid (E),  Margarita Ruiz-Altisent, Constantino Valero
Horiculture Research International, East Malling, (UK) - David Johnson, Colin Dover



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