Texture and mouthfeel

©Robert Sekuler revised 2004
Includes material edited from
"Texture and Mouthfeel: Making Rheology Real"
written by Ray Marsilli, 1993, for Weeks Publishing Company
Northbrook, IL 60062

It's 1938 in Austria, just before World War II. Maria Rainer, a less than stellar candidate to become a nun, is advised by her Mother Superior to take a break from the Abbey, and work temporarily as a governess for a family near Salzburg. It's an opportunity to find out whether Maria is truly ready to become a nun, or whether she should take another, more worldly path. The family comprises a widower, Captain von Trapp, and his seven children. But then, you probably know this story from the film starring Julie Andrews.

One evening there is a ferocious thunder storm, which scares the von Trapp children. They come running in their p.j.'s to their governess, who calms their fears by encouraging them to think of "nice things...daffodils, green meadows, skies full of stars, raindrops on roses, and whiskers on kittens." Maria breaks into song, "My Favorite Things":

"...bright copper kettles and warm woolen mittens
Brown paper packages tied up with strings,
These are a few of my favorite things.
Cream-colored ponies and crisp apple strudels
Doorbells and sleighbells and schnitzel with noodles
Wild geese that fly with the moon on their wings
These are a few of my favorite things...
When the dog bites, when the bee stings, when I'm feeling sad
I simply remember my favorite things and then I don't feel so bad."
(Richard Rodgers and Oscar Hammerstein II, ©1959)

I'm not afraid of thunder and lightning, but to misquote Maria's song from The Sound of Music, here are a few of my own favorite things...

You can certainly make up an analogous list of your own. These terms (firmness, crunchiness and so on) represent integral parts of the pleasure of food, but none of them pertain directly to what the textbook calls "taste" (or "smell"). No, these important sensory experiences are something altogether different. To people who study food and eating, these important variables are known as texture and mouthfeel characteristics, and they can make food appealing or not.

Texture is one of the major criteria that consumers use to judge the quality and freshness of foods. When a food produces a physical sensation in the mouth (hard, soft, crisp, moist, dry), the consumer has a basis for determining the food's quality (fresh, stale, tender, ripe).

Cucumber pickles, for example, are evaluated for quality by the consumer primarily on the basis of texture. In one survey, 400 consumers were asked, "What should a good pickle be like?" Almost 90% answered that a good pickle should be crisp, firm and hard.

Although it may be one of the most important organoleptic properties, a food's mouthfeel is probably the least understood and most neglected by food developers. ("Organoleptic" is a term used in the food and fragrance industries; it means "related to stimulation of sense organs"). When creating a new food product or redesigning an existing one, food developers must worry about the food's rheological properties. (Rheology is the division of physics that deals with with the deformation and flow of matter.)

Subtle changes in a food product's formulation can change mouthfeel significantly. Simply taking out sugar and adding a high-intensity sweetener can cause noticeable alterations in mouthfeel, making a formerly-good product unacceptable to consumers. Sugar not only sweetens; it also builds body and viscosity in a food product, and leaves a slight coating on the tongue. And reducing salt levels in soup changes not only taste, but can alter mouthfeel --for the worse.

Analyzing texture
A major challenge facing food developers is how to accurately and objectively measure texture and mouthfeel. Texture is a composite property related to a number of physical properties (e.g., viscosity and elasticity), and the relationship is complex. Describing texture or mouthfeel in a single value obtained from an instrument is impossible. Mouthfeel is difficult to define. It involves a food's entire physical and chemical interaction in the mouth -- from initial perception on the palate, to first bite, through mastication and finally, the act of swallowing.

In the early 1960s, researchers at the General Foods Corporation developed one of the first significant systems for scientifically classifying food rheological properties. And what General Foods Corporation finds interesting is important for all of us to know. Following several centuries of mergers and acquisitions, in 1989 General Foods became Kraft General Corporation, which is responsible for a huge proportion of all manufactured food products available in American supermarkets.

General Foods' Texture Profile Analysis (TPA) technique forms the basis of most standard methods of mouthfeel analysis used today. A. S. Szczesniak proposed a classification of food texture based on rheological principles that could be monitored by both instrument-based as well as psychophysical methods of texture characterization. She classified the textural characteristics of food into mechanical, geometrical and "other" properties. The mechanical properties were subdivided into five primary parameters (hardness, cohesiveness, viscosity, elasticity and adhesiveness) and three secondary parameters (brittleness, chewiness and gumminess). The geometrical characteristics were divided into two general groups -- those related to size and shape of particles and those related to shape and orientation. The "other" characteristics included moisture content, oiliness and greasiness. Several other classification schemes have been proposed since. They all have their advantages and disadvantages, but Szczesniak's is about the best of them.

Instrument-based measurements
Yes, sensory tests and verbal descriptor terms are highly useful in understanding product rheology. But sophisticated instruments designed for rheological food measurements are becoming more and more common, easier to use and better able to provide reliable, meaningful data. "Meaningful" means that the data from these physical instruments correlate very well with the sensory judgments rendered by humans. Instrumental methods to measure mouthfeel are based on a science called "rheology," a relatively young branch of physics that measures the deformation and flow of materials.

The challenge confronting food designers who want to quantify mouthfeel characteristics using an instrumental technique: How to take instrument readings -- measurements of forces, distances and other data that look like numbers from a physics experiment-- and relate them to something meaningful and relevant to what people actually experience when they taste, chew and swallow a food product. That's easy to say, but pretty hard to do.

Correlating sensory data with instrument readings
The process of sensory evaluation starts with a well trained sensory panel --a group of humans who will judge sensory qualities. To carry out a meaningful texture profile analysis, a panel of judges needs to have knowledge of the texture classification system, the use of standard rating scales and the correct procedures related to the mechanics of testing. Panelist training should start with a clear definition of each attribute. Furthermore, the techniques used to evaluate the food product should be explicitly specified, explaining how the food product is placed in the mouth, whether it is acted upon by the teeth (and which teeth) or by the tongue and what particular sensation is to be evaluated. Panelists should be given reference standards for evaluation so they can practice their sensory evaluation techniques and the use of scales. (Examples of attribute definitions and techniques for evaluating mechanical texture characteristics appear in the accompanying tables.)

The panel is given a list of rheological attributes that are relevant and important to the products they'll be judging. These attributes might include firmness, hardness, cohesiveness, chewiness, elasticity, springiness, etc. The next step is to perform an organoleptic sensory evaluation in which the trained panelists assign intensity levels on various descriptors/texture attributes. For example, for evaluating the texture of pickles, firmness may be considered one important attribute. In this case, panelists could be asked: "On a scale where 1 equals extremely soft and 9 equals extremely firm, how would you rate the firmness of pickles A, B and C's?"

The following table gives a small sample of the qualities that a panel might be instructed to evaluate. Notice that the meaning of each quality is not left to panel member's imagination, but is defined fairly rigorously. (Note: A more complete set of qualitites is given at the end of the page.)

Sensory Techniques for Evaluating Food's Texture Characteristics
Place spoon with sample directly in front of mouth and draw liquid from spoon over tongue by slurping, evaluating the force required to draw liquid over tongue at a steady rate.
Sensory Characteristic
Technique for Evaluating Characteristic
Hardness Place sample between molar teeth and bite down evenly, evaluating the force required to compress the food.
Cohesiveness Place sample between molar teeth; compress and evaluate the amount of deformation before rupture.
Springiness Place sample either between molar teeth (if it is a solid) or between the tongue and the palate (if it is a semi-solid) and compress partially; remove force and evaluate the degree and quickness of recovery.
Adhesiveness Place sample on tongue, press it against the palate and evaluate the force required to remove it with the tongue.
Fracturability Place sample between molar teeth and bite down evenly until the food crumbles, cracks or shatters, evaluating the force with which the food moved away from the teeth.
Chewiness Place sample in the mouth and masticate at one chew per second at a force equal to that required to penetrate a gum drop in 0.5 seconds, evaluating the number of chews required to reduce the sample to a state ready for swallowing.
Gumminess Place sample in the mouth and manipulate with the tongue against the palate, evaluating the amount of manipulation necessary before the food disintegrates.

After the taste panel has been instructed and rated various samples, instrument readings of the food product are made. The instrumental technique must duplicate as closely as possible how the mouth manipulates the particular food product. The instrument should apply the same amount of force in the same direction and at the same rate as the mouth and teeth do during chewing.

Note that for some particular food, springiness may be more important than fracturability. Or it may be that hardness is more important than brittleness. For example, in the case of pickles, crispness is usually considered to be the most important texture attribute. This is a complex characteristic related to hardness, elasticity and cohesiveness; viscosity and adhesion are less important. So pickles are not just cucumbers that have sat for a while in brine: a host of complex sensory and physical variables go into making a good pickle.

And sometimes there is no good correlation of any type between instrument readings and taste panel scores. The problem is that no instrument can manipulate a food product precisely the same way as the human mouth does during mastication. For example, an instrument may compress some food product between two flat plates, but a human would be biting down with incisors, which are sharp, rather than flat. And the result is different. In fact, what an instrument measures may not relate at all to what the tongue perceives. For example, adding a tiny amount of pectin to fruit juice is readily perceived by a human consumer but is nearly impossible to measure with a viscometer. (Pectin enhances the body of the juice.)

Nevertheless, instruments that employ advanced sensor technologies, powerful computer capabilities and sophisticated, versatile software are invaluable toosl for mouthfeel evaluation. They make it possible to establish quite strong correlations between results from taste paneland results from physical instruments. Today's instruments can perform an entire TPA and are capable of measuring dozens of rheological food properties. Some instruments measure stress rheometrics such as compression and elasticity. Others specialize in biting force. Many people perform compression or puncture tests. But these tests still aren't quite as useful as the TPA.

Why not forget instruments entirely and rely exclusively on organoleptic taste paneling? The answer is simple. Once good correlation has been established between instrumental data and organoleptic data, the instrumental approach offers several advantages over sensory paneling. Compared to sensory paneling, instrumental results are obtained in much less time, at less cost and are more objective and accurate. Furthermore, instruments can be used in situations that are impractical for sensory paneling -- for example, as a quality control tool to check rheological properties throughout the processing cycle. Instrument readings and rheological evaluation can easily be made at numerous steps and stages during the product design process.

Measuring mouthfeel
The first instruments used for measuring food rheometrics were giant monstrosities originally designed by mechanical engineers to evaluate the strength of construction materials including metals and composites. Now several smaller, more versatile instruments have been designed specially for use in evaluating food products. One of these newer devices comes with 25 different mechanical probes, including various sizes of needles, cones, cylinders, punches, knives and balls. It can measures qualities such as adhesion, breaking point, cohesion, creep, crispiness, density, extrudability, film strength, hardness, lumpiness, rubberiness, slipperiness, smoothness, softness, spreadability, springback, tackiness, tensile strength and nearly every other known rheological property of food.

When Szczesniak first proposed a classification of food texture based on rheological principles in the early 1960s, it was enthusiastically accepted. Then interest by industrial food technologists waned, and texture/mouthfeel studies of food systems were primarily conducted by university researchers. But with the advent of the reduced-fat/no-fat trend, food developers again became interested in measuring mouthfeel characteristics of the new products they were developing. Although several polysaccharides and/or protein blends have been found to closely mimic mouthfeel attributes of fat, consumer sales for many lowfat/no-fat food products (for example, dairy products and frozen novelties) have been disappointing. Consumers have complained about the taste of many of these products, and studies show that consumers are reluctant to sacrifice taste for healthy eating. When fats and oils are taken out of foods (or reduced in volume), the food changes in both flavor (taste and smell) and in mouthfeel.

In conventional foods, fat is both a source of flavor and a carrier of flavor. The chemicals responsible for the flavor of fat can be added as synthetic flavor cocktails, but fat's flavor release characteristics are difficult to mimic. When fats are removed from a product, lipophilic ("fat-loving") flavoring agents like vanilla and chocolate are released from the product and into the mouth and nasal cavity more quickly than when fat is present. As a result, an initial high burst of flavor is perceived, but it then it dissipates very quickly. As a result, the perceived flavor of a product can be dramatically changed when a fat-free product is developed, even though the same flavor composition is used.

Although understanding and measuring mouthfeel and texture have been an important part of developing fat-reduced products, rheological studies have not provided all the answers. The flavor-release problem needs to be better understood and addressed before significant flavor improvements can be made.

More complete set of Terms Used in Sensory Texture Profiling
Adhesiveness Force required to remove the material that adheres to a specific surface (e.g., lips, palate, teeth).
Bounce The rate at which the sample returns to the original shape after partial compression.
Chewiness Number of chews (at 1 chew/sec) needed to masticate the sample to a consistency suitable for swallowing.
Coarseness Degree to which the mass feels coarse during product mastication.
Cohesiveness Degree to which the sample deforms before rupturing when its bitten with molars.
Denseness Compactness of cross section of the sample after biting completely through with the molars.
Dryness Degree to which the sample feels dry in the mouth.
Fracturability Force with which the sample crumbles, cracks or shatters. Fracturability encompasses crumbliness, crispness, crunchiness and brittleness.
Graininess Degree to which a sample contains small grainy particles.
Gumminess Energy required to disintegrate a semi-solid food to a state ready for swallowing.
Hardness Force required to deform the product to given distance, i.e., force to compress between molars, bite through with incisors, compress between tongue and palate.
Heaviness Weight of product perceived when first placed on tongue.
Moisture absorption Amount of saliva absorbed by product.
Moisture release Amount of wetness/juiciness released from sample.
Mouthcoating Type and degree of coating in the mouth after mastication (for example, fat/oil).
Roughness Degree of abrasiveness of product's surface perceived by the tongue.
Slipperiness Degree to which the product slides over the tongue.
Smoothness Absence of any particles, lumps, bumps, etc., in the product.
Springiness Degree to which the product returns to its original size/shape after partial compression (without failure) between the tongue and palate or teeth.
Uniformity Degree to which the sample is even throughout.
Uniformity of Chew Degree to which the chewing characteristics of the product are even throughout mastication.
Uniformity of bite Evenness of force through bite.
Viscosity Force required to draw a liquid from a spoon over the tongue.
Wetness Amount of moisture perceived on product's surface.
How does your next meal stack up??
Here's a simple, fast exercise to try. It doesn't claim to be scientific, but it certainly does claim to being fun. And it will definitely make you think about your food in a new way. When you eat your next meal, have in front of you a copy of one of the tables above ("More complete set of Terms Used in Sensory Texture Profiling" or "Sensory Techniques for Evaluating Food's Texture Characteristics"). Take one or two of the solid foods in that meal, and rate each according to the scales provided in the table. Use a scale 1-5 for each rating, where 5 means a lot of the property, and 1 means none. For example --Saltine cracker from a freshly-opened package might rate 4 or 5 on moisture absorption, and also score high on adhesiveness, though not as high as certain brands of peanut butter would; a juicy, fresh Gala apple might rate highly on moisture release; a Gummy bear (well-named) might score 5 on gumminess. When you're done, send your data to the instructor.