Chapter 3 Surface Composition of Industrial Spray-Dried Dairy Powder
Spray-dried dairy powders are important ingredients in the manufacture of many food and dairy products due to their various functional and nutritional qualities. Some of the properties of these powders that are important in their storage, handling and final application (e.g. wettability, dispersibility, flowability and oxidative stability) are expected to be determined largely by the surface composition of the powder. Therefore, an understanding of the mechanism behind the formation of the surface composition of the powder and the ability to control the surface composition will be very useful in the improvement of product quality and the development of new products.
In order to understand the mechanism behind the formation of the surface composition, it is important to understand the influence of composition of the drying liquid on the powder surface composition. Faldt  has perform a study on the influence of components of the drying liquid on the powder surface composition of spray-dried ‘model’ emulsions (lactose, sodium caseinate or whey proteins, soybean oil) using electron spectroscopy for chemical analysis (ESCA). The study has shown that the composition of the drying liquid has a strong influence on the powder surface composition. In particular, if the solution contains surface-active components, such as proteins, there is a transport of surface active components to the air-liquid interface of the drying droplets, and it accumulates at the droplet surface and thus appears on the powder surface after the drying is completed. However, the investigated model emulsions may not be directly comparable to milk. Milk is a more complicated system containing many components including minerals. Also, milk protein casein is present in an aggregated state, so-called casein micelles, and milk fat is a heterogeneous mixture of various triglycerides with a melting range from −40 to +40oC , which are not directly comparable to sodium caseinate and soybean oil, respectively. Faldt  have estimated the surface composition of spray-dried whole milk powder using ESCA. The surface of spray-dried whole milk powder has been found to be largely covered by fat (~55%) after spray-drying process. This result indicates that the surface activity of protein may not be the only surface formation mechanism of ‘real’ spray-dried dairy powders and additional mechanisms may exist. To better understand how the surface composition of ‘real’ spray-dried dairy powders forms, more quantitative knowledge on the surface composition of various spray-dried dairy powders is needed. However, no or little work examining the surface composition of spray-dried dairy powders has been carried out to date.
The aim of the present study was, therefore, to investigate the surface composition of various industrial spray-dried dairy powders manufactured for consumer use, so as to provide more fundamental information to study the mechanism behind the formation of the powder surfaces. The information of the surface composition of powders would also be useful in predicting the behaviour of powders during its storage, handling and final application. Of special interest in this thesis was the fat content on the surface of powders. Spray-dried skim milk powder, whole milk powder, cream powder and whey protein concentrate were selected to represent a range of fat contents from 1 wt.% to 75 wt.%. In addition, the usefulness of ESCA measurements in direct analyzing the surface composition of dairy powders was demonstrated.
MATERIALS AND METHODS
Four industrial spray-dried dairy powders (skim milk powder (SMP), whole milk powder (WMP), cream powder (CP) and whey protein concentrate (WPC)) were obtained from a local dairy company. The powders were commercial products that had been freshly manufactured and packed for consumer use. The capacities of the dryers in which the powders were made were at least 4 t powder/hr. The composition of the powders used is shown in Table 3-1. The lactose, protein, fat and moisture content were determined by the T-Chloramine T-test, Kjeldahl method, Accelerated Solvent Extraction (ASE) method and gravimetric method, respectively. Petroleum ether (boiling point, 40-60oC) from BDH laboratory supplies (Poole, UK) and ethanol (99.5%) from Panreac Quimica Sa (Barcelona, España) were used as extractants.
Extraction of free-fat
The free-fat extraction procedure used in this experiment follows that extensively used by Buma  with some modification. One gram of the powder was added to 40 mL of petroleum ether (b.p. 40-60oC) and was shaken frequently by hand for 48 hrs. The powder and the solvent were first separated by filtration through filter paper (No. 4, Whatman, Maidstone, Kent, UK). The powder residue was further washed with 2 x 2 mL of petroleum ether (b.p. 40-60oC) and then dried under vacuum at room temperature. The filtrate solution containing the extracted fat was allowed to evaporate until the extracted fat residue achieved constant weight. The extracted fat value was then recorded as g free-fat/g fresh powder.
Extraction of total fat
Extraction of total fat was made with ethanol for various extraction times (10 min, 24 hrs and 48 hrs). One gram of the fresh powder was added to 40 mL of ethanol, and was shaken frequently by hand. If the extractions were extended to 20 hrs, the suspension was left overnight without agitation. After the required extraction time, the powder and the solvent were first separated by filtration through filter paper (No. 4, Whatman, Maidstone, Kent, U.K.). The powder residue was further washed with 2 x 2 mL of ethanol and then dried under vacuum at room temperature. The filtrate solution containing the extracted fat was allowed to evaporate until the extracted fat residue achieved constant weight. The extracted fat value was then recorded as g fat/g fresh powder.
Removal of surface free-fat
In order to remove only the (very) surface free-fat and minimize the removal of outer layer and capillary free-fat from powders, only a brief wash with organic solvent was carried out. One gram of the fresh powder was accurately weighed on a filter paper (No. 4, Whatman, Maidstone, Kent, UK), and washed with 5 mL of petroleum ether (b.p. 40-60oC). This step typically took approx. 10 s. The powder residue was dried under vacuum at room temperature, and the filtrate solution containing the extracted fat was allowed to evaporate until the extracted fat residue achieved constant weight. The extracted fat value was then recorded as g surface free-fat/g fresh powder. This procedure was repeated until a sufficient amount of powders for the analyses had been obtained.
Electron spectroscopy for chemical analysis (ESCA)
Electron spectroscopy for chemical analysis (ESCA) was used to analyze the surface composition of the powders. The underlying principles of the technique and its application to dairy powders have been described in Chapter 2. The ESCA measurements were made with an XSAM 800 photoelectron spectroscope (Kratos Analytical, UK). The instrument used a non-monochromatic Al Kα X-ray source. The pressure in the working chamber during analysis was less than 1×10-7 Torr. The take-off angle of the photoelectrons was perpendicular to the sample. The analyzer operated with a pass energy of 65 eV. The step size was 0.1 eV, and the dwell time was 1000 ms. The powders were loosely packed in aluminium sample holders, and the surface was levelled. The analyzed area of the powder was a region of 5 mm x 8 mm. The surface composition of the powders was calculated according to the method described in Section 2.3.4.
Scanning electron microscopy (SEM)
The powder samples were mounted on aluminium stubs using a double-sided adhesive tape. Excess particles were removed by directing a jet of dry air at the surface of the stub. The samples were then coated with platinum in a Polaron SC7640 sputter coater (VG Microtech, England) and were examined with a Philips XL30 S-FEG SEM (Holland) operating at 5 kV accelerating voltage.
Determination of flowability
The flowability of powders was determined by measuring the angle of repose (a static measure of relative flowability). The angle of repose for each of the powders was measured using the simple equipment shown in Figure 3-1(A). This equipment is similar to that described by Kaye . Ten grams of powder was carefully placed in the top box of the equipment with the trap door closed. The trap door was then opened allowing the powder to flow downwards and to form a heap. This method allowed for the measurement of the drained angle of repose (α) and the poured angle of repose (β), as shown in Figure 3-1(B). Since the poured angle of repose (β) measured by this method was similar for most of the powders, the drained angle of repose (α) was measured for comparison using horizontal still photographs and a protractor. More free-flowing powders tend to have lower drained angles of repose.
Chapter 1. Project Overview
1.1. Problem definition
1.2. Project objectives
1.3. Thesis outline
Chapter 2. Electron Spectroscopy for Chemical Analysis (ESCA) for the study of dairy powder surfaces
2.2. Basic principles of ESCA
2.3. Application of ESCA to spray-dried dairy Powders
Chapter 3. Surface composition of industrial spray-dried dairy powders
3.2. Materials and methods
3.3. Results and discussion
Chapter 4. Distribution of milk components within spray-dried dairy powders
4.2. Materials and methods
4.3. Results and discussion
Chapter 5. Melting characteristics of fat present on the surface of fat-containing powders
5.2. Materials and methods
5.3. Results and discussion
Chapter 6. Development of surface composition during manufacture
6.2. Materials and methods
6.3. Results and discussion
Chapter 7. Effects of spray-drying conditions on the surface composition
7.2. Materials and methods
7.3. Results and discussion
Chapter 8. Changes in surface composition during long-term storage
8.2. Materials and methods
8.3. Results and discussion
Chapter 9. Surface formation mechanisms of industrial spray-dried dairy powders
9.2. Literature search
9.3. Summary of the findings in this work
9.4. Possible surface formation mechanisms
9.4.3. During storage
Chapter 10. Conclusions
10.1. General conclusions
10.2. Recommendations for future research
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Surface Composition of Industrial Spray-Dried Dairy Powders and Its Formation Mechanisms