Shedding New Light on your Breakfast… using laser diffraction for particle sizing in food

Most food items on the breakfast table are in particulate form at one time or another during their production process, either in powder form or as emulsions. With its ability to measure both liquid and dry dispersions and its wide, nanometer-to-millimeter measuring range, Anton Paar’s Particle Size Analyzer (PSA) is ideally suited to the requirements of production and quality control in the food industry. Wake up to the benefits of laser diffraction by looking at a few of its foodstuff applications.

Introduction

Particle size in foodstuff not only affects most aspects of the production process, such as transport, storage or shelf life, but also crucially influences organoleptic properties, such as taste and mouthfeel. Here we look at a few selected food industry applications using Anton Paar’s Particle Size Analyzer (PSA), which measures particle size by laser diffraction technology.

Laser diffraction is based on the observation that the angle of light diffracted by a particle bears a direct correspondence to its size. The size of the particle and the angle of the diffracted light have an inversely proportional relationship, i.e., the angle decreases as particle size increases. In the PSA, dispersed particles (in dry or in liquid form) are directed towards a laser beam, which gets diffracted by them. The resulting laser diffraction pattern is then analyzed by a mathematical model, producing a particle size distribution.

Milk: Know the Particles, Know the Quality

Milk is an emulsion of butterfat droplets within an aqueous solution of carbohydrates, minerals and several proteins, the most abundant being casein.

Casein forms micelles in solution, which vary in diameter from about 100 to 200 nm, while butterfat droplets are mostly in the micrometer size range. The size of these particles can vary, which strongly affects the milk’s mouthfeel as well as its colloidal stability.

Here we used the PSA in liquid mode to compare the size of particles in commercial whole milk preserved by different methods. Fresh pasteurized milk was compared to ESL (Extended Shelf-Life) milk and to UHT (Ultra High Temperature) milk.

As shown in Figure 1, the particle size distributions for pasteurized and ESL milk displayed a single peak, indicating a relatively homogeneous particle size. The mean diameter was 1.66 μm for the pasteurized milk, suggesting that the butterfat droplets dominated the distribution. The mean particle size was only very marginally decreased in ESL milk (1.58 μm).

In contrast, mean particle size was significantly lower in UHT milk (1.2 μm) while the span, which gives a measure of the broadness of the distribution, was enhanced compared to pasteurized and ESL milk. This indicated that the butterfat droplets in UHT milk have been broken down by the heat treatment and that the resulting emulsion was more heterogeneous than pasteurized milk.

In all, the results suggested that, compared to pasteurization, the ESL treatment only very marginally modi ed the organoleptic properties of milk. This was not the case for the UHT treatment which strongly modified the size distribution and, presumably, also the taste and mouthfeel of the product.

Figure 1: Volume-weighted particle size distributions of pasteurized, ESL and UHT whole milk samples. Results are expressed as mean particle diameter (nm) and span, which is calculated as (D90 – D10)/D50.

Coffee: The Right Grind Makes the Good Cup

It does not take a professional barista to know that the particle size of ground coffee, together with the brewing method, crucially influences the final taste of the infusion. Grinding must be perfectly adapted to each brewing method and, for industrially ground coffee, the size of particles must be tightly monitored.

While filter coffee brews slowly and benefits from a coarse grind, espresso coffee, which is brewed quickly and under pressure, requires a finer grind. The fine grind exposes more particle surface to the hot water and enables a quicker flavor extraction. However, very fine particles tend to release bitter flavors, and oppose more resistance to the water flow. Therefore, a homogeneous (monomodal) size distribution is desirable for most brewing methods, while a mixture of large and small coffee particles must be avoided.

Here we show the results of a ground coffee sample destined for filter brewing. The sample was dispersed in dry form using the PSA’s Venturi system, under low air pressure (500 mb). As shown in Figure 2, the coffee sample displayed a clearly monomodal size distribution, with a D50 value (i.e., median size of particles) of 836 μm. This corresponded to the coarse grind expected for filter coffee. The low span value (0.9) confirmed the narrowness of the distribution and the monomodal nature of the sample.

Together, the large and highly homogeneous particle size and the apparent lack of small particles suggest that the sample is of good quality and well suited to the lter brewing method.

Figure 2: Volume-weighted particle size distribution (grey bars) and undersize (red curve) of ground filter coffee. The dry sample was dispersed under low pressure airflow.

Sugar: The Delicate Delicacy

The most commonly used form of sugar in the kitchen is sucrose, a disaccharide of glucose and fructose. It is found in the tissues of most plants but is overwhelmingly produced from sugar cane and beetroot. Extraction takes place in liquid form, and after refinement and evaporation, crystallization is induced in the supersaturated syrup by the introduction of sugar crystals. The resulting product, termed granulated sugar, has a particle size in the lower millimeter range which is strongly influenced by the different manufacturing steps.

Finer forms of sugar are also required for many baking applications. Caster sugar, or fine granulated sugar, is produced by pulverizing granulated sugar to reduce the particle size. Powdered (or icing) sugar is the nest form of commercial sugar and is produced by milling granulated sugar.

The close monitoring of particle size during sugar production is therefore of paramount importance, and the PSA is an ideal candidate for the task. One of the advantages of the PSA
is its ability to disperse powder samples using two distinct methods. In the so-called Venturi mode, the powder is fed into a compressed air chamber and ejected through a Venturi tube at a controlled pressure, which can be set by the user between 100 and 6000 millibars.

For particles larger than 300 μm, the free-fall mode can be used as alternative to the Venturi mode. In that case, the sample is fed into a manifold whose opening is positioned directly above the laser beam. The particles then simply fall in front of the detector and no pressure is applied.

We measured a commercial powdered sugar sample, which had an expected particle size below 100 μm, using the Venturi dispersion mode. We determined the minimal pressure capable of generating an adequate dispersion to be 500 mb. As shown in Figure 3 (upper panel), the median particle size using these conditions was 33 μm, with a span value of 2.55 indicative of a relatively broad distribution. Increasing the air pressure to 2000 mb significantly decreased the median particle size (20.6 μm) while increasing the span (3.5), clearly indicating that the high pressure led to a breakage of sugar particles.

Figure 3: Volume-based particle size distributions (grey bars) and undersize (red curve) of powdered sugar dispersed by Venturi system at 500 or 2000 mb.

Caster sugar, with an expected particle size above 300 μm, was measured using both dispersion methods. Measurements performed using the free-fall mode returned a clearly monomodal distribution and a median particle size of 760 μm (Figure 4, upper panel). However, using the Venturi mode even at a relatively low pressure (1000 mb) led to a more than 50% drop in median particle size. Measuring at high pressure (4000 mb) led to an even more marked particle size reduction, with the D10 value dropping by 95% compared to the free-fall mode, indicative of a dramatic degradation of the sample.

Figure 4: Volume-based particle size distributions (grey bars) and undersize (red curve) of caster sugar dispersed using the free-fall mode or the Venturi mode at different pressures.

The PSA’s strong modularity in terms of dry dispersion method is therefore particularly advantageous when measuring fragile powders such as sugar crystals, which are highly susceptible to breakage.

Conclusion

With its extended measuring range (0.4 μm to 2.5 mm) and the variety of dispersion methods available (liquid, Venturi, free-fall), the PSA is ideally suited to perform particle sizing for the food industry. Developed by the pioneers of the laser diffraction technology in the 1960s, the instrument has benefited from 5 decades of continuing research and development and has acquired a strong reputation for robustness and reliability.

Article written by:
Nathalie Etchart, Daniel Paul, Carina Burgstaller, Aleksandra Mitrovic

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