Why is it important?
Texture is a very important character of every food we eat or drink. To many people, it is even more important than taste! Think of all the ways we describe a food’s texture. The students in the class could probably come up with twenty or more words just to describe the foods in their lunchboxes. "Crunchy, chewy, crispy, juicy, squashy, runny, solid, hard, soft, soggy, firm, creamy, fatty" and so on.
The components and ingredients within foods plus the processes they go through determines their texture. Take the example of making a fruit smoothie, which is easy to make at home in a blender and found on many supermarket shelves. Depending on the type of fruit, how pulpy and pureed the fruit is and how much water is in there will determine how thick the final smoothie is. Most commercial products proudly claim to contain only fruit (and water), so all the texture comes from the fruit and the process. Different smoothies can vary in their thickness. If one is so thick that it requires a spoon to eat, then it is really more of a soft-solid type product than a drink.
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How we assess a food’s texture
The human body is very adept at evaluating a food’s texture. We use not only the sense of feeling in our mouths – or "mouthfeel", as food professionals refer to it - but also our other senses to evaluate the texture of foods. We make decisions about foods before we actually eat them; the appearance of an apple’s skin can tell us how we expect the texture to be when we come to eat it – if the skin looks wrinkly then the apple will probably have lost its crunchy bite; the ripeness of an avocado is best judged by feeling it in our hands- gently squeezing it will tell us if its ready to eat that day or not until a few days later. The colour of many fruits indicates their texture – green bananas will be too hard to eat but brown ones will be mushy! When we do actually eat the food, we also use another sense to tell us something about the texture – our ears. We hear how crunchy an apple is as we bite into it and we expect to hear the crispy sounds of crisp snack products. Much of the time, this is an automatic sub-conscious response and we don’t really think about it. All of our senses work together to provide us with the total picture of what we expect that familiar food. Maybe we only think more consciously about it when we try a food for the first time. We use an assessment of food texture, along with the other attributes of food, such as smell and taste, to decide how fresh, ripe and generally safe our food is to eat.
Food texture takes on a great importance from very early on in our lives. Babies are gradually weaned onto 'solids' over many months because it is the texture which defines how suitable a food is for a baby. With increased better control over the mouth and the incremental appearance of teeth, the infant moves from a solely liquid texture of milk through a "pureed thick liquid" consistency to a "soft mushy" texture to a "soft solid" onto finally a full solids texture. It is quite important for babies to encounter these textures at the appropriate time in order to prepare them for the many different textures in later life. The baby food industry puts a great deal of research and development effort into ensuring that the texture or consistency of baby foods is appropriate and appreciated for the specific age of the baby. Many babies are fed on commercially produced baby foods at some stage of their early lives. If the baby rejects the texture (or also taste) then they also reject the product and with it the possibility of deriving the much needed energy and nutrients from that food.
It's not just babies for whom teeth are a consideration in the textures that can be eaten. Humans are omnivores and that means we are able to eat both animal and plant-based foods. For this purpose we need both grinding teeth (molars) for the chewy, coarse vegetables and also sharp, tearing teeth (canines) for meats. This can be seen in dogs and cats and other avid meat eaters with their pronounced canine teeth whilst sheep and cows are vegetarians with a great set of molars.
So how sensitive is the mouth at detecting texture? Quite! Food science students at university learn that the human mouth can usually detect ice crystals in ice cream which are about 40μm (called micrometers or microns). One micron is a 1/1000th of a millimeter. That’s pretty small! So if you subject your ice cream to temperature fluctuations, like leaving out of the freezer for too long so it starts to melt and then re-freezing it, you often notice that it becomes a bit "grainy". This graininess you feel in your mouth is actually the presence of ice crystals that have become too large (they grow very slowly in the freezer helped on by temperature fluctuations). Ice cream manufacturers go to great efforts to ensure that the ice cream they sell has the perfect ice crystal profile which contributes to mouth feel. The bigger international ice cream companies have whole R&D centres around the world dedicated to research on ice cream.
Its not just humans who are fussy about texture - ask any cat owner! This presents special challenges for pet food manufacturers who have to ensure that any ingredients they add to a meat product mimic the real thing in terms of texture. So any added "jellies" in a tinned pet food should melt at about the same temperature as gelatin, the gel naturally found in meat, so that the cat gets the same texture at mouth temperature – not an easy task for many plant-based gelling agents.
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Using our senses to judge foods forms an important evaluation process in the food industry called sensory evaluation. Food companies and research organizations all around the world use real people and their senses to tell them something about their products. These can be relatively simple tests in small factory labs right through to highly complex tests using dedicated sensory faciities where factors like lighting and smells are controlled and sophisticated statistical analyses are used on the data. Sometimes panels of testers are screened (to check their suitability to detect the specific attribute under test) and trained for many weeks before they are actually given the tests. This increases the reliability of the data. This is needed because we are all individuals and our judgment of a food’s texture is unique and can be influenced by what else we have eaten, what mood we are in and how much time we have to think about it! As such precise sensory evaluation requires careful training and selection of panelists (the taste testers) and control of the surrounding environment, the desired outcome is very specific and accurate information. Texture is often the focus or an important aspect of sensory tests. This can be in isolation of taste or together with other factors, such as taste in the overall product. For example, a company might want to see how their crispy snack product compares to the market leader for crispiness and if it is the texture which is driving the product's success or something else.
Sensory testing may be performed in many ways but one popular method which is often used by the food industry is called the "triangle test". A description of this is given in 'Real Industry Case Study - Fat Replacers' of the 'Help for Teachers' section. Whatever test is chosen, certain scientific principles are applied to make the test as fair and unbiased as possible, and to produce data which can be trusted. Such practices include removing any name identification of the products under test and replacing this with a random 3-digit number so that testers are not influenced by their expectations of a known product. Texture may also be evaluated during consumer testing, which is often carried out by many companies at later stages to get wider information on their products.
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In addition to sensory testing, food texture is also measured by special equipment in research laboratories and factory quality assurance (QA) labs. Texture analysers are machines that can press, pull, pierce, squash, twist and crush samples of food in a way which tries to mimic the end use as closely as possible. Special attachments have been developed to test for particular applications, especially in the meat area. Texture analysers measure food texture in a scientific, non-biased way that can be repeated to give standardized assessment methods. In many cases, these tests have been developed to try to mimic our senses to make the test as applicable to the product as possible, for example, to represent a biting action or a chewing action.
The texture analyzer above from Stable MicroSystems is used primarily for semi-solid or solid foods but more liquid foods can also be measured by machines called viscometers or rheometers. Rheometers are complex machines which have been developed to measure the rheological properties of materials in many industries, not just the food industry, and are often found in research organisations. More simple viscometers are also useful in factories and test kitchens to get quick indications of the product. You can think of viscosity as essentially the liquidness of a material, how "runny" or how thick it is. For older or more able students, there is a fuller explanation of rheology in the extensional section further down the page.
Suffice to say at this stage scientists have measured viscosities of many food materials as standards. For example, we now know that water is assigned a viscosity of 1mPas at room temperature (20oC). The unit "mPas" is called the milli-pascal-second but you do not need to worry about that at this stage, just that for water it is measured to be 1 unit. Olive oil is about 100 times thicker at 20oC (its viscosity is about 100mPas) and golden syrup is about 10 000 times thicker! (or about 10000 mPas). Just think of the differences when you try to pour water, olive oil and golden syrup out of a bottle. You can wait a long time for the syrup to make its way out of the bottle, especially if you have stored it in the fridge! Viscosity is greatly affected by temperature which is why it is always important to specify the temperature when talking about viscosity. Materials generally get thicker when they are colder and more liquid as they warm up.
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Examples of manufactured textured products
Texture is so important in our perception of foods that if we try to copy a food then we need to get the texture absolutely right, even if not the flavour. The most common example of this in the case of meat analogues, alternatives made to replace meat, which are eaten by both vegetarians and non-vegetarians. One example is a product called textured vegetable protein (TVP) made from soya beans. It has the fibrous texture of meat but a totally bland flavour. Because it is produced from soya beans, it has a good protein content and amino acid profile. Its bland texture also means that it makes a good bulking ingredient because it is less expensive than meat.
Quorn is another product you may have seen or tried. It has the texture of chicken but is actually a myco-protein or a tiny fungus grown through a fermentation process. It has no flavour of its own but like the TVP picks up the flavours of other things it is cooked with (for example in a curry or a chilli). In both these products there has been no attempt to recreate the taste of meat but just to perfect the texture to make it convincingly like meat.
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Special considerations for chocolate texture
Most people enjoy eating chocolate from time to time or even all the time! As well as the taste, the texture of chocolate is also very important. What ways can you think of to describe the texture of chocolate? "Smooth, velvety, creamy, soft, hard, crispy…"? Some people prefer their chocolate straight out of the fridge – this can give a different texture to the product at room temperature. So why is this temperature issue so important in chocolate?
The answer lies with the most important ingredient of chocolate, the cocoa butter. This fat component has a very precise melting point range which corresponds to about body temperature. This is why chocolate readily melts in your mouth but not at room temperature. Scientists have used rheometers, the sophisticated measuring machines described earlier, to study how cocoa butter melts and it is known that it changes from solid to liquid all at about 32o-36oC. This is because the smaller parts of the fat, called the triglycerides, are all of the same type and therefore behave about the same way and melt at about the same temperature. But these triglycerides also explain another important textural feature of chocolate. Have you ever taken chocolate out of the fridge and noticed it has a white grainy texture instead of the smooth glossy texture you expect? This is because of some changes that have taken place to the form of the fat crystals during the cold storage and they have crystallized out on the surface. This harmless white coating is known as chocolate "bloom" but some people mistake it for mould growth. This is because triglyerides are polymorphic, which means that can exist in several different crystalline arrangements, each with a characteristic melting point. For students who want to know more, then the three principle forms are known as α (said as "alpha"), β (beta), β’ (and "beta-prime"). When a melted triglyceride cools rapidly it solidifies in the lowest-melting, unstable α-form. If this is slowly heated, it will melt and then resolidify in the β’ form. Repetition of this procedure will bring about a transition to the final, stable β form). Milk fat can prevent this blooming so this extra ingredient is often added in small amounts in plain chocolates. (It accounts for about 25% of the total fat in a typical milk chocolate).
Only one of the six states that cocoa butter can exist in produces the right properties needed for the final eating chocolate. Making sure that the chocolate ends up in this desired state is the job of the chocolate maker who uses a process called tempering to keep the liquid chocolate just below the temperature of the desired form. The chocolate is then stirred for some time at this temperature in order to obtain a high proportion of the fat as very small crystals of the desired type when it is finally set (or solidified) in the mould. If you do any cake decorating at home then the products you buy for this purpose are often modified to make them easier to handle. But in exchange for the added convenience, the texture may sometimes be a bit greasy.
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Research on texture and chocolate
Food texture has been a focus of many research teams from universities and food companies in the UK and around the world over the years. Today, much research is undertaken through large consortium groups where universities, other research centres and commercial food companies all work together to try and find out the answers to questions and to better understand a problem. Often, this is enabled with the help of funding from external bodies, governments, and EU research funding streams. Many larger, international food companies also undertake in-house research in their own R&D centres.
One area of research which many people are interested in is that of texture changes on low-fat products. Fat contributes a very important role to a foods texture and taking it away changes the mouthfeel and taste of most products quite dramatically. It can also change how the food responds to cooking and storage. Food microbiologists play an important role here in measuring any changes to the products microbial stability – for example, if it supports the growth of any more food poisoning bugs – when products are reformulated and fat levels reduced.
One research group in the UK, Leatherhead Food International (LFI), is even investigating textual and structural changes in low fat foods during chewing. In other words how the food breaks down in the mouth when it’s a low-fat product.
Chocolate has also been the subject of some very recent research. Researchers at the University of Birmingham are also interested in what is needed to produce a low-fat product, in this case a low fat chocolate! This has been traditionally very difficult to achieve so of course the group are excited to think that they may have come up with the answer.
They believe the key to making this work lies with the physics of the chocolate microstructure. The researchers make a water-in-oil emulsion, where tiny droplets of water are suspended in the surrounding cocoa butter oil and because the water droplets are very small, the mouth does not feel them.
But it is early days yet and the research team still have lots more research to do. They hope to publish the work and patent it some time in 2009. So watch this space for more news!
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Extension information - The really technical stuff!
When we talk about a food’s texture what we are actually talking about is the food’s rheology. Texture describes a physical property of a material related to its rheology, or how a material deforms or flows. All materials can be described in terms of their rheological properties and foods are no exception. Foods are quite special though in that they are usually made up of lots of other things or ingredients, which together give an overall, complicated – but tasty! – material.
Rheologists define materials along a spectrum from true liquids (like water) through to true solids (like ice) with a whole array of things in the middle. Most foods fall into this in-between category and it is more usual to use the term texture to describe the food, especially since most foods tend to be a least a little bit solid. But even liquid foods like milk can still be described in terms of their texture – full fat milk has a "creamier" texture than skimmed milk, which is more "watery". You may have heard of the term viscous (or viscosity) when describing liquids. Rheologists actually use the term viscous to describe the liquidness (or "slippiness") of a material. A feature of a liquid is that once it moves it stays there. You may not cry over spilt milk but you certainly have to clean it up! The liquid will not move back to where it came from of course! But at the other end of the spectrum are materials which do just that – they recover back to their original shape. Think of what happens when you drop a rubber ball onto a hard floor. If you’re lucky you might be able to catch it at about the height you dropped it. It’s the same idea as this. This describes what rheologists call an 'elastic character of solids', where the material deforms back to the starting point with complete recovery of energy. So if we have viscous (or liquid-like) at one end of the texture description scale and elastic (or solid-like) at the other end then most things actually fall in the middle in a big area called viscoelastic!! In other words, they are both a little bit liquid and a little bit solid.
However rheology is a complicated subject and no more needs to be said at this stage. Students who go on to food science or engineering or materials science at university will revisit it at some stage.