10.10.2008 Doctoral dissertation: Tyrosinase- and laccase-enzymes suitable for modifying food

Thesis: Tyrosinase and laccase as novel crosslinking tools for food biopolymers

Enzymes are natural and sustainable tools to modify the structural properties of food products. VTT's Research Scientist, M.Sc. (Tech.), Emilia Selinheimo showed in her D.Sc. (Tech.) thesis work that tyrosinase- and laccase-enzymes are suitable for modifying the structure of wheat bread. She defended her thesis "Tyrosinase and laccase as novel crosslinking tools for food biopolymers" at the Helsinki University of Technology on 10th October 2008.

Health-promoting foods, i.e. low-calorie, low-fat, low-additive products, are becoming more and more important in the food market. However, consumers still make purchase decisions based mainly on the palatability of the product, not its healthiness. Hence, the challenge in the food industry is to make healthy food products with excellent sensory properties.

Enzymes offer a possibility for the specific modification of the textural characteristics of food products. The nutritional value of the products can also be improved with enzymes. Emilia Selinheimo showed in her work that tyrosinase- and laccase-enzymes are capable of crosslinking milk- and cereal-based biopolymers, such as proteins and carbohydrates. Crosslinking is described as formation of strong covalent bonds between the biopolymers. In the reactions catalysed by tyrosinase and laccase the covalent bonds are formed via phenolic compounds. Thus, the initial raw-material components of foodstuff can be exploited for improving the textural characteristics of the product.

Emilia Selinheimo characterized in her work the biochemical properties of a novel fungal tyrosinase from Trichoderma reesei (TrT). In addition, she investigated the applicability of tyrosinase for crosslinking of cereal- and milk-derived proteins. The substrate specificity and protein crosslinking ability of TrT were compared to other tyrosinases of plant and fungal origin. Furthermore, the suitability of TrT and laccase from Trametes hirsuta (ThL) was examined for hetero-crosslinking of cereal-based carbohydrates and milk-based proteins, and for improving wheat breadmaking quality.

The novel Trichoderma reesei tyrosinase was discovered to be a secreted enzyme, which is not typical among the reported tyrosinases. TrT showed the highest activity and stability in the neutral and alkaline pH range and retained its activity relatively well at temperatures of 40 ºC and below. TrT had broad substrate specificity, oxidizing several mono- and diphenolic compounds, and had good ability to crosslink milk α-casein and wheat gluten proteins. However, TrT was unable to crosslink xylan, the dietary fibre component of oat. On the other hand, laccase from Trametes hirsuta (ThL) was shown to be able to polymerize this xylan from oat.

Despite the notable differences between the action mechanisms of these oxidative enzymes, both ThL and TrT were observed to be able to catalyze oxidative hetero-crosslinking of milk α-casein and oat xylan. In wheat breadmaking, both enzymes increased the bread volume and bread crumb softness. The effects of laccase in dough and bread are suggested to be due mainly to polymerization of wheat xylan, whereas the effects of tyrosinase are proposed to result from polymerization of wheat gluten proteins.


More information:

Research Scientist
Emilia Selinheimo,
Tel. +358 20 722 7135
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Link to the thesis (full-text, pdf): http://www.vtt.fi/inf/pdf/publications/2008/P693.pdf

Suomenkielinen väitöstiedote: http://www.vtt.fi/vtt/081008_vaitos_selinheimo.jsp?lang=fi