14 research outputs found
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The influence of surface functional groups on β-lactoglobulin adsorption equilibrium
Interactions between proteins and contact surfaces can have
important implications in the food industry. Such interactions
contribute to the course of fouling of membrane surfaces and they
appear to mediate bacterial and spore adhesion to some degree as
well. In addition to protein and solution properties, interfacial
behavior is strongly influenced by contact surface properties. Among
these, hydrophobicity and the potential to take part in acid-base
interaction have received considerable attention, but in a quantitative
sense we know very little about their respective influences on protein
adsorption. It was the purpose of this research to quantify the
equilibrium adsorptive behavior of the milk protein β-lactoglobulin as
it is influenced by the presence of different contact surface functional
groups.
Monocrystalline and polished silicon surfaces were modified to be
hydrophilic by oxidation and hydrophobic by silanization with dichlorodiethylsilane (DDES), dichlorodimethylsilane (DDMS), and
dichlorodiphenylsilane (DDPS), each used at concentrations of 0.82,
3.3, and 82 mM. Surface hydrophobicities were evaluated with contact
angle methods. Adsorption isotherms were constructed after allowing
each modified silicon surface to independently contact β-lactoglobulin
(0.01 M phosphate buffer, pH 7.0) at concentrations ranging between
200 and 2000 mg/L for eight h at room temperature. Surfaces were
then rinsed and dried. Optical properties of the bare- and
film-covered surfaces, necessary for calculation of adsorbed mass,
were obtained by ellipsometry.
Plots of adsorbed mass as a function of protein concentration
exhibited attainment of plateau values beyond a protein concentration
of about 200 mg/L. At high silane concentration, the plateau values
associated with surfaces exhibiting ethyl groups were observed to be
greatest followed by those exhibiting phenyl, methyl, then hydrophilic
(OH) groups. At the low DDMS and DDES concentrations (0.82 and 3.3
mM), adsorbed mass did not increase beyond that value recorded for
the hydrophilic surface. This is likely due to some critical spacing of
methyl and ethyl groups being required to produce a favorable
hydrophobic effect on adsorption. For surfaces treated with
dichlorodiphenylsilane, adsorbed mass increased with silane
concentration. Apparently, a favorable acid-base interaction effected by
the hydrophilic surface is inhibited by the presence of small amounts
of methyl and ethyl groups, but somewhat less inhibited by the
presence of phenyl groups because the latter have the ability to
undergo acid-base interaction
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The influence of preadsorbed milk proteins on adhesion of Listeria monocytogenes to silica surfaces
β-lactoglobulin (β-Lg), bovine serum albumin (BSA), α-lactalbumin
(α-Lac), and β-casein were adsorbed onto
silanized silica surfaces of low and high hydrophobicity
for 8 h, and β-Lg and BSA for 1 h. The surfaces were
incubated in buffer for 0, 5, 10, or 15 h and then contacted
with Listeria monocytogenes for 3 h. Cell adhesion was
quantified using image analysis. Following 8 h of protein
contact, adhesion to both surfaces was greatest when β-Lg
was present and lowest when BSA was present. Preadsorption
of α-Lac and β-casein showed an intermediate effect on cell
adhesion. Adsorption of β-Lg for 1 h resulted in lower
numbers of cells adhered as compared to the 8 h adsorption
time, while the opposite was observed with BSA, but adhesion
to BSA was observed to decrease slowly with film age to
values comparable to the 8 h tests.
The adsorption of BSA and β-Lg to both surfaces was
also carried out where each protein was allowed to contact
the surface in sequence and simultaneously. In sequential
tests performed with an 8 h contact/protein, cell numbers on
each surface were near that expected for the bare
hydrophobic surface when β-Lg contact preceded introduction
of BSA, whereas adhesion was reduced to values below that
expected for the bare hydrophilic surface when BSA preceded
β-Lg contact. In short-term sequential tests (1 h
contact/protein), adhesion was lower than that recorded on
bare hydrophilic surfaces in each case. Adhesion to each
surface following contact with an equimolar mixture of β-Lg
and BSA was lower than that measured on the bare hydrophilic
surface in each case, with adhesion following 1 h contact
being greater than that following 8 h contact. Adhesion
following competitive adsorption was greater to hydrophobic
than to hydrophilic surfaces. These results were explained
with reference to the surface passivating character of BSA,
and its ability to rapidly attain a nonexchangeable state
upon adsorption, relative to β-Lg