Abstract
The oxidative enzymes tyrosinase and laccase, as well as the
acyltransferase transglutaminase (TG), are capable of creating covalent
cross-links in proteinaceous substrates. These enzymes differ from each other
on the basis of their different reaction mechanisms and the amino acid
residues with which they react. TG and tyrosinase are well-known protein
cross-linkers, and TG is already used industrially in the food sector. The
commercial feasibility of tyrosinase is under assessment, whereas the
effectiveness of laccase is not known because the reaction mechanism of
protein modification by laccase is still poorly understood. The effects of
tyrosinase-, laccase- and TG-catalysed protein modification were studied in
different meat protein systems from the myofibrillar proteins of chicken
breast muscle to heated ground chicken breast meat systems containing
different amounts of meat and salts. The study was focused on the effects of
the enzymes on the thermal behaviour and gel formation properties of
myofibrils, and on the textural and water-holding properties of the heated
meat systems. The cross-linking efficiency of a novel tyrosinase from the
fungus Trichoderma reesei was compared to that of the commercial tyrosinase
from the mushroom Agaricus bisporus. Trichoderma tyrosinase was found to be
superior compared to the Agaricus enzyme in its protein cross-linking
efficiency and in the incorporation of a small molecule,
dihydroxyphenylalanine, into a complex proteinaceous substrate, wool fibre.
Agaricus tyrosinase was found to have a propensity towards oxidation of small
tyrosine-containing substrates. All three enzymes, although having different
reaction mechanisms, affected the same myofibrillar proteins. Myosin and
troponin T were found to be most sensitive to enzymatic modification, whereas
actin was clearly more resistant. Tyrosinase, laccase and TG all polymerised
myofibrillar proteins, but only laccase was found to cause protein
fragmentation. The fragmentation products originated from myosin and troponin
T. In the differential scanning calorimetric (DSC) measurements alteration
of the peak temperature of myosin or actin transitions was not observed.
However, tyrosinase and TG decreased, whereas laccase increased, the heat of
myosin transition. The heat of actin transition was increased with all three
enzymes. Tyrosinase and TG improved the gel formation of a 4% myofibrillar
suspension at 0.35 M (2%) NaCl. With both enzymes the gel formation increased
along with increasing enzyme dosage, showing a positive connection between
covalent cross-link and gel formation. Laccase was able to increase the gel
formation slightly only when the NaCl concentration was increased to 0.60 M
(3.5%). With an excessive laccase dosage the gel formation declined markedly
due to protein fragmentation. Tyrosinase, laccase and TG had different
effects on the texture and water-holding of the heated chicken breast meat
homogenates. The homogenates were prepared free of phosphate (75% meat, 2%
NaCl), with a low meat content (65% meat, 2% NaCl, 0.34% phosphate), with a
low salt content (75% meat, 1% NaCl, 0.34% phosphate) or with low amounts of
both NaCl and phosphate (75% meat, 1% NaCl, 0.17% phosphate). Tyrosinase
improved the firmness of the homogenate gels free of phosphate and with a low
amount of meat. TG improved the firmness of all studied homogenates.
Laccase weakened the gel firmness of the low-meat, low-salt and
low-salt/phosphate homogenates and maintained the firmness on the control
level in the homogenate free of phosphate. Tyrosinase reduced the weight
loss in the homogenates containing a low amount of meat and a low amount of
NaCl, whereas TG and laccase were not able to decrease the weight loss of any
homogenate. TG was the only enzyme that could positively affect the
firmness of the homogenate gel containing both low NaCl (1%) and phosphate
(0.17%) amounts. In pilot scale the test products were made of coarsely
ground chicken breast fillet with a moderate amount of salt (1.2-1.8% NaCl),
a meat amount 70-80% and a TG dosage 0-20 nkat/g protein (0-0.2% of the meat
mass). All three factors, meat, salt and TG contents, favoured the
development of firmness of the test products. The evaporation loss decreased
slightly along with increasing TG and NaCl amounts in the experimental
conditions used, indicating a positive interaction between these two
factors. In this work it was shown that tyrosinase, laccase and TG affected
the same myofibrillar proteins. However, these enzymes had distinguishable
effects on the gel formation of a myofibril system as well as on the textural
and water-holding properties of the finely ground meat homogenates,
reflecting distinctions at least in the reaction mechanisms and target amino
acid availability in the protein substrates for these enzymes.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 20 Jun 2007 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-7022-5 |
Electronic ISBNs | 978-951-38-7023-2 |
Publication status | Published - 2007 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- proteins
- cross-linking
- enzymes
- tyrosinase
- laccase
- transglutaminase
- Trichoderma reesei
- food
- meat
- chicken breast
- myofibril
- thermal behaviour
- gel formation
- structure
- texture
- water-holding