Consequences of matrix structural changes on chemical and functional stability on enzymes as affected by electrolytes

MAZZOBRE, M. FLORENCIA; SANTAGAPITA, PATRICIO R.; GUTIERREZ, NORA; BUERA, M. PILAR

Córdoba, Argentina

Congreso; Congreso Internacional de Ciencia y Tecnología de los Alimentos; 2006

Ministerio de Ciencia y Tecnología de Córdoba y Universidad Nacional de Córdoba

The conservation of labile biomolecules is generally performed in dehydrated or frozen media. An equilibrium state does not exist in these systems, but they can reach several states of metastability. The conservation of desirable properties in foods and ingredients is then governed by conditions of metastability, often based in the maintenance of amorphous metastable properties of the systems.The objective of present work was to analyze how the enzymatic stability in sugar systems depends on ice formation conditions during freezing and on the crystallization of solutes in dehydrated systems. Both processes produce a segregation of components in the amorphous phase. The effect of salts on the physical properties of sugar systems and their relationship to enzyme stabilization were also studied.Honey of multifloral origin, soy and malt extracts, and b-galactosidase from Aspergillus oryzae, were employed as enzyme sources. Solutions containing the corresponding enzyme extract in presence of 20% (w/v) of trehalose or sugar-salt mixtures (NaCl, CaCl2, KCl, MgCl2.6H2O, Na3C6H5O7.2H2O, Mg(C2H3O2)2 or KC2H3O2, were prepared. For cryo-concentrated systems, solutions were stored at -26°C. To obtain amorphous dehydrated systems solutions were freeze-dried, re-humidified to 11-43% relative vapor pressure and stored at 55°C. Enzyme activity (amylase, urease, aspartate amino transferase and b-galactosidase) and thermal transitions (glass transition, sugar or water crystallization/melting) were analyzed in all systems after storage. In frozen systems, the amount of ice formed in the diluted enzymatic extracts was higher than in more concentrated ones, negatively influencing enzyme stability. The amount of water associated to the amorphous frozen matrix increased when salts were present. Thus, enzyme stability impaired due to the increased molecular mobility, and to the high concentration of salt in the unfrozen matrix. When about 30-50% trehalose crystallized in dehydrated systems, the protective effect of the sugar was lost, and the enzyme become rapidly inactivated in almost all salt containing systems. The results showed that interactions of organic or inorganic salts (commonly present in food formulations) with the excipients, enzymes and water are complex and for an effective enzyme protection the amount of ice formed in frozen systems, the crystallized sugar in dehydrated systems and the concentration of salt in the amorphous phase should be considered.