The most important physical properties of food quality are probably those related to consumer perception. Freshness of bread is commonly evaluated by lightly squeezing the loaf on the shelf. Its density is evaluated by feeling its weight, from which a consumer may imply something about chewiness.
When shopping, we commonly evaluate our potential purchases by examining their physical properties while handling them. Feel and appearance affect our initial purchasing decision, so it is imperative that manufactured products be produced with acceptable values for these physical properties. These values can be measured to quantify what consumers find acceptable about a particular food product.
A customer’s decision to buy the same food item again and again involves a more complex set of physical properties than those that can be evaluated simply by handling the food product. The properties of flavor, aroma, audible crispness, and mouth feel all become important. Producers must be able to measure and control all of these properties within a range of acceptable limits, or they won’t succeed in the marketplace.
The idea of controlling physical properties implies some means of measuring them. For food products there are historically two means of measuring properties: sensory methods and instrumental methods. Rigorous sensory methods are well developed by professional sensory scientists and are used by many food companies, usually larger ones. These companies can also afford lab instrumentation to measure many types of food properties, each of which requires a different lab instrument and method. Recently, the cost of some of this equipment has dropped to within the reach of small and even startup food companies. The best example of this is the uniaxial compression tester known as a texture analyzer (see Figure 1).
Instruments that compress and/or pull apart items are known as uniaxial testers. Many companies that manufacture this type of device produce a range of force-measurement capability in such instruments. The food industry is concerned with relatively low force levels when using these machines for testing. Typical forces relate to squeezing a food object with your hand, biting into an apple with your front teeth, or chewing a piece of meat several times before swallowing. Consequently, this particular type of uniaxial tester has become known as a texture analyzer.
The first of the basic techniques for conducting a texture test involves choosing a probe that can penetrate into a food item. The choice of probe (cone, cylinder, punch, blade, ball, or wire) depends on the shape of the food item and the nature of the consumer action that is being simulated. For example, the cone or blade probe can mimic the front or eye teeth biting into a candy bar; the wire can cut cheese or butter. The control parameters are the speed with which the probe moves into the food item and the penetration distance once the probe makes contact. The measured parameter is the resistance the probe experiences as it pushes down into the item, recorded as grams of force.
Texture analyzers can work in standalone mode and report the maximum force, or peak load, measured by the probe. Alternatively, they can work under software control and show the force-versus-time or force-versus-distance graph of the probe’s movement into the food item.
Food scientists have agreed upon a number of physical properties that relate to textural properties. These measurements are accomplished using a texture analyzer and customized applications software that calculates sensory-related properties from the load profile data generated by the machine. Firmness, cohesiveness, springiness, chewiness, and resilience are five useful parameters, defined as follows:
- Firmness: The maximum force required to compress a food between the teeth. For example, how firm does the product feel upon the initial bite?
- Cohesiveness: The property that allows baked products to be handled, sliced, and served without crumbling.
- Springiness Index: The ratio of the height to which the sample springs back after the first bite relative to total compression distance of the bite. This calculation can apply to how springy the sample feels to the touch as well as in the mouth.
- Chewiness Index: The energy required to chew a solid food to the point required to swallow it.
- Resilience: How the sample recovers from compression relative to the distance it is compressed and the speed with which it was compressed.
Let’s see how this works with a few common food products:
Most authorities agree that the gluten-free market is one of the fastest-growing food group segments. While the texture properties of wheat-based baked products are difficult to mimic without gluten, new product development in the gluten-free market is making rapid improvements. Most gluten-free products are made with blends of rice flour and tend to be much more dense and less compressible than wheat flour products.