Food Product Design: New Technologies - June 2004 - Why Measure Particle Size?

June 2004

Why Measure Particle Size?

By Patrick O'HaganContributing Editor

Now, more than ever, consumers are paying great attention to dietary supplements as a means of achieving optimum health, disease prevention, and, of course, a reduction in obesity. This has led to a substantial amount of research into the formulation of new fortified food and beverage products.

Much of this extra nutritional value comes from ingredients in powdered form. This allows for reduced shipping costs, increased stability and ease of use for processing. Since powders are inherently particulate systems -- that is, they consist of an ensemble of discrete particles -- the most important physical parameter with regard to handling them is their particle-size distribution.

Particle-size distribution can influence flow out of storage bins, blending of different components, compaction and the segregation of a mixture. Detailed information about particle size can determine how to design process or storage equipment, which raw materials to use, how to mix components together, and how long they can sit without separation or caking.

Not only is particle size important in powder handling, it also significantly influences important properties essential to food products in general: taste, smell, texture and appearance. For example, many food products, such as spreads and beverages, are processed into emulsions. Particle-size knowledge can help determine the stability of these emulsions. Foams, an important characteristic of beer and coffee drinks, are actually air particles (bubbles) in a liquid matrix. The particle size of the bubbles will determine how the foam forms and how long it lasts.

Particle size can influence the dissolution rates of nutritional supplements and, thus, how much of the nutrient is adsorbed into the body. This, in turn, can influence the overall efficacy of such products. Particle sizing is even used to monitor process end points -- grinding, homogenization or other size-reduction processes -- and to quality-control final products.

Considering the major influence the properties of powders can have in final food products and during food processing, it is important for food scientists and engineers to have an understanding of how particle sizing is done, what techniques are available, what particle-size analyzers actually measure and how the results are communicated and interpreted.

Sizable technology There are literally dozens of technologies to measure particle size and particle-size distributions. It would be impossible in the context of this article to discuss even the most common technologies. However, as a means of introduction, the following particle-sizing techniques are in common use.

· Sieves measure particle size from 20 to 5,000 µm. · Optical microscopy measures optical contrast from 1 to 200 µm. · Single-particle optical sensing measures light obscuration from 0.5 to 2,500 µm. · Laser diffraction measures light scattering from 0.1 to 2,000 µm. · Dynamic light scattering measures Brownian motion from 0.001 to 5 µm. · Centrifugal sedimentation measures sedimentation from 0.01 to 30 µm. · Gravity sedimentation measures sedimentation from 0.5 to 100 µm.

Most of these techniques do not measure particle size directly but rather a physical or optical property related to particle size. It is important to appreciate that the reason so many different techniques are used for measuring particle size is because there are so many applications. Some techniques provide certain types of information -- particle counts or molecular weights, for example -- while others handle certain materials best. So, the suitability of a certain technology must be evaluated with regard to the material tested as well as the information required.

Microns of milk Milk, for example, when separated and dried, can be an important source of many nutritional components. Some powdered dietary supplements on the market consist of almost pure protein derived from milk. These can increase protein intake without the fat that comes from eating meat, or they might be tailored to provide specialized health benefits. They may be formulated with other components, such as vitamins and minerals. These supplements are in powdered form that the end-user usually disperses in a liquid, such as water, and consumes. Even though the main purpose of the powder is as a nutritional supplement, success in the market results from a beverage with a flavor and mouthfeel that appeals to the customer.

One of the problems with beverages derived from reconstituting or dispersing powders is a "gritty" or "chalky" texture imparted by large particles that do not break up or dissolve. The human tongue can feel particles as small as 20 µm. Thus, to produce a beverage from a powder that will be perceived as a true liquid, no particles large enough to be felt by the tongue should remain. Even small numbers of these particles can cause the consumer to reject the product.

Consider the analysis of the volume-weighted particle-size distributions of two milk-protein nutritional supplements in powder form: one generic and one name-brand product. In this case, the ability to detect small amounts of large particles is the main goal of the analysis. Laser-diffraction measurements produced particle-size distributions that were quite similar for both samples, with just a slight shift to a larger mean diameter for the generic product. However, the results from single-particle optical sensing (SPOS) indicated a larger shift, which primarily came about due to the sensitivity of this particular measurement to small populations of oversized particles. So, the difference in the two products is not in the overall distribution, which is just about the same, but in the amount of large particles. The larger concentration of aggregates or oversized particles might not dissolve as well or might impart a texture that would result in an inferior product. The manufacturer of the name-brand product either ground the powder more efficiently or classified it in some way.

Homogenized stability Homogenization is a process that is affected by particle size. Homogenization forces particles dispersed in a liquid under high pressure through a small orifice. The combination of shear forces and impact forces reduces large particles and aggregates to smaller particles. Homogenization can make stable oil-in-water emulsions or disperse solid particles for beverages.

According to "A Multi-Tool Approach to Colloid Stability: SPOS and Separation Analysis" in 2001 Fine Powder Processing International Conference Proceedings, one cannot always infer stability from particle size data. Also, according to Driscoll et al in the American Journal of Health-System Pharmacy (1995, 52:623-634), small concentrations of large particles in oil-in-water emulsions can indicate a loss of stability.

These concepts can be illustrated in another example, this time featuring two samples of solid suspensions made by homogenization. One sample was determined to be "bad," that is, it did not meet stability specifications. To the dynamic light-scattering instrument, both samples appeared quite similar, with a slightly lower mean diameter for the "bad" sample. When tested with the SPOS method, which has the sensitivity to detect small amounts of large or oversized particles, the true difference is apparent. Clearly, the difference between stable and not stable is a few large particles, which become even more large particles through agglomeration, which eventually leads to complete separation of the oil and water phase.

To expand on this further, a beverage emulsion underwent SPOS testing after various passes through a homogenizer. Particles greater than 1 µm in size are two to three standard deviations away from the mean of the main peak, which is at about 0.35 µm. So, the SPOS technique is looking at a very small part of the overall distribution. But, according to Turbitt et al, this range is where colloidal stability is either achieved or not. It took six passes through the homogenizer to reduce all the particles to smaller than 2 µm. It is worth noting that light-scattering data indicated that, from passes one through six, the mean diameter did not change significantly.

The manufacturing of emulsions is an example where utilizing multiple particle-size measurements is the best approach. First, monitoring the homogenization process can help ensure that it achieves the specified mean diameter. Wide-dynamic-range particle sizers of the light-scattering type are appropriate tools for this. After the correct mean diameter is achieved, a method like SPOS can make sure that the number of passes was sufficient to remove oversized particles, which act as sites of nucleation for further agglomeration -- in other words, to stabilize the emulsion. No technology exists to measure the mean diameter of an emulsion and, at the same time, provide quantitative information on the presence of a small number of large droplets. This requires two separate measuring tools.

Particle size can come into play in many aspects of food product formulation. It influences the processing, the handling, the shelf life, and finally, the eating of products. Depending on the specific food item, particle size can figure into the taste, color, texture and smell of the final product. These are the characteristics of foods that the customer cares the most about, and they will determine whether a product becomes economically successful or not. Given the influence particle size has on the manufacturing of food products and their final properties, it should be obvious why we measure particle size.

Patrick O'Hagan is national sales manager with Particle Sizing Systems, New Port Richey, FL. He can be contacted at [email protected] or 727/846-0866. Kerry Hasapidis, vice president; Heather Helsing, sales engineer; and Greg Pokrajac, Asian sales manager, all from Particle Sizing Systems, also contributed to this article.

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