Effect of storage on Texture: Dispersions, Emulsions and Colloidal Systems

Dispersions, emulsions, and colloidal systems undergo textural changes during storage as with any other food group, but the nature of these changes is different due to their unique structure. A dispersion is a two-phase system made up of a continuous and a dispersed phase. Colloids are a type of dispersion, but with a dispersed component too small to be detected under the light microscope. Emulsions are colloidal dispersions of liquids in liquids. The physicochemical laws and factors governing the stability of these systems are well known.

Three of the most common changes to emulsions on storage are creaming, flocculation and drop coalescence:

• Creaming refers to the separation into a concentrated layer of dispersed drops and a small volume of continuous phase. It can be upward or downward, depending on the density difference between the two liquid phases. Creaming can be prevented by eliminating these density differences.
• Flocculation is aggregation of droplets, which entraps the continuous phase liquid. The aggregate breaks into individual drops under high shear conditions, except in the case of very concentrated systems.
• Drop coalescence follows flocculation when, during storage, the continuous phase film between the drops drains away and the protective emulsifier layer around the drops ruptures. It is associated with fat separation and leads to product rejection.

Flocculation increases emulsion viscosity, while drop coalescence decreases it. Total breakdown leads to two distinct layers.

Mayonnaise

Mayonnaise is an oil-in-water emulsion consisting of oil as the dispersed phase and a continuous aqueous phase consisting of vinegar, egg yolk, salt, sugar, and spices. Egg yolk and mustard act as emulsifying agents. Normally, in an emulsion, the phase in greater quantity is the continuous phase; this is reversed in mayonnaise to give the product its characteristic texture. Such an “unnatural” emulsion tends to be unstable and difficult to prepare.

Back Extrusion Rig

Mayonnaise is particularly unstable in cold and frozen storage. If the oil used is not properly winterised (low temperature solid phases removed), fat crystals formed at refrigeration temperature will break the emulsion. The emulsion will also break on freezing, due to the puncturing by the ice crystals of membranes covering the oil droplets.

The Back Extrusion Rig is ideal for the measurement of mayonnaise consistency. It is comprised of a sample container centrally located beneath a disc plunger. The disc plunger performs a compression test, extruding the product up and around the edge of the disc. Results relate to measurements of viscosity. Using the back extrusion principle (i.e. using a disc on the probe adapter) the consistency can also be determined directly in containers out of the running production. This rules out pre-stressing of the material caused by transfilling.

The force profile measured as the probe enters the sample relates to the sample firmness (maximum force) and consistency (positive area under the graph). As the probe withdraws from the sample, the maximum negative force is recorded as cohesiveness and the negative area under the graph as the work of cohesion.

Comparison of consistencies of two moisturising cream types
when subjected to backward extrusion 

Salad dressings

Salad dressings may be divided into several categories: spoonable, i.e., mayonnaise-like, and pourable (bottled) in either an emulsified or separating form. The first two types are oil-in-water emulsions. The third type is an unstable mixture of oil and water that must be shaken prior to use. Only the emulsion-type salad dressing will be considered here.

Spoonable salad dressing typically contains about 50% less oil than mayonnaise. The desired consistency is achieved through the presence of gelatinised starch or other water-binding carbohydrates. Pourable salad dressings contain 40–60% oil, depending on the type. A number of reduced, low, and zero fat salad dressings are currently available in the marketplace, responding to the consumer demands for less fat in the diet. Hydrocolloid gums are used as emulsifiers and also as thickening agents and substitutes for the rheological properties of fat in pourable salad dressings.

In general, the texture of properly prepared salad dressings is quite stable on storage. Any instabilities will involve structural changes in the emulsion and the gum system. Major textural changes take place in bottled salad dressing during the initial week of storage, showing increases in yield stress and consistency. These changes are caused by structure build-up due to the hydration of xanthan gum and coalescence of oil droplets. This relates occasional problems experienced by consumers with “pourable” dressings not being “pourable” due to improper formulation/processing and to quality control that takes place too soon after processing.

Forward Extrusion Cell

The Forward Extrusion Cell is suitable for measuring the texture of more viscous salad dressings. This rig measures the compression force required for a piston disc to extrude a product through a standard size outlet in the base of the sample container. The complete sample container is located into a centralising insert fitted into the Heavy Duty Platform and a piston disc is attached to the loadcell using a probe adapter. In this test, the mean extrusion force is calculated and recorded as the product’s ‘firmness’.

Mustard

Prepared mustard is a smooth, spreadable paste consisting of ground mustard seed (or other form of mustard), vinegar, spices, and other flavouring materials. It contains about 79% water and about 5% oil derived from mustard seed. Changes to mustard during storage include syneresis (an expulsion of liquid), and a decrease in yield stress and consistency. The decrease in yield stress indicates that storage weakens the sample’s internal structure – flow and spreading require less force.

The Forward Extrusion Cell is suitable for the measurement of smooth mustard texture. However, if granular mustard is to be tested, the Back Extrusion Rig can be put to use with the selection of a smaller extrusion disc (i.e. creating a larger annular gap). Products containing particulates cannot be tested using the Forward Extrusion Rig as erroneous data is produced when the particulates attempt to extrude through the annulus.

Dispersions, emulsions and colloidal systems may all have a basic structure in common, but their textures vary from runny liquids to hard solids. The perfect Texture Analysis measurement exists for all samples upwards from viscous liquids.

Reference: Food Storage Stability, Chapter 8 “Effect of storage on texture” – Alina S. Szczesniak

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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