Shape and size matter when analysing food particles

It is vital that food manufacturers have a greater understanding of the influence of both particle shape and size on ingredient properties. Helen Metcalfe reports.

Many food products and their ingredients exist in particulate form ranging from powders to emulsions, suspensions and pellets. The shape and size distribution of particles can have an important impact on many aspects of food including: taste, texture, appearance, stability, processability and functionality of the final product.

Until relatively recently, the food industry primarily focused on traditional sieving, as the most common technique for particle sizing. While sieving is still widely used to determine particle size in the food industry today and indeed, still one of the methods used by researchers at Campden BRI, there is an increasing awareness and acceptance that particle shape also has a significant bearing on food products' characteristics.

The shape of particles not only affects the mouth-feel and taste of food but also affects how certain products behave during processing. The bulk density, compressability and flow-ability of raw materials are highly dependent on particle shape as well as size. For example, how a powdered ingredient, such as flour, flows or how much water is absorbed during the processing of a dough or a batter, can have an impact on processing. This could potentially lead to interruptions in production or technical issues with equipment.[Page Break]

Analytical instrumentation

Proper particle characterisation and analysis requires accurate and reliable analytical instrumentation. The challenge for food manufacturers is to select the best techniques and the optimum technology available to accurately determine particle shape and size.

There are issues with some methods of measurement, such as laser diffraction, which measures the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample.

While laser diffraction can determine the size of particles ranging in size from hundreds of nanometres up to several millimetres in size, it assumes all particles are spherical in shape, which is not always the case. Therefore, with this technique, it is impossible to accurately determine the size of irregularly-shaped particles.

Campden BRI, as part of its latest installation of £1.4m worth of analytical and processing equipment, has recently installed a brand new, leading-edge particle analyser that can accurately measure the shape and size distribution of particles from two microns (µm) up to thirty millimetres (mm).[Page Break]

How it works

Dynamic image analysis using a pulsed laser allows fast quantification of particle shape and size distribution. Due to the unique way in which the machine operates, scientists at Campden BRI can now analyse the shape of particles, as well as their size.

Dispersion is key to the way the system works because it allows a large number of particles to be imaged in a short space of time without any 'overlaps' occurring.

The innovative, new particle shape and size analyser uses an extremely short exposure time capturing up to 450 images per second to allow fast methods of dispersion to be used while still providing clear images from particles travelling at high speeds guaranteeing proper dispersion of agglomerated, fine, and cohesive powders.

As images of the particles are captured in freefall, their orientations are completely random; this provides a much more accurate representation of particle size than can be obtained by static image analysis systems, yet the speed at which images are captured means particle edges are sharp and are independent of their position in the measuring zone.

Particle sizes between 2µm and 30mm can be measured and primary measurement data can be evaluated in individually definable formats. Rapid image capture, allowing the recording of up to 450 images per second, ensures high statistical security of measurement results in short analysis time.[Page Break]

Due to the equipment's range, particle size analysis can be performed on a wide range of food products from very fine powders to larger objects such as cereal grains.

The flexibility of the system allows rapid analysis of a range of samples with the modular system allowing quick swaps between different types of analysis.

Large particles can be dispersed using a vibratory feeder and gravity disperser allowing particles to be imaged with little or no overlap, whereas small particles can be dispersed under a compressed air system where a short exposure time provides clear images from particles travelling at high speeds.

The pressure used in the system can be adjusted for agglomerates in order to look at the whole particle or, by introducing more energy, breaking it up into its component parts.

In contrast to other approaches, the particles are not biased by the orientation in which they are presented.[Page Break]

Liquid emulsions

The analyser also accommodates measurements of particle size in wet systems. This not only means that particles suspended in liquids can be measured, but also bubbles and droplets in the liquid are quantifiable. Liquid emulsions and slurries can all be analysed.

Particle shape can be measured by considering two or more measures of particle size and studying the ratio between them.

These data provide much more information about the particles' properties and from this, inferences can be made about the properties of the substance, as different particle shapes can have an impact by, for example, altering the surface area to volume ratio of the particle.

This information complements particle size information to provide a lot more detail about the particles in question and allows a better understanding of how the size and shape of the particles contribute to specific qualities in the end product.[Page Break]

Practical applications

There are a range of practical applications in the food and beverage industry for the accurate analysis of particle shape and size. In terms of quality control, it is of great importance in assuring consistently safe and high quality food and drink products.

For example, in more complex products, such as 'three-in-one' coffee drinks which consist of a number of product ingredients such as coffee granules, milk powder and sugar, which all have different particle shapes and sizes, the analyser can provide invaluable data to help with quality control measures.

It also helps with ingredient specifications in new product development. For example, knowledge of the basic physical properties of sugar is of great importance in determining its usefulness in food formulations and the quality attributes and acceptability of foods containing these components.[Page Break]

Diverse products

Among some of the specific uses to which Campden BRI is putting the new particle shape and size analyser is the analysis of products as diverse as wholemeal flour and cream.

Recent research work has involved the analysis of the particle size distributions in brown and wholemeal flour and has highlighted the effectiveness of the analyser at identifying the differences between different flours prepared by different processes.

Potential baking industry applications could be looking at the differences between the sizes of bran particles in different types of flour and identifying which flours might perform best in their process.

Campden BRI research has shown that the analysis of cream samples has significant potential, as fat globule size has a major influence on the functional properties of the cream and therefore the way it can be processed.

There is also the capacity to investigate the effect of different processing regimes on globule size distribution.

The range and potential applications of the equipment to powders, mixtures, slurries and emulsions is wide and varied - from quality control and ingredient specifications to product development. Campden BRI is also investigating the application of particle shape and size analysis to other non-food applications.

Campden BRI is on hand to offer food manufacturers both large and small, an analytical service to provide a greater understanding of how particle shape and size affects product attributes and performance. In addition, interested parties are invited to approach Campden BRI with ideas for applications for their products and potential exploratory research work.

Helen Metcalfe, Research Scientist, Campden BRI, Chipping Campden, Gloucestershire, UK.

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