Abstract
Milk proteins, sodium caseinate (CN) and whey protein
isolate (WPI) are used in food industries as emulsifiers.
The stability of an O/W emulsion is dependent on the
electrostatic and steric repulsion provided by the
interfacial proteins against droplet aggregation or lipid
oxidation. Therefore, modifications of the surface charge
or the interfacial conformation of protein emulsifiers
are expected to enhance their emulsifying properties and
emulsion stability. In this present work, sodium
caseinate and whey protein isolate were modified by
different chemical and enzymatic approaches. The modified
proteins were characterized using multiple techniques,
and the effect of these modifications on emulsifying
properties of proteins and emulsion stability were
investigated.
Succinylation converts the positively charged amino
groups into negatively charged carboxyl groups, lowering
the isoelectric point (pI) of protein. The ethylene
diamine (EDA) modification worked in the opposite way,
leading to an increased pI. The extent of these two
modifications was studied using SDS-PAGE and MALDI-TOF
mass spectrometry. The pI of succinylated and EDA
modified milk proteins was studied using zeta-potential
measurement. As a result, the succinylation to full
extent altered the pI of CN from 4.2 to 2.7, and the EDA
modification shifted the pI of CN and WPI from 4.2 to 9.4
and from 4.9 to 9.5 respectively. The pH stability of
emulsion made with the modified milk proteins was
monitored by following the increase of particle size
during storage. The results suggested that succinylation
and EDA modification could enhanced the emulsion
stability at pH 4-7 by increasing the electrostatic
repulsion between droplets.
Regarding the enzymatic modification of milk proteins,
the laccase and transglutaminase (Tgase) catalyzed
cross-linking were applied on WPI and CN respectively. In
order to improve the reactivity of WPI towards the
laccase, a vanillic acid modification was carried out to
incorporate additional methoxyphenol groups into the
protein surface. The crosslinking of vanillic acid
modified WPI (Van-WPI) by laccase was studied using
SDS-PAGE. The extent of cross-linking of Van-WPI was
found to be significantly higher compared to the
unmodified WPI and the combination of WPI and free
phenolic compound as a mediator. The effect of laccase
catalyzed cross-linking on storage stability was
investigated by visual observation and confocal
microscopy. The post-emulsification cross-linking was
proven to enhance the stability of the emulsions prepared
with Van-WPI during the storage. The reduced droplet
coalescence could be most likely attributed to an
extended interfacial protein layer formed via the
interaction between the adsorbed proteins and
non-adsorbed proteins in the water phase. In contrast
with the limited extent of crosslinking of WPI by
laccase, CN was extensively cross-linked by Tgase. The
physical stability of emulsions was studied by measuring
the increase of particle size during storage, and the
oxidative stability was evaluated by following the
formation of fatty acid hydroperoxides and volatile
compounds in different stages of the lipid oxidation. The
pre-emulsification cross-linking showed no obvious
influence on the physical stability of CN emulsion but
significantly improved its stability against lipid
oxidation. The improvement of oxidative stability of
emulsions could be contributed to a thicker and denser
interfacial protein layer and thus increases the amount
of anti-oxidative groups located at the interface and
provides a stronger barrier against competitive
adsorption by oil oxidation products.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
|
Supervisors/Advisors |
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Award date | 11 Sept 2015 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-8333-1 |
Electronic ISBNs | 978-951-38-8334-8 |
Publication status | Published - 2015 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- milk proteins
- emulsifier
- protein modifications
- cross-linking
- physical stability
- oxidative stability