Compression Testing using a Texture Analyser – Calculating Fundamental Parameters

Compression testing involves a self-supporting sample being compressed under a flat probe large enough to cover the whole sample. 

The loading arm (attached to the probe) moves down at a constant speed to deform the sample, first deforming it elastically then plastically (if it is not completely brittle). If the force required to break the sample is within the limit of the load cell, fracture may occur. However, this is not always the case in compression as many samples are stronger (or tougher) in compression than in tension, especially ceramic samples.

Three Point Bend Testing using a Texture Analyser – Calculating Fundamental Parameters

When a customer is presented with an elongated or flat food sample, one of the first things they might do is snap it between their hands. 

During this process, they are subconsciously assessing whether the food product is brittle or ductile, stiff or compliant, and strong or weak. Consequently, bend testing is a technique used very frequently by Texture Analyser users, covering everything from the snap of tablets to the toughness of dog chews. 

The analysis of this type of testing is often limited to looking at a force peak and maybe the distance to fracture. It is a very useful test for monitoring quality of irregular objects. However, this test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample dimensions. “Flexure” and “bending” have the same meaning and are often used interchangeably.

Tensile Testing using a Texture Analyser – Calculating True Stress and True Strain

The calculation of parameters from the stress-strain graph of a tensile test has already been covered in a previous blog post. 

The calculations considered were those most often used when referring to stress and strain, and to give them their full name they would be called “engineering stress” and “engineering strain”. It is usually safe to assume that every time stress and strain are mentioned in the literature, this refers to the engineering values.
 
However, as the load on a sample increases, the cross-section over which the force is applied changes (it gets thinner). If the engineering stress is used (taking into account the initial area), the stress is underestimated. True stress solves this issue by using the instantaneous area over the course of application of load so that as the cross-section changes, the value of stress is calculated using the new cross-sectional area.
Additionally, whereas engineering strain is the amount that a material deforms per unit length, true strain is the natural log of the current length over the original length.

Tensile Testing using a Texture Analyser – Calculating Fundamental Parameters

Tensile testing involves a sample held in two grips a set distance apart. The loading arm (attached to the top grip) moves up at a constant speed to deform the sample, first deforming it elastically then plastically. If the force required to break the sample is within the limit of the load cell, fracture will occur.

It is a very useful test for monitoring quality of irregular objects, such as the toughness of pizza or the texture of fish. However, this test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample’s dimensions. “Dogbone” shaped specimens are often used in tension, with two wide sections tapering to a narrower central section.

Indentation Testing using a Texture Analyser – Calculating Fundamental Parameters

Although it has long been known that some materials are harder than others, indentation tests to find quantitative hardness values only came about in the 1800s. 

Once it was established as a valuable technique, hardness testing machines started to appear on the market early in the following century. Old fashioned indentation testing involved the application of a weighted probe onto a flat sample surface that was left for a set time period. The hardness of the sample was calculated from the area of the residual dent left in the sample. “Instrumented” (computer controlled) indentation testing has now been in use for many years, and involves the collection of force, displacement and time data, which is why the Texture Analyser is so well-suited to this type of measurement.

Plasticity Measurement of Soft Solid Foods

Plasticity is a property shown by many materials, such as polymers, metals and the majority of foods. It is the ability of a material to undergo permanent deformation. 

If you press your finger on a spring, it returns to its original shape. If you made that spring out of cheese, it would not spring back as far – it would remain squashed. When stretching a sample, plasticity is seen on the force-distance graph as a change in gradient after the initial linear section (straight line). The point where the gradient changes is known as the ‘yield force’. If this point is reached and more force is applied, the sample will be permanently changed. Before this point, the behaviour is elastic and spring-like.

Food Texture: A video, a webinar, latest research and capturing more

Explaining texture preference

Understanding texture preference and the trends that may result in consumer behaviour change is an area of great opportunity. However, remaining agile before a consumer shift occurs can be a challenge.

A new webinar will address this challenge and will offer:

• A view on techniques to forecast upcoming influences on food trends 

• A perspective on the linkage of newly monitored issues to texture and the process used to deliver higher value commercial insights 

• Expertise on driving rapid product development through localising relevant texture trends from around the world

Speakers: Janet Carver, Culinology Group; Mary Lynne Shafer, Ingredion; Susan Badarroco, Culinary Tides.

Watch this webinar to learn more...

Optimise your Salmon texture testing

Fish is loved by consumers – it is unique, nutritious and well-balanced. The four quality elements of food are texture, appearance, flavour and nutrition.

Compared with the meat of livestock, tissue of fish is more soft and delicate in its chemical composition.

With the development of people's living standards and of the food processing industry, texture has attracted more and more attention. 

A recently published paper describes several texture parameters, analyses various factors affecting fish tissue texture especially the protein composition of fish muscle tissue from 3 aspects – physical factors, chemical factors and post-mortem changes – and mainly reviews the effects of different processing methods such as heating, freezing, salting and smoking on texture of fish meat.