When crunch comes to chomp

Instron Pty Ltd
By Richard McManuis, Product Manager, Instron Pty Ltd
Saturday, 06 August, 2005

Meeting customer expectations for product quality is essential to success in any competitive industry. In the grain-based food industry, change is constant, and the need to maintain quality while satisfying customer demands for new products is an ongoing problem. Add to this the need to reduce costs, and you have a clear view of the daily challenges facing the grain-based food industry. Texture analysis of cereal, bread, biscuits, snacks, pasta and other products can help the grain-based food industry meet these challenges.

Texture analysis can be done using a sensory panel - a group of people selected to taste food and provide feedback - or by instrumental methods. Compared with sensory panels, which are costly and time consuming, instrumental methods can save time, reduce costs and provide more consistent, objective results. However, since it is difficult for machines to simulate biting and chewing, the need for sensory panels as a correlative test method will continue for the foreseeable future.

The modern texture analysis instrument evolved from simple mechanical devices operated by hand pressure. While good for their time, these devices produced varied results. There was no control of the rate at which force was applied so viscoelastic properties varied widely. Measurement accuracy was often influenced by the skill of the operator. And transcription errors occurred when calculating results and making reports. Today, instruments with digital control circuits apply force at precise rates. Electronic force and deformation measurement systems provide accurate, repeatable results. Texture analysis software performs complex calculations in seconds and produces detailed reports. Today, improved texture analysis instruments allow researchers to develop new test methods and automate the data reduction and reporting process. Quality assurance professionals also benefit from the improved repeatability that allows more accurate trend analysis reports.

Instrumental texture analysis is a cost-effective way to provide repeatable results quickly. In a typical analysis or test, a sample is placed into the test area of a texture analyser, and force is applied using a probe, knife or fixture. As force is applied, the resulting sample deformation is recorded. Force versus deformation data is analysed. Results provide information such as firmness, hardness and fractur-ability of the specimen. Since biting and chewing compress and shear foods, most texture methods are designed to apply compressive force. The type of probe, knife or fixture used determines whether the principle stresses in the sample are compression or shear.

Is the bread fresh?

In a common instrumental texture test for bread firmness, a flat-end probe having an area of 10 cm2 is used to deform a slice of bread. The probe is moved at a constant rate, usually between 10 and 50 mm/min to compress the bread. The force required to deform the slice by 25% of its original thickness is used as a measure of firmness. A modern texture analyser, such as an Instron Model 5542, can measure the original slice thickness and automatically compress it by the desired percentage. This test can be repeated at regular intervals within hours to days of baking to determine time-dependent properties, like 'staling' or shelf life information. Time-dependent texture analysis is also important for cereal. Unlike the staling test for bread, the analysis for cereal is done over a span of minutes, or the time the cereal takes to soften in milk.

Another common analysis or texture test used for more brittle foods is the three-point bend or snap test. A cracker, biscuit or muesli bar is placed across two support anvils, 25 to 50 mm apart, forming a bridge. A third anvil is used to apply force to the centre of the bridge until it breaks or snaps. The force at which the sample snaps is a measure of brittleness.

Snack foods are often judged by their 'crunchiness'. Many snack foods consist of irregularly sized and shaped pieces. To produce more consistent texture results, most methods specify simultaneous testing of many pieces to average or normalise geometric differences. A specific number of pieces (greater irregularity requires more pieces be used) are placed between two flat, 150 mm diameter platens in the texture analyser. The platens compress the sample pieces at a constant rate. The results usually include force at the first peak, average force over a range of deformation, and force at final deformation.

For some snack foods, an individual piece may be used as a sample. For example, a tortilla chip can be placed on a ring-shape or three-legged support. A spherical shape probe is then used to apply force to the chip at a constant rate. The force at fracture is used as a measure of crispness.

Testing your noodle

Although most texture tests are done using compression loading, there are occasions where tensile loading is desired. Texture of cooked pasta is often determined from a tensile test. Each end of the pasta noodle is wrapped around a mandrel style grip several times. Friction holds the noodle to the mandrel while tensile force is applied. The texture analyser records force and extension data from which tensile strength and percent elongation results are calculated.

Pasta stickiness is another example of a tensile test for texture. In this test, a cooked, flat noodle is held horizontally between a base plate and an upper plate with an aperture in the centre. A flat-end probe is lowered through the aperture to establish a small compression force on the noodle. As the probe is pulled away from the noodle the tensile force is recorded as a measure of stickiness. Data from this test can be used for process control in production areas and to determine whether the product will meet customer acceptance criteria.

The stickiness test is also useful for measuring dough. For example, in making pies and other filled products, the two layers of dough must stick together until baked. If the dough has too little stickiness the filling will leak, and if it has too much the dough will cling to the processing machinery.

Designing a test procedure

To produce meaningful results, there are numerous considerations: the design of a texture analysis method; accuracy of the instrument used; selection of results calculations; and type of probe or fixture chosen. When designing an analysis method one should consider which texture properties are sought. For example, compressing the entire surface of a sample between platens will produce compressive stress but very little shear stress. However, using a probe to compress a small area within the perimeter of the sample will produce more shear stress. Which method to use should be determined by the type of stress that most closely approximates the stresses from biting, chewing, processing or other conditions the sample will experience.

Table 1 serves as a guide for selecting an instrument to meet texture analysis needs.

Preparation and handling of the samples is equally important. For example, firmness results can vary widely if bread samples are inadvertently stacked on one another or handled roughly before analysis. Sample moisture content should be maintained as specified in the analysis method, as well. Hourly or daily temperature and humidity variations in the area where analysis is performed can also increase varied results. Eliminating variables in sample preparation, handling and analysis methods ensures that the results represent differences between samples.

Texture analysis is a useful tool for monitoring product quality and providing an objective measure of customer acceptance. It can be used to shorten new product development cycles, and instrumental methods are faster than sensory techniques. Texture analysis can also be used for process control feedback. For example, variations in baking temperature or other process controls can be detected and the information can be used to adjust or modify processing parameters.

Recent years have brought many changes that indicate texture analysis is enormously beneficial to the grain-based food industry. This type of analysis is needed to maintain overall quality of foods as ingredients change, as competition increases, and as new products are developed to meet customer demands. The increased popularity of bagels during the late '90s opened the market to new suppliers. This type of increase in competition forced higher standards of quality - standards that texture analysis can ensure. Falling prices of ready-to-eat cereals drove cost reduction efforts to salvage profits. Implementation of texture analysis assisted in keeping quality high and costs low.

Whether companies are meeting consumer demand for healthier foods by fortifying them with nutraceuticals and organic ingredients, or using antioxidants to increase shelf life, texture analysis is the key to maintaining high quality and low costs. Many companies realise they can no longer rely on a seasoned employee to assess texture quality the old way - by poking, stretching, squeezing, etc. The subjectivity inherent in this approach and the fact that testers cannot work around the clock or be at multiple sites has given companies the justification needed to incorporate instrumental texture analysis into R&D and quality programs.

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