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Study of the combined effect of the freezing process and storage conditions on the quality of frozen products. : Application to porous matrices

Etude de l'impact couplé du procédé de congélation et des conditions d'entreposage sur la qualité des produits surgelés. : Application aux matrices poreuses

Author(s) : ZENNOUNE A.

Type of monograph: Doctoral thesis

Summary

During the freezing of food products, an ice crystalline structure is developped and continues to evolve throughout the cold chain, particularly during the storage stages when the products are likely to be subject to temperature variations. The phenomena of crystallisation and recrystallisation will contribute to modify the microstructure of the products and thus alter their quality. In this context, the main objective of this thesis is to study the coupled impact of freezing and storage conditions on the evolution of the microstructure of a frozen sponge cake, in order to better understand the mechanisms of crystallisation and recrystallisation as well as the formation and location of ice in a porous matrix.Two types of sponge cake were studied: a non-reactive model sponge cake and a reactive reference sponge cake. Their manufacturing protocol was validated by a series of measurements of thermo-physical properties (densities, water content, porosity). DSC measurements were carried out to determine the freezing point and the fraction of freezable water in the baked sponge cake. Different techniques were then used to characterise the evolution of the microstructure of the frozen sponge cake during the cold chain: cryo-MEB and X-ray microtomography. Concerning microtomography, two types of tests were performed: (i) samples of model sponge cake were frozen at two different rates (slow and fast freezing) and then stored for two weeks under stable or fluctuating temperature conditions. These samples were then visualized in the frozen state at very high resolution using a CellStat thermostat cell installed at the ANATOMIX beamline of synchrotron Soleil; (ii) In the case of the reference sponge cake, the evolution of the microstructure of the same sponge cake sample during its freezing and storage (with temperature fluctuations) could be followed continuously for 6 days using the CellDyM thermostat cell installed on a laboratory microtomograph. In this case, a specific image processing method based on Digital Volume Correlation (DVC) was implemented to segment the images and follow the evolution of the microstructure.The results show that rapid and slow freezing of the model sponge cake result in (i) small and coarse ice crystals within the starch matrix and (ii) more of less thicker ice layers at the pore interfaces. In the case of slow freezing, the majority of the ice forms at these interfaces reflecting greater water migration from the matrix to the pores. During storage, the crystalline microstructure changes regardless of the initial freezing rate and storage conditions. However, the microstructure changes are more important when the products are frozen slowly regardless of the storage conditions. Furthermore, our results show that although the benefits of fast freezing are reduced at the end of the two-week storage period, the products still have a better quality than the slow-frozen samples. Finally, the results obtained with the CellDyM cell allowed to highlight different phenomena, such as the shrinkage of the porous matrix during freezing and thermal cycling as well as the formation and growth of crystals at the pore interface which are preferential sites for ice nucleation. All these original results open many perspectives in terms of modelling and optimisation of cold chain parameters to preserve product quality.

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Pages: 216 p.

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Details

  • Original title: Etude de l'impact couplé du procédé de congélation et des conditions d'entreposage sur la qualité des produits surgelés. : Application aux matrices poreuses
  • Record ID : 30032289
  • Languages: French
  • Subject: Technology
  • Publication: AgroParisTech - Antony - France
  • Publication date: 2022/03/31

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