Let’s do lunch

21 May 2013 by Evoluted New Media

Materials testing is a laboratory technique that knows almost no bounds. By the addition of the appropriate sample grips, fixtures and accessories, a single test instrument can be transformed into making a multitude of tests. You need proof? Let us test your lunch for you…

Many people may be settling down over lunch to read their copy of Laboratory News. Those of you who have popped out for a sandwich, packet of crisps, yoghurt and a piece of fruit might find it surprising that a universal materials tester could have been involved in testing every part of your lunch, including the credit or debit card you used to pay for it!

[caption id="attachment_33016" align="alignleft" width="200" caption="Figure 1. Single and twin column materials testing machines"][/caption]

More about ‘lunch testing’ later, but first, let’s take a look at the basic principles of materials testing. Universal materials testing machines allow precisely controlled tensile or compression forces to be applied to the sample under test over a controlled period of time. Precision loadcells measure the material under test as a function of force against extension and time. Materials testers are available in a variety of sizes, and ar

e defined by the maximum force that they can apply (Figure 1). Bench mounted single column instruments can typically provide a maximum force from 1 kN (225lbf) to 5 kN (1124 lbf). For higher forces, twin column instruments are needed, going to floor-mounted versions capable of delivering a maximum force of hundreds of kN. Robust, high stiffness load frames are at the core of each materials testing system, with modern linear guide technology, pre-loaded ball screws and advanced software compensation ensuring high displacement precision. Depending on the machine, elongations of between 1 micron and 2.5 m (98.4 in) can be measured, meaning tests can be performed on anything from a human hair to large components as diverse as complete car seats, large cardboard cartons, mattresses and large diameter pipes. Although all materials testers basically apply a controlled compressive or elongation force, the secret to the versatility lies in the sample holders and jigs and fixtures. For example, support a sample at either end and apply a compression force to the middle and the system measures bend strength! Table 1 indicates just some of the tests that can be performed using a materials tester.

As materials testing is a well-established technique, it comes as no surprise that a vast range of internationally recognised standards (ASTM, ISO, FINAT etc) exist. Most modern machines will have a number of these standard operating methods stored in a software library so that they can automatically be called up when required. Although materials testing is well known as a laboratory technique, it can also be used throughout the design, development and manufacturing process, from the evaluation of raw material to the finished product. It can be used to test pretty well any material you can think of. One special branch of materials testing is found in the food industry, where the instrument can be used for texture analysis.

Texture analysis testing evaluates the mechanical and physical properties of raw ingredients, food structure and for pre- and post- Quality Control checks. Texture testing has applications across a wide range of food types, including bakery, cereals, confectionery and snacks, dairy, fruit and vegetables, gels, meat, poultry and fish, pasta and even pet food. Since texture is a characteristic relating to sensory perception, it is a property that can be measured easily by mechanical methods in units such as force, distance and time. In texture testing, standard tests such as compression, tension and flexure are used to measure hardness, crispiness, crunchiness and other properties listed in Table 2.

[caption id="attachment_33017" align="alignleft" width="200" caption="Figure 2. Compression testing of bread"][/caption]

Comparison of the results from texture analysis together with trained human sensory panels has shown that the measurements correlate well with the various sensory attributes associated with textural quality. For natural products, such as fruit, vegetables, meat and fish, texture can be related back to the way the product is grown, or reared, while for processed food, the texture can be used to optimise the process. Texture analysis can highlight quality improvement opportunities throughout the supply chain and the production process. At the research and development stage, new or alternative ingredients can be compared with existing ingredients. In production, texture analysis is used for the measurement and control of process variations such as temperature, humidity and cooking time. As with ‘conventional’ materials testing, the key to the versatility of the texture analysis technique is the availability of huge number of grips and fixtures in a variety of sizes, gripping surfaces, styles and capacities.

Lunch testing

As mentioned above, a materials tester could have been used to test virtually everything associated with that lunch, which you may have finished by now!

Let’s start with the credit card used to pay for it. A special jig with an upper wedge grip and lower fixture containing rollers is available for peel strength testing of laminated layers of standard plastic identification cards. The jig allows 90? peel tests to be made at forces up to 200 N (44.96 lbf) for the measurement of peel strength between card layers in accordance with ISO/IEC 10373-1:2006 (E). The strip to be tested on the card is attached to an adhesive tape loop. This is passed through the roller assembly and held securely by the upper clamp so that the face of the card rests on the underside of the rollers. The grip therefore clamps the prepared card test layer so that a peeling angle of 90° is maintained during measurement. Up to four test strips per card may be prepared and tested.

[caption id="attachment_33018" align="alignleft" width="200" caption="Figure 3. Cutting resistance of cheese"][/caption]

Moving on to the sandwich, this probably arrived in plastic or cardboard packaging, and during design and manufacture, these packages could have been tested for properties such as tensile strength, puncture resistance, heat bond peel strength and tearing strength. The bread in the sandwich can be tested for firmness according to AACC (74-09) (Figure 2) and the spreadability of butter, margarine and other spreads can be tested. The cutting resistance for a filling such as cheese could be measured to ISO16305 (Figure 3).

For crisps, measurement of the crispiness and fracturability of crisps can be made using a penetration test (Figure 4), whilst burst strength, opening strength, seam strength, tear and peel are all parameters that can be measured on the crisp packaging.

For yoghurts, the consistency of yoghurt can be measured and a special jig is available for the measurement of the peel strength of yoghurt pot lids. When designing food container lids, a balance must be struck between a strong effective seal that prevents food contamination, and one that is easy to peel back. This jig consists of an angled platform that is positioned on a base plate and locked securely into position to suit the container. An upper fixture, connected by a non-elastic cord to the testing machine, grips the foil lid and the machine measures the peel forces in tension up to a maximum 500 N (112.5 lbf).

Finish off the lunch with a healthy fruit option, and texture analysis could also have played a role, since there are many factors that affect the texture of fruit and vegetables. The time of harvest and storage conditions have an effect on the rate of softening. Texture analysis can help determine the physical properties of fruit and vegetables, and how they change during ripening.

There are plenty of other potential applications for materials testing and texture analysis even within the ‘lunch’ example we have given, so it is clear that this equipment offers extraordinary versatility. It is no exaggeration to describe it as a universal technique.

[caption id="attachment_33019" align="alignleft" width="200" caption="Figure 4. Fracture testing of crisps"][/caption]