Study of the interactions between milk proteins and hydroxyapatite particles : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Riddet Institute, Palmerston North, New Zealand
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Date
2016
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Massey University
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Abstract
Hydroxyapatite (HA) and other insoluble calcium salts added to calcium-fortified milks are
often described as inert, as they do not cause any protein aggregation and heat instability
during heat treatment of the milk. However, it is well-known that proteins can interact with
HA. The adsorption of milk proteins on HA has been demonstrated in many systems, for
example in chromatography, bioceramic and dentistry applications, and has been shown to
have consequence on the colloidal stability of HA, but has never been studied in food
systems.
The main objective of the present study was therefore to explore the adsorption of milk
proteins onto HA particles under a range of physico-chemical conditions. The consequences
of these interactions on the colloidal properties of the HA particles and on the stability of the
milk proteins were investigated.
It was shown that the five individual milk proteins aS-casein, ß -casein, ?-casein, ß-
lactoglobulin and a-lactalbumin adsorbed onto the HA particles. A Langmuir model was
used to fit the adsorption data and determine the affinity constant and maximum surface
loads of the different proteins. The adsorption of the different milk proteins onto HA
particles was found to be of competive nature. ß-casein and aS-casein were always preferred
for adsorption over ?-casein, ß-lactoglobulin and a-lactalbumin. This was attributed to the
presence of phosphoserine clusters in ß-casein and aS-casein, forming many anchor points
capable of binding to the calcium sites of HA. ß-casein and aS-casein also adsorbed to higher
maximum levels compared to ?-casein, ß-lactoglobulin and a-lactalbumin. Both ß-Casein
and aS-casein were considered to self-associate or associate together in the adsorbed layer,
therefore forming a thick layer onto the HA surface. Conversely, ?-casein, ß-lactoglobulin
and a-lactalbumin adsorbed to lower maximum amounts and had lower affinities for HA,
which was attributed to adsorption in a monolayer through their carboxyl groups binding to
the calcium sites of HA.
The amount of protein adsorbing to the HA surface was affected by the physico-chemical
properties of the solution such as pH and ionic strength, for all proteins. Decreasing pH and
increasing ionic strength decreased the electrostatic repulsive forces between HA and the
proteins and the electrostatic repulsive forces within the protein molecules, which allowed
more protein to adsorb onto the HA surface. Milk serum ions such as calcium, phosphate
and citrate bound specifically onto HA particles, therefore competing with the milk proteins
for adsorption.
In milk, it was shown the addition of HA in milk disrupted the mineral equilibrium and the
milk protein phase. When HA particles were added to milk, the milk serum ions bound to
the HA surface. This caused the colloidal calcium phosphate to be released from the casein
micelles and the casein micelles to dissociate. Therefore the casein micelles did not bind as
intact micelles but as individual molecules or small aggregates onto the HA particles.
The adsorption of milk proteins onto HA particles affected the colloidal properties of the HA
particles in suspension. The adsorption of both caseins and whey proteins onto HA particles
resulted in the particles becoming negatively charged, thus improving their suspension
stability. Whey protein adsorption probably provided only electrostatic stabilisation,
whereas casein adsorption also provided steric stabilisation.
Overall, this work has provided a detailed understanding of the interactions between milk
proteins and HA particles. Calcium fortification of milk using insoluble calcium salts such as
HA should be approached using an awareness of these interactions, as they may have
consequences on the stability of calcium fortified milks.
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Keywords
Milk proteins, Hydroxyapatite, Milk, Heat treatment