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Particle Size in the Food Industry

Whether as powders, suspensions, or emulsions, many food items are present in particulate form at one time or another during their production process.

For powders, particle size affects bulk density. By doing so, it also affects powder flow properties. In a similar manner, particle size strongly influences shear viscosity in suspensions. This shear viscosity, in turn, affects the pumping, mixing, and transportation of the raw materials.

Particle size has a strong influence on the storage and stability of food products. While powdered food items can undergo caking, emulsions (e.g., milk) can have stability issues if the droplet size increases, or it isn’t thoroughly controlled. 

Texture and mouthfeel play pivotal roles in the acceptability of food products. The particle size has a major influence on their organoleptic properties. Since the human tongue is capable of detecting particles of just a few microns in size, tweaking the size of particles can have a big impact on product acceptance.

In order to determine the sizes of small particles (0.3 nm to 10 µm), such as coloring and flavoring components, the Litesizer is the instrument of choice with its dynamic light-scattering technology. You can analyze larger sizes (40 nm to 2.5 mm) with PSA, our laser diffraction instrument.

The perfect coffee taste

Figure 1: Volume-weighted particle size distribution of coffee. CC BY 4.0 licensed

 

A huge variety of coffee beans, various blends, and different roasting techniques and degrees influence the taste of coffee. In addition, the particle size of coffee is crucial. In order to get the perfect coffee flavor and taste, you also need to optimize the particle size for each coffee and brewing method. Espresso, for instance, uses a very fine coffee powder, while filter brewing or French press preparations use higher particle sizes. 

To be able to reproduce high quality ground coffee, the particle size needs to be controlled regularly after grinding. The Particle Size Analyzer (PSA) uses laser diffraction technology to determine particle sizes between 40 nm and 2.5 mm and covers the optimal range of coffee powder. Fast and easy measurements of the dry powder ensure reproducible high product quality and the perfect coffee taste.

The graph compares the particle size distribution of an espresso powder with the one for filter coffee. The main fraction for espresso is below 1 mm with a peak at 600 µm. In contrast, particle sizes for coffee powder used for filter coffee or coffee crema are much higher. Here, the peak is at 900 µm with a significant fraction above 1 mm. 

Milk and milk powders

Figure 2: Volume-weighted particle size distribution of milk. CC BY 4.0 licensed

For milk, creaming is a shelf-life-limiting factor. Creaming amount and visual appearance are influenced by fat globule size, storage time and temperature, package size and shape, and milk quality. The particle size of milk fat globules usually ranges from 0.1 µm to 20 µm with an average size of 3 µm to 4 µm (1). Low-density fat globules float to the surface, a process called “creaming.”  Shelf life is prolonged by homogenization, which leads to a unified and reduced homogenized milk fat globule size. Here, homogenization efficiency can be evaluated by D[5,3] and D90 values.

The graph compares the particle size distributions of homogenized elongated shelf life milk (ESL) with fresh pasteurized milk. These were determined by laser diffraction using the PSA liquid mode. For ELS milk, a monomodal particle size distribution is shown with a peak size around 1 µm, which represents the fat globule size in homogenized milk. For fresh milk, in contrast, two peaks are present with a fat globule fraction between 2 µm and 10 µm. These differences in milk particle size are represented by D[5,3] and D90 values. 

The Underestimated D[5;3]: Determining Homogenization Efficiency of Milk by Laser Diffraction

Milk powder should dissolve quickly, have a good flowability, and the particles shouldn’t stick together. Using the PSA Dry dispersion unit lets you determine milk powder particle size and enables quality control.

Characterization of Milk Powder – Chapter 1: Dissolution of Infant Formula and the Final Milk Product

Milk Powder Characterization – Chapter 2: Packaging, Transport, and Storage

1. Tetra Pak. Optimized homogenization of milk technology, measurement and processing strategies. s.l.: Tetra Pak International S.A., 2019

Flour

Figure 3: Volume-weighted particle size distribution of flour. CC BY 4.0 licensed

Finding the flour that is most suitable for an application isn’t easy. Flour is available based on different cereals and in a huge variety of grinding degrees that are optimized for specific applications, like baking a cake or some bread. Flour grain size not only influences mouth-feel and appearance of the final product but also batter or dough properties such as density and viscosity, and the final product volume as well as quality or softness.

Particle size determinations allow quality control but also optimization of the manufacturing process regarding temperature and humidity. Detecting the % of very fine particles helps to judge the risk for dust explosions during manufacturing. 

The graph shows the particle size distribution of unconditioned flour with a size range of 80 µm to 500 µm and the influence of 35°C and 95 % humidity (conditioning). Due to swelling and aggregating particles in the range of 200 µm to 900 µm, particle size dramatically increases with increasing humidity.

Food Characterization

Chocolate

Figure 4: Volume-weighted particle size distribution of chocolate. CC BY 4.0 licensed

 

When it comes to chocolate, you expect a particular soft, smooth, and creamy mouthfeel – not to mention a typical flavor. Creating such a distinct mouthfeel requires a defined texture and melting characteristic of each chocolate. The particle sizes of the ingredients, along with the fat and lecithin content, significantly influence these characteristics. But so do other properties, such as yield stress, viscosity, firmness, hardness, and the melting index. For this reason, determining the particle size of chocolate on a regular basis during the manufacturing process ensures both intermediate and final products are always high-quality. What’s more, it also lets you optimize the process and the final product.

The particle size of a typical chocolate ranges from between 1 µm and 80 µm. That’s why laser diffraction instruments, such as the Particle Size Analyzer (PSA), are optimal for measuring the particle size at different stages of the manufacturing process. This is true for a laboratory setting or at the production line.

The graph shows a typical particle size distribution of white and milk chocolate analyzed in a defined triglyceride-solvent. White chocolate contains a higher number of large particles due to its higher sugar content and the missing cocoa.

Bittersweet Symphony: Optimized particle sizing in the chocolate manufacturing process

Sugar

Sucrose is the most commonly used form of sugar in the kitchen. Liquid plant extracts were refined and crystallized with the resulting product. We call this product “granulated sugar.” The grain size of sugar in mm defines its application field, which makes intense quality control indispensable. Coase sugar crystals up to 2.2 mm are excellent on patisserie products, such as cookies, cakes, and sweet breads. For biscuits, a medium size of 600 µm to 800 µm would be the right choice for the perfect mouthfeel. A caster sugar particle size between 150 µm 450 µm is used for bakery applications, such as smooth doughs, and is made by pulverizing granulated sugar. By milling granulated sugar, a powdered sugar particle size or icing sugar particle size of around 50 µm is produced, which is the finest grain size of sugar that’s commercially available. Since sugar dissolves rapidly in water particle size, conducting determinations by laser diffraction in dry mode give you fast and easy quality control of sugar grain size.

Particles for Breakfast: Using Laser Diffraction for Particle Sizing in Food

Flavored emulsions

Figure 5: Volume-weighted particle size distribution of flavored and colored emulsions. CC BY 4.0 licensed

Food flavor is of upmost importance for food quality, and is something consumers don’t want to compromise on. Flavor emulsions can be used as food excipients to improve the taste of many food products. They typically contain water, essential oils, and emulsifiers. The stability of these raw materials has to be ensured in order to prevent degradation during processing and to increase the shelf life of the final product.

With the Litesizer, we measured a flavor and colored emulsion after the production (Fresh) and after one day of storage at room temperature (One-day storage). After one day, particle size increased significantly from 155 nm to 2587 nm, which indicates that the flocculation process had started. Therefore, the mixture became more turbid and the measured transmittance decreased.

Infused beverages

Figure 6: Volume-weighted particle size distribution of infused beverages. CC BY 4.0 licensed

Plant extracts or other-health promoting substances such as vitamins are popular supplements in a variety of food and beverage products. These have gained significant followings in the health and wellness industries. Since a majority of these extracts contain hydrophobic components, emulsifiers are used to create stable “oil in water-emulsions” in order to overcome the mixing limitations. This method increases both the solubility and the bioavailability of the hydrophobic components. In this regard, particle size is critical for body absorption and bioavailability in general. It is also important for the storage and transport stability of the beverage. 

Control of droplet size via dynamic light scattering enables optimized beverage formulations, bioavailability, and storage conditions. The graph compares two infused beverages using different emulsifiers. The beverage with acacia gum shows a lower droplet size, which indicates  increased stability and body absorption, compared to the beverage that contains xanthan gum.

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