Table Of ContentFOOD SCIENCE AND TECHNOLOGY
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Food Texture
and Viscosity:
C o n c e pt a nd M e a s u r e m e nt
Malcolm C. Bourne
Department of Food Science and Technology
New York State Agricultural Experiment Station
and
Institute of Food Science
Cornell University
Geneva, New York
ACADEMIC PRESS
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COPYRIGHT © 1982, BY ACADEMIC PRESS, INC.
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Library of Congress Cataloging in Publication Data
BOURNE, MALCOLM C.
FOOD TEXTURE AND VISCOSITY.
(FOOD SCIENCE AND TECHNOLOGY)
INCLUDES BIBLIOGRAPHICAL REFERENCES AND INDEX.
1. FOOD TEXTURE. 2. VISCOSITY. 3. FOOD--ANALYSIS.
I. TITLE. I I. SERIES.
TX531.B685 66V. 07 82-6711
ISBN 0-12-119060-9 AACR2
PRINTED IN THE UNITED STATES OF AMERICA
82 83 84 85 9 8 7 6 5 4 3 2 1
To my beloved wife, Elizabeth
Preface
This book is intended for those who want to know more about the texture and
viscosity of food and how these properties are measured. It draws together
literature from many sources including journals in chemistry, dentistry, en-
gineering, food science, food technology, physics, psychology, and rheology.
The Journal of Texture Studies and scientific and trade journals dedicated to
special commodity groups, books, proceedings, and commercial literature have
also been utilized.
The treatment is descriptive and analytical but not mathematical. Equations
are given only when they illuminate the discussion and then in only the simplest
form. Their derivations, however, are not given; this is not a mathematics
textbook.
Chapter 1 defines texture terms, discusses the importance of textural prop-
erties of foods, points out the present status of food texture and viscosity
measurements in the food industry, and gives a brief history of early develop-
ments in the field. Chapter 2 describes physical interactions between the human
body and food—a necessary background for the ensuing chapters. Chapter 3
discusses the principles of objective methods of texture measurements (including
ideas that have yet to evolve into commercially available instruments) and pro-
vides a foundation for the following chapter. Chapter 4 describes commercially
available instruments and their use. Chapter 5 discusses the various types of
viscous flow, followed by a brief description of commercial viscometers. Chap-
ter 6 describes sensory methods for measuring texture and viscosity. Chapter 7
outlines a system for selecting a suitable instrument and method with the mini-
mum of time cost. The Appendix lists names and addresses of suppliers of
instruments for those who are interested in purchasing equipment. I have no
vested interest in any corporation that sells texture-measuring instruments and
xi
xii Preface
have endeavored to be unbiased in describing commercial instruments and to
make the list as complete as possible.
Many people will read this book selectively. The practicing food technologist
and quality controller might want to concentrate on Chapter 4 and the latter part
of Chapter 5, where instruments are described. The professor and college student
might spend more time on Chapter 3 and the first part of Chapter 5, which
describe principles. The sensory specialist will find Chapter 6 the most interest-
ing. The researcher wanting to establish a texture laboratory will find Chapter 7
most useful.
I have expressed my own opinions, attitudes, and interpretations in this
volume. For example, my personal conviction that empirical tests have been
responsible for most of the successes in food texture measurements is reflected in
the extended discussion of empirical methodology and the brief discussion of
fundamental tests. Even if subsequent reports show the guidance to be wrong at
times, I hope most readers will find useful the methods and yardsticks offered.
I have received help from many sources in the preparation of this book . A
number of individuals and organizations provided figures or compiled tables;
their contributions are noted where the figure or table appears. I particularly
appreciate the assistance of C. Cohen, J. M. deMan, W. F. Shipe, A. S.
Szczesniak, and P. W. Voisey, each of whom critically read one or more chap-
ters in the draft stage and made numerous suggestions for improvement. S.
Comstock has faithfully assisted with experiments in my laboratory for many
years. R. Bowers made a major contribution by typing the manuscript. I sin-
cerely thank all of them for their contributions.
CHAPTER 1
Texture, Viscosity, and Food
Importance of Textural Properties
The four principal quality factors in foods are the following:
1. Appearance, comprising color, shape, size, gloss, etc., is based on optical
properties and a visual manifestation of size and shape.
2. Flavor, comprising taste (perceived on the tongue) and odor (perceived in
the olfactory center in the nose), is the response of receptors in the oral cavity to
chemical stimuli.
3. Texture is the response of the tactile senses to physical stimuli that result
from contact between some part of the body and the food.
4. Nutrition.
Other factors, such as cost, convenience, and packaging, are also important
but are not considered quality factors of foods. Of the above listed the first three
are termed ''sensory acceptability factors" because they are perceived by the
senses directly. Nutrition is a quality factor that is not an acceptability factor
because it is not perceived by the senses.
The sensory acceptability factors of foods are extremely important because
people obtain great enjoyment from eating their food and, furthermore, the
enjoyment of food is a sensory pleasure that we can appreciate from the cradle to
the grave. One prominent food scientist has said, and with good reason, "To
most people, eating is a very personal, sensual, highly enjoyable experience;
enjoyment here and now, with little worry about long-term consequences"
(Clausi, 1973).
1
2 1. Texture, Viscosity, and Food
The importance of texture in the overall acceptability of foods varies widely,
depending upon the type of food. We could arbitrarily break it into three groups:
1. Critical: Those foods in which texture is the dominant quality characteris-
tic; for example, meat, potato chips, and celery.
2. Important: Those foods in which texture makes a significant but not a
dominant contribution to the overall quality, contributing, more or less equally,
with flavor and appearance; for example, most fruits, vegetables, bread, and
candy fall into this category.
3. Minor: Those foods in which texture makes a negligible contribution to the
overall quality; examples are most beverages and thin soups.
The importance of texture in foods was indirectly pointed out by Schiffman
(1973), who fed 29 different foods to people who had been blindfolded and asked
them to identify the foods based only on flavor. The samples had been pureed by
blending and straining in order to eliminate textural clues. Some of the data from
Schiffman's work are shown in Table 1. It is remarkable to discover how poorly
many foods are identified when their texture and color are concealed and flavor is
the only attribute that can be used to identify the food. Young adults of normal
weight were able to identify correctly only 40.7% of the foods used in the study.
It is surprising to find, for example, that only 4% of the respondents could
TABLE 1
PERCENTAGE OF CORRECT IDENTIFICATION OF PUREED FOODS"
Normal weight Obese Normal weight
Food (young) (young) (aged)
Apple 81 87 55
Strawberry 78 62 33
Fish 78 81 59
Lemon 52 25 24
Carrot 51 44 7
Banana 41 69 24
Beef 41 50 27
Rice 22 12 15
Potato 19 69 38
Green pepper 19 25 11
Pork 15 6 7
Cucumber 8 0 0
Lamb 4 6 —
Cabbage 4 0 7
Mean for 29 foods 40.7 50.0 30.4
°From Schiffman (1973), with permission from author.
Importance of Textural Properties 3
identify cabbage correctly by flavor only, 15% for pork, 41% for beef, and 51%
for carrots.
Szczesniak and Kleyn (1963) gave a word association test to 100 people to
determine their degree of texture consciousness and the terms they used to
describe texture. Seventy-eight descriptive words were used by the participants.
These authors concluded that texture is a discernible characteristic, but that it is
more evident in some foods than others. Foods that elicited the highest number of
texture responses either were bland in flavor or possessed the characteristics of
crunchiness or crispness.
Yoshikawa et al. (1970a,b,c) conducted tests in Japan that were similar to
those conducted by Szczesniak's group in the United States. They asked 140
female college students to describe the texture of 97 foods and collected 406
different words that describe textural characteristics of foods. These studies
showed the importance of textural properties as a factor in food quality and the
great variety of textures found in food. The 10 most frequently used words in
these two studies are listed in Table 2. It is interesting to notice that 7 of these 10
words are common to both lists, although the Japanese culture and food habits
are substantially different from those of North America. It is also interesting to
note that the Japanese used 406 descriptive words as compared to 78 words in the
United States.
Perhaps the richer textural vocabulary of the Japanese is due partly to the
greater variety of textures presented in Japanese cuisine, making them more
TABLE 2
MOST FREQUENTLY USED TEXTURE WORDS' a
United Statesb Japan/
Crisp Hard
Dry Soft
Juicy Juicy
Soft Chewy
Creamy Greasy
Crunchy Viscous
Chewy Slippery
Smooth Creamy
Stringy Crisp
Hard Crunchy
78 words 406 words
aIn descending order of frequency.
^Szczesniak and Kleyn (1963).
cYoshikawa et al. (1970a).
4 1. Texture, Viscosity, and Food
sensitive to subtle nuances in textures, and partly to the picturesque Japanese
language which uses many onomatopoeic words. For example, Yoshikawa et al.
(1970a) assign to each of the following expressions the meaning of some form of
crispness: kori-kori, pari-pari, saku-saku, pori-pori, gusha-gusha, kucha-
kucha, and shaki-shaki.
In a second study (Szczesniak, 1971), a word association test was given to 150
respondents and the results were similar to the first study. This test again showed
that texture is a discernible characteristic of foods and the awareness of it gener-
ally equivalent to that of flavor. This study also found that women and people in
the higher economic brackets showed a higher level of awareness of the textural
properties of foods than did the general population.
Szczesniak and Kahn (1971) conducted in-depth interviews with homemakers
and found that texture awareness in the United States is often apparent at a
subconscious level and that it is taken more or less for granted; however, when
the textural aspects did not come up to expectations, there was a sharp increase in
the awareness of the texture and criticism of the textural deficiencies. The au-
thors state that
If the texture of a food is the way people have learned to expect it to be, and if it is
psychologically and physiologically acceptable, then it will scarcely be noticed. If, however,
the texture is not as it is expected to be ... it becomes a focal point for criticism and rejection
of the food. Care must be taken not to underestimate the importance of texture just because it is
taken for granted when all is as it should be.
Szczesniak and Kahn (1971) also reported that time of day exerted a strong
influence on textural awareness and flavor. At breakfast, most people prefer a
restricted range of familiar textures that lubricate the mouth, remove the dryness
of sleep, and can be swallowed without difficulty. New or unfamiliar textures,
and textures that are difficult to chew, are not wanted at breakfast.
People are willing to accept a wider range of textures at the midday meal just
so long as it is quick and easy to prepare and not messy to eat. After all, this is a
practical meal with a limited time for preparation and consumption.
Texture is most appreciated and enjoyed at the evening meal. This is the time
for relaxation, which comes after the day's work and, for most people, is the
largest meal of the day when several courses are served and a wide range of
textures is expected and relished. The appetizer (nondemanding textures and
flavors that stimulate the flow of saliva) is perceived as a preparation for the main
meal which follows, and this in turn features a great variety of textures, including
some items that require considerable energy to chew. No texture seems to be
completely inappropriate for the main course so long as there are several con-
trasting textures.
The dessert features textures that require low energy for mastication and
restore the mouth to a relaxed and pleasant feeling. This is the time for "fun"
foods that are easy to manipulate and leave a nice feeling in the mouth. Soft,
