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Aalto

Reactivity of Lipids During Cereal Processing

Pekka Lehtinen

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 12th of June, 2003, at 12 noon.

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Dissertation is also available in print (ISBN 951-22-6476-5)

Abstract

The study elucidates factors affecting the reactivity of lipids in multiphase food systems, such as processed cereal food products. By using oat and oat products as model materials, both enzymatic and non-enzymatic lipid reactions were studied in aqueous suspensions and in dry flour. The information obtained can be used to improve existing cereal processing schemes and to develop new processing technologies for obtaining high quality food products with enhanced shelf stability.

In aqueous cereal processes the most important factors affecting the rate of enzymatic lipid reactions were identified to be the availability of lipid substrate to lipolytic enzymes and the total lipolytic enzyme activities present in the system. The ratio between the lipid substrate and flour material was critical in determining the reaction kinetics. When the ratio of lipid to flour was low, the reaction kinetics was set by the availability of substrate whereas, in the opposite case, the reaction kinetics was controlled by lipolytic activities. Different cereals processes represent examples of these two cases. In different cereal materials, the change from substrate limited to an enzyme limited kinetics occurred at different lipid to flour ratios. A simple oat fractionation scheme was identified where the substrate limited kinetics persisted also at high lipid concentrations. Such a fraction is well suited as a processing aid for aqueous processes in which the reactivity of lipids needs to be controlled.

In water activities that are encountered during storage of dry flour, two reactions of lipids were identified in processed oat: enzymatic hydrolysis of acylglycerols, and non-enzymatic oxidation of unsaturated fatty acid moieties acylated to polar lipids. The general practice to prevent enzymatic reactions by hydrothermal inactivation was noticed to actually provoke non-enzymatic lipid oxidation. This, in turn, leads to the development of oxidative rancidity upon prolonged storage of oat products. Thus, in order to prevent the unwanted reactions of lipids upon storage at low water activities, the careful control of hydrothermal treatment is crucial and should meet two requirements: elimination of the endogenous lipolytic activity, and prevention of the subsequent lipid oxidation.

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

  1. Lehtinen P. and Laakso S., 1997. Antioxidative-like effect of different cereals and cereal fractions in aqueous suspension. Journal of Agricultural and Food Science 45, pages 4606-4611.
  2. Lehtinen P. and Laakso S., 1998. Effect of extraction conditions on the recovery and potency of antioxidants in oat fibre. Journal of Agricultural and Food Science 46, pages 4842-4845.
  3. Lehtinen P. and Laakso S., 2000. Inhibition of linoleic acid oxidation by fatty acid binding oat protein fraction. Journal of Agricultural and Food Science 48, No. 11, pages 5654-5657.
  4. Heiniö R.-L., Lehtinen P., Oksman-Caldentey K.-M. and Poutanen K., 2002. Differences between sensory profiles and development of rancidity during long-term storage of native and processed oat. Cereal Chemistry 79, No. 3, pages 367-375. © 2002 American Association of Cereal Chemists. By permission.
  5. Lehtinen P., Kiiliäinen K., Lehtomäki I. and Laakso S., 2003. Effect of heat treatment on lipid stability in processed oats. Accepted in Journal of Cereal Science 37, No. 2, pages 215-221. © 2003 Elsevier Science. By permission.

Keywords: Avena sativa, oat, oxidative rancidity, hydrolytic rancidity, lipase, lipoxygenase, shelf-life

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© 2003 Helsinki University of Technology

Last update 2011-05-26