The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Effects of Prefreezing Treatments on the Structure of Strawberries and Jams

Marjaana Suutarinen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Chemical Technology for public examination and debate in Auditorium KE 2 (Komppa Auditorium) at Helsinki University of Technology (Espoo, Finland) on the 19th of April, 2002, at 12 noon.

Overview in PDF format (ISBN 951-38-5980-0)   [717 KB]
VTT Publications 462, ISSN 1455-0849

Dissertation is also available in print (ISBN 951-38-5979-9)
Copyright © 2002 VTT Technical Research Centre of Finland
VTT Publications 462, ISSN 1235-0621
VTT-PUBS-462
TKK-DISS-1604

Abstract

Different chemical components and their locations in strawberry (Fragaria x ananassa) achenes, vascular bundles and cortical cell walls were studied by means of bright-field microscopy using different staining systems and by Fourier transform infrared (FT-IR) microscopy. The structural changes in pretreated strawberry tissues during freezing and thawing were studied by means of physical and chemical analyses as well as by bright-field and FT-IR microscopy, and by confocal laser scanning (CLS) microscopy. In addition, traditional jams made from pretreated frozen strawberries were investigated by means of physical and chemical analyses and by sensory evaluation.

Basic microstructural studies showed that all the cell walls were complex and consisted of several components. Lignin was an important component of achenes and vascular tissues, whereas the cortical cell walls contained mainly cellulose, pectin and some protein. Cellulose and pectin were also detectable in the intercellular spaces of cortex, in the vascular tissue and pith. Protein was deposited inside the cortical cells, as well as in the vascular tissue and pith. The microscopical methods used gave comparable results.

According to microscopical studies both the pretreatments with calcium chloride (CaCl2) and crystallised sucrose as well with CaCl2 and pectin methyesterase (PME) in a vacuum affected the microstructure of strawberry tissues. These pretreatments especially affected pectin, protein, lignin and structural carbohydrates in the vascular tissue and cortex compared to the untreated reference samples. The use of a vacuum appeared to make the pretreatment solutions absorb more efficiently to the cortex and pith, thus improving the stabilisation particularly of pectin and structural carbohydrates.

Firmness of thawed and particularly of jam strawberries pretreated with CaCl2 and PME in a vacuum was higher than that of other pretreated or untreated berries. In all the prefreezing treatments studied, dipping of strawberries into a CaCl2 solution with PME in a vacuum resulted in a significantly different sensory profile than was found in the other jams. The sensory attributes wholeness of the berries (p<0.001), firmness, clarity and evenness of the jam medium (p<0.001), softness of the berries (p<0.001) and faultlessness of odour and flavour (p<0.001) in particular were statistically significantly different among the strawberry jams. Sensory quality was perceived to decrease during 4 months of storage, even though the shapes of the sensory profiles of the studied jams did not change significantly from those evaluated after 2 weeks storage.

For achieving high quality jams the pretreatment time should be short (about 5-15 min), the temperature low (below 20 °C), the vacuum level high (pressure less than 10 kPa), the CaCl2 concentration moderate (about 1%) and the dosage of PME comparatively low (about 50-100 μkat/kg strawberries).

This thesis consists of an overview and of the following 5 publications:

  1. Suutarinen, J., Änäkäinen, L. and Autio, K. 1998. Comparison of light microscopy and spatially resolved fourier transform infrared (FT-IR) microscopy in the examination of cell wall components of strawberries. Lebensm.-Wiss. u. -Technol. 31, 595-601.
  2. Suutarinen, J., Heiska, K., Moss, P. and Autio, K. 2000. The effects of calcium chloride and sucrose prefreezing treatments on the structure of strawberry tissues. Lebensm.-Wiss. u. -Technol. 33, 89-102.
  3. Suutarinen, J., Honkapää, K., Heiniö, R.-L., Autio, K. and Mokkila, M. 2000. The effect of different prefreezing treatments on the structure of strawberries before and after jam making. Lebensm.-Wiss. u. -Technol. 33, 188-201.
  4. Suutarinen, J., Honkapää, K., Heiniö, R.-L., Autio, K., Mustranta, A., Karppinen, S., Kiutamo, T., Liukkonen-Lilja, H. and Mokkila, M. 2001. Effects of calcium chloride-based prefreezing treatments on the quality factors of strawberry jams. J. Food Sci. Accepted to be published.
  5. Suutarinen, J., Honkapää, K., Heiniö, R.-L., Mustranta, A., Liukkonen-Lilja, H. and Mokkila, M. 2001. Modeling of calcium chloride and pectin methylesterase prefreezing treatments of strawberries and jams. J. Food Sci. Accepted to be published.

Keywords: calcium chloride, pectin methylesterase, sucrose, vacuum, strawberry, structure, prefreezing, jam

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© 2004 Helsinki University of Technology

Last update 2011-05-26