Spray dried milk-protein stabilized emulsions with high oil content : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
This study explores the behaviour of oil droplets in milk protein-stabilized
emulsions during spray drying. The impact of preheat treatment on the stability of
oil droplets during drying in milk protein-stabilized emulsions in maltodextrin was
also observed, using a variety of techniques such as particle size analysis, various
microscopy techniques and sodium dodecyl sulphate polyacrylamide gel
electrophoresis. In the last section of the study, the stability of the powdered
emulsions was investigated against oxidative deterioration when soybean oil was
replaced with fish oil in the emulsion formulation.
The results showed that spray drying and redispersion of the powdered emulsions in
water (at similar total solids content) caused a shift in the droplet size distribution to
larger values for all emulsions made using low concentration of whey protein
isolate or sodium caseinate (0.5-2.0%, w/w w.b.), in comparison with their
respective parent emulsions. However, the droplet size distribution was affected
only very slightly by spray drying when the protein concentration was above 2.0%
(w/w). This minimum concentration of protein that was required to produce
emulsions that were stable during the spray drying process was 3.0% (w/w) for the
emulsions prepared using aggregated milk protein products as compared with 2.0%
(w/w) for the NaCas- and WPI-containing emulsions.
It was suggested that the amount of unadsorbed protein in the bulk phase of the
parent emulsions play a crucial role in stabilizing the oil droplets during spray
drying. When the surface of the oil droplet is saturated with protein molecules and
the bulk phase of the emulsion has sufficient unadsorbed protein, the oil droplet is
stable during drying. However, for emulsions with a low concentration of
unadsorbed protein in the bulk phase (= 1.0% for WPI or NaCas emulsions),
protein molecules could potentially migrate from the surface of the oil droplet to
the air?water interface, causing “gaps” in the oil droplet interface and leading to
coalescence and/or bridging flocculation.
Emulsions containing low levels of maltodextrin showed marked coalescence
during spray drying and redispersion even at a WPI concentration of 10.0% (w/w).
Above a critical concentration (12.0%, w/w), maltodextrin appeared to stabilize
proteins at the interface and provide adequate rigidity to the matrix perhaps by
forming a glass, under the drying conditions.
In whey protein-stabilized emulsions made with preheat treated protein solution
(above 70ºC), a shift was observed in average droplet diameter towards the larger
size range, because of droplet coalescence as a result of spray drying. This was
thought to probably be a result of protein aggregation in emulsions, which
adversely affected the ability of proteins to stabilize the emulsion droplets during
spray drying and further emphasized the crucial role of monomeric whey proteins.
A reduction in the non-adsorbed monomeric whey proteins as a result of preheat
treatment led to oil droplet coalescence during drying. The stability of the emulsion
made with pre-heat treated whey proteins was noticeably improved when NaCas
was added to the emulsion either before or after the homogenization step. This
improved stability was believed to be a result of the steric effect of caseins that
prevented large-scale aggregation of whey proteins.
The stability of emulsions during drying as shown by the change in the average
droplet diameter before and after drying showed a negative correlation with
oxidative stability of these emulsion where soybean oil was replaced with fish oil.
The protein content and preheat treatment also showed a positive impact on the
oxidative stability of spray-dried emulsions.
Overall, the finding from this systematic study has advanced the understanding of
the mechanisms of the stability of oil droplet during drying as well as the impact of
emulsions components and processing conditions. This may help to design
emulsion formulations and processes and extend the applications of milk-protein
stabilized powdered emulsions with high oil content.