Effect of degritting on particle size of extracted concentrates

In order to study the effect of centrifugation on particle size distribution of extracted polyphenolic concentrate, three different samples were analyzed (Fig 3.1). It was found that centrifugation had significant effect on particle size distribution of the samples.

Sample had larger particle size prior to centrifugation (P1). Sample spun at 10,000 rpm (P3) contained more particles with smaller size when compared to the sample spun at 5,000 rpm (P2). In other words, more particles with larger size were removed from the concentrate as centrifugation rotational speed was increased. This resulted in the shift of particle size distribution curve to the left (lower diameters) which can be explained by Stoke’s Law. General equation of Stoke’s Law (Leung, 2007) can be expressed as:

𝑉_𝑠𝑜 = ^{𝜌𝑆−𝜌}_18𝜇^{𝐿 𝑔𝑑2} (16)

where, Vso is separation velocity, ρS density of solid, ρL density of liquid, g centrifugal acceleration, d diameter of the particle, and µ viscosity of liquid. Subscript “o” in

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separation velocity stands for separation of an individual particle with no interaction with other particles in an ideal dilute solution. In this analysis all parameters were the same for three samples except centrifugal acceleration. As angular velocity increased critical diameter of the particle to remain in the suspended form decreased.

Fig. 3.1 Effect of degritting on particle size distribution of concentrated polyphenolic extracts P1, P2 and P3.

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It can be seen from the Equation (16) that P3 had smaller particles in the suspended form when compared to P2 during centrifugation. Undoubtedly, P1 contained larger suspended particles when compared to P2 and P3, since no centrifugal acceleration was applied on it. Hence, sample spun at the higher angular velocity was comprised of particles with smaller Sauter mean diameter (D[32]) (Table 3.1; Table B.1). On the contrary, span of P3 had the highest value and it was significantly different (p≤0.001) from that of P1 and P2 samples (Table B.2). This can be explained by analyzing its particle size distribution curve (Fig 3.1). Since most of the particles of P3 are smaller this shifted the particle size distribution curve to the left (smaller size), therefore difference between d(v,90) and d(v,10) is high (Equation (4)). Definitely, this difference is lower for P2 and P1. In addition, d(v,50) is the lowest for P3. Removal of the large particles by centrifugation from P3 made it possible to detect smaller particles which could be shadowed during particle size analysis and thus, not present in the results of P1 and P2. Therefore, particle size distribution curve of P3 shifted to the left.

Table 3.1 Influence of angular velocity on purification of dispersion

* Columns having different letters (a, b & c) are significantly different (p ≤ 0.05).

Specific surface area was also significantly different (p≤0.001) for all samples (Table B.3). The reason for this is its indirect proportionality to D[32] (Table 3.1). The highest

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specific surface area was reported for P3. Consequently, it can be understood that filtration process was insufficient for the removal of particles with the diameters in the range of 1-100 µm from the polyphenolic extract, so extra treatment had to be applied.

The origin of these particles which were present in the extract can be organic or inorganic. Soluble and insoluble tissues of pomace with different size and geometry could be extracted during maceration. Drying of the pomace under the sun could lead to contamination with particles from the environment such as mineral crystals or dust suspended in the air. These particles with size under the critical diameter of suspension could remain in the extract.

3.1.2 Effect of degritting of concentrates on particle size distribution of emulsions Two different core materials (EPP and PEPP) were used to prepare emulsions. Core materials were entrapped in two different coating materials including 10% MD and combination of 8% MD and 2% GA. Results of the particle size analysis of the emulsions are given in the Table 3.2.

Table 3.2 Particle size analysis results of emulsions prepared with different coating materials and powder types

* Columns having different letters (a & b) are significantly different (p ≤ 0.05).

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Purification of polyphenolic extracted concentrates had significant (p≤0.001) influence on D_[32] (Table B.4), span and specific surface area values of emulsions (Table B.5;

Table B.6). Emulsions prepared with PEPP contained smaller particles when compared to emulsions prepared with EPP. Most of these particles were in the nano range, resulting from the D[32] values of P3. Therefore, degritting was found to be a critical parameter in the preparation of nano-emulsions. Coating materials used in this study had no significant influence (p>0.05) on the particle size distribution of the emulsions.

Similarly, it can be seen on the Fig.3.2 that particle size distribution curves are very similar for emulsions prepared with different coating materials. On the contrary, they appear very different when emulsions containing PEPP were compared with emulsions containing EPP.

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Fig. 3.2 Particle size distribution of micro-emulsions (containing EPP) prepared with 10% MD (solid line) and 8% MD-2% GA (dotted line), nano-emulsions (containing PEPP) prepared with 10% MD (dashed line) and 8% MD-2% GA (dash dotted line)

Similar to extracted polyphenolic concentrates, degritting had significant (p≤0.05) effect on the span values of the emulsions. High span value of P3 resulted in the high span values of the emulsions prepared with PEPP. These emulsions contained large range of particles with different sizes. Most of these particles were in the nano range, and there was continuous and approximately equal percent volume range of large particles (Fig 3.2) which had remained in the concentrate after degritting, and were present in the PEPP. However, particle size distribution appeared more narrow (lower span values) for emulsions prepared with EPP, but still contained large range of

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particles with different diameters. Jafari et al. (2007a) reported that homogenization in blender and ultrasonication increased span values of the sub-micron emulsions. In addition, since PEPP contained lower amount of impurities, energy density of ultrasonication of total solids in the emulsions was higher than that of the emulsions prepared with EPP, leading to more disruption and formation of smaller particles.

Gordon and Pilosof (2010) reported that ultrasonication for 10 min caused the formation of many small particles. Specific surface areas of the emulsions prepared with PEPP were higher and significantly (p≤0.001) different when compared to emulsions prepared with EPP (Table 3.2), due to the indirect proportionality to Sauter mean diameter.

3.1.3 Effect of degritting of concentrates on particle size distribution of capsules