Many workers have observed that under standardized conditions only part of the fat present in spray-dried milk can be extracted by fat solvents. This fat is usually called 'free fat' and has been related to other powder properties which are of practical importance.
Contradictory results obtained by different workers have raised doubts concerning the practical significance of free fat. Moreover the explanation generally accepted in the literature, that free fat is 'unprotected fat' mainly situated on the particle surface, is not altogether satisfactory.
An experimental study of the properties of free fat and its relation to powder properties of practical significance was carried out. The results of this study were described in a series of papers in Netherlands Milk and Dairy Journal
and are collected in this thesis.
In Chapter 1 a general outline of the problem and a brief review of the literature are given.
It seemed desirable to investigate the influence of the experimental conditions during the determination of the free-fat content because earlier authors used widely varying conditions. In Chapter 2 it is shown that the extraction time and temperature influence considerably the amount of fat which is extracted but its influence may depend strongly on the powder under investigation.
Because it was not clear which experimental conditions yielded free-fat contents relevant to other powder properties, two methods were usually applied, differing widely in contact time and extraction temperature.
Particle size is of major importance for the physical properties of most powders. In Chapter 3 two methods are described to determine the mean particle size and the particle size distribution of spray powders, viz gas permeametry and microscopic counting, respectively. With these methods the influence of certain process parameters on particle size and the influence of particle size on the free-fat content of spray-dried whole milk were studied. It is shown that the mean particle size increases inversely with Vp, p being the spray pressure. Orifice diameters appeared to be of minor importance. Increasing the dry-solids content of the concentrated milk in the range of 30-45 % resulted in a reduction of the number of small particles. No influence of the particle size on the fat content and the moisture content was observed, but both the mean particle density and the free-fat content strongly increase with decreasing particle size.
It is suggested that the influence of some processing parameters on the free-fat content of spray-dried whole milk may be attributed to their influence on particle size.
With 20 powders, widely differing in free-fat content, the possible relationship between free fat and other powder properties of practical importance is investigated (Chapter 4). From the results the following conclusions were drawn: 1. There is no influence of the free fat on the development of oxidation flavour during 6 months' storage at 30°C in the presence of air; 2. The solubility of spray-dried whole milk is usually better if its free-fat content is higher, but probably there is no causal relationship between the two quantities; 3. Below a free-fat content of 20 % (of the fat) there is a tendency for the dispersibility to increase with decreasing amount of free fat. This is not the case above 20 % free fat. Because both particle size and homogenization may effect dispersibility as well as free-fat content, no causal influence of free fat on dispersibility could be observed; 4. Neither cream rising nor foam formation during reconstitution is materially influenced by the amount of free fat in the powder. Churning of the butterfat by vigorously stirring during reconstitution was observed only with powders containing much free fat, prepared from unhomogenized concentrated milk. No quantitative correlation was observed between the amount of churned butterfat and the free-fat content of the powder.
Chapter 5 treats the stickiness of spray-dried whole milk and a simple test to determine the cohesion of such powders is described. Good agreement between this cohesion and the visual observation of stickiness was found. It is shown that the cohesion of spray-dried whole milk increases with decreasing particle size as expected. Unexpected, however, was that the cohesion is independent of fat content in the range of 20-45 % fat, and of free-fat content. In spite of this, experiments at different temperatures showed that the milk fat is an important factor in the cohesion, and it is suggested that only a small amount of surface fat on the particles is already sufficient to give cohesion to whole milk powders. Probably fat contents higher than 20% result in more surface fat, but this does not increase the cohesion.
It is shown that an increase of moisture content decreases the cohesion in the range of 2-4%, but a strong rise of cohesion occurs in the range of 4-7%, probably due to increased plasticity of the powder particles and to lactose crystallization.
In Chapter 6 it is shown that the free-fat content decreases considerably when the moisture content of spray-dried whole milk is increased in the range of 2-8 %. The effect appeared to be reversible. It is suggested that part of the fat is extracted from the milk powder particles via capillary pores or cracks, which close when the powder absorbs water.
With a scanning electron microscope photographs were taken of dried milk particles without water being used, this to avoid artefacts. The method is described in Chapter 7 and a few photographs are shown.
From measurement of the penetration speed of nitrogen into the powder particle porosity factor was calculated (Chapter 8). There appeared to be a strong correlation (r = 0.94) between the free-fat content and this particle porosity. From this observation it is concluded that the particle porosity is of major importance for the extraction of fat from milk powders.
Photographs taken with a scanning electron microscope show cracks and pores in particles with high porosity, whereas in less porous powder particles only surface folds and occasional cracks could be observed.
Possible explanations of particle porosity are discussed.
Chapter 9 contains the fat globule size distributions in concentrated milk and in milk reconstituted from whole milk spray powders with various free-fat contents as determined with a Coulter counter. It is concluded that the spraying of concentrated milk causes a considerable size reduction of the fat globules. Some coalescence of fat globules takes place during drying or during reconstitution of milk from the powders. A rather good relationship is obtained between the free-fat content of the powders and the percentage of fat present as fat globules>2 μm in milk reconstituted from the same powders.
The phenomenon that only part of the fat in spray-dried whole milk can be extracted by fat solvents is usually explained by the assumption that this 'free fat' is unprotected surface fat.
In Chapter 10 it is shown that surface fat in such powders is dissolved in a few seconds by organic solvents. The amount of fat extracted by the majority of the extraction methods with much longer contact times applied until now, consists for only a minor part of surface fat in many cases. Experiments with spray powders consisting only of lactose and anhydrous butterfat show that fat globules without protein membranes can be protected by the amorphous lactose against fat solvents.
A new model for the occurrence of free fat in spray-dried whole milk, based on the accessibility of the fat in the powder particles, is proposed.
The relationship between fat content and free-fat content of spray-dried whole milk is investigated, and the curves which were obtained are explained with the new model.
Finally the practical consequences of the model are discussed and compared with the results of earlier experiments. The conclusion is that only the surface fat is relevant because of its possible influence on dispersibility and the cohesion of spray-dried whole milk. This fat can be determined approximately with a very short extraction (e.g. 10 seconds) at room temperature.