Handbook of Food Science, Technology, and Engineering. Volume 1-4, part 3

586 Pages·2016·4.03 MB·English

by  Hui Y.H.

Vegetable: Horticulture and Processing 170-7
products like potato tubers seriously injured during trans- vegetables in an atmosphere that differs substantially from
portation and distribution may be lost completely for human normal air in respect to the proportion of nitrogen,oxygen
use. Potatoes can be stored for about 1 year with small and carbon dioxide. The composition of the atmosphere
losses,due to recent developments in storage technology. may be altered by restricted venting of the storage room or
Refrigeration or low temperature storage can most the container by scrubbing the atmosphere of carbon diox-
effectively extend the shelf life of horticultural produce ide or oxygen or by adding individual gases to the container
and reduce their post-harvest losses. Refrigeration is pro- while reducing the proportion of others. This method if
duced mechanically by the evaporation of a compressed, combined with refrigeration retards respiratory activity and
liquefied gas (e.g., ammonia, Freon) in a closed system. may delay softening, yellowing, changes in quality and
Refrigeration is the most widely used short-term preser- other deteriorative processes by maintaining an atmosphere
vation method for a variety of vegetables. It is used of higher carbon dioxide and oxygen than in normal air.
throughout the distribution chain by commercial firms, The limits of tolerance vary according to a function of the
retail outlets and the consumer. Vegetables can be kept in temperature,duration of storage,concentrations of supple-
good condition for several months in cold storage. mental gases,type and physiological age,and anatomy and
Although most vegetables are processed right after har- morphology of the vegetable produce involved.
vest,asparagus,carrots,bell peppers,onions,potatoes and CA storage has proved beneficial to extend the life for
others are sometimes stored under controlled temperature vegetables that deteriorate rapidly or those that complete
and humidity for some time prior to processing in order to ripening after harvest. Temperature and duration of stor-
suit the operating capacity of the plant. The recommended age and commodity to be stored are the interdependent
temperature and humidity for storage prior to processing factors, each influencing the decision of use of the gas
are designed to suit the operating capacity of the plant. mixture in CA storage.
Leafy vegetables should be promptly cooled to about
32F and held there until processing. Wilting and disorders
increases in severity as temperature increases. Vacuum
VII. VEGETABLE PROCESSING
cooling is the most common method of pre-cooling
TECHNOLOGIES
although hydro-cooling is used on some leafy vegetables.
High humidity (about 95%) is essential to prevent wilting. Processing begins in the field with many vegetables.
Root crops such as beets,carrots,radishes and turnips, This is true with mobile cutters for greens,viner/shellers
are generally stored at 32°F to retain their quality. Decay for peas and beans, juice extractors for tomatoes, and
can cause substantial losses in root crops during even a graders/washers for many vegetables. Advantages are
week or ten days if they are held above 40F. Rapid cooling that harvesting/processing can be stopped or started at
of root crops to storage temperature is not as critical as for any point to accommodate variation in the field. There is
leafy vegetables. However, roots should be brought into no lag in time in which freshly harvested sometimes
desirable temperature range promptly after harvesting due badly bruised product can deteriorate while waiting for
to respiration can rapidly reduce quality. A high relative the next step in processing and most of the refuse includ-
humidity is essential for storage of cold requiring roots if ing vines, shells, leaves and other organic waste is scat-
desiccation is to be avoided particularly during prolonged tered over the field to be incorporated into the soil. There
storage of root crops. are several steps common to the preparation of most veg-
Storage at a few degrees above their freezing point is etables for processing.
desirable for all of the commonly grown leaf,stem,and bud Harvest in the immature tender stage,before any por-
vegetables and, for the most part, the root vegetables. tion becomes fibrous and tough. Corn should be full
Storage life of the leafy vegetables is only a few days,even grown,but tender; peas and beans should be green; stalks
in optimum environments. The root crops,including carrots, of asparagus, stems of greens and shells of snap beans
parsnips,turnips,beets and horseradish,which are adapted should be without fiber. There is a tendency for most veg-
to storage for several months,are commonly held in venti- etables to be harvested after the peak of quality.
lated storage. Other vegetables, such as potatoes, carrots, Grade to remove trash, over mature, diseased, insect
sweet potatoes,turnips,dry onions and cabbage are usually infested vegetables,and other materials that would impart
stored under ventilation and may be refrigerated to extend an off flavor to the product. This may be with a roller
their storage life beyond their growing season. Vegetables grader/sizer, a blower, or rod/shaker, followed by hand
including cucumbers, eggplant, green beans, okra, sweet inspection. Bruising or cutting which might cause loss of
potatoes, squash and tomatoes are native to subtropical or juices should be avoided.
tropical areas are not adapted to low temperatures. Weigh to ascertain pay rate,yield and production rate.
Controlled Atmosphere (CA) storage is one of the most Rinse in water to remove surface dirt, insects and small
important technologies in vegetable storage systems. trash not removed by the blower. A detergent may be used
Controlled atmosphere storage is a system for holding fresh on vegetables taken from the soil,such as potatoes,sweet
170-8 Handbook of Food Science, Technology, and Engineering, Volume 4
potatoes and turnips and leafy vegetables like spinach and 5. Defective parts become more visible so the
turnip greens,provided it is thoroughly rinsed off. product can be sorted more effectively.
Prepare as required for individual vegetables. This 6. The microbial status is improved because veg-
includes peeling,shelling,shucking,trimming and cutting etative cells,yeast and mold are killed.
and dicing. 7. Cooking time of the finished product is short-
Vegetable processing steps include handling,transporta- ened. If the blanching periods for the individual
tion,refrigeration,molding,washing,trimming,blanching, components in mixtures are adjusted,the heat-
freezing,canning,drying,packaging,use of chemicals,radi- ing requirement for the preparation of the fin-
ation,storage and ultimately cooking or heating. ished product is equalized.
Vegetables are processed into more stabilized prod-
ucts that can be used the whole year round in the world When water or steam is used for heating,leaching of
market by canning, freezing, dehydrating and ionizing vitamins, flavors, colors, carbohydrates, and other water-
radiation. Some processed products are canned soups, soluble components takes place. If products are going to
chutney, catsup, instant dehydrated mashed potatoes, etc. be frozen after blanching, a chilling step will generally
The processing of vegetables is as follows: take place before transporting the product into the freezer.
A good blanching technique must fulfill the following
1. Removal of water- to reduce moisture content of essential demands:
vegetables to lower activity of microorganisms;
2. Heat sterilizing- to destroy or inactivate enzymes 1. A uniform heat distribution to the individual
and microorganisms (Suitable packaging pre- units of the product.
vents entry of microorganisms into processed 2. A uniform blanching time to all units of the
products.); product.
3. Lowering temperature- to slow down most bio- 3. No damage to the product during the entire
chemical reactions that can deteriorate food; blanching and cooling process.
4. Providing a chemical environment that will not 4. A high product yield and quality.
permit certain deteriorative actions to proceed; 5. Low consumption of energy and water.
5. Sterilizing with ionizing radiation to destroy 6. Tough, reliable design, giving an operation
enzymes and microorganisms. without problems even when running in contin-
uous three-shift operation. This also includes
In most cases,combinations of preservation technolo- quick and proper cleaning.
gies are recommended. For example, heat sterilizing 7. The design must also provide a pleasant work-
effectively destroys microorganisms, then sealing in a ing environment for the staff without any
container prevents further contamination, and storage of unnecessary noise, heat radiation, and loss of
the canned product under refrigeration reduces deteriora- steam and process water with foam and product
tion normal for any canned product,cold storage of dried particles flushing on the floor.
products extends their storage life,etc.
When blanching vegetables before freezing,inactiva-
tion of the enzymes is dependent upon both time and tem-
A. BLANCHING
perature of the heat treatment. Other effects of heating,
Blanching can fulfill one or several of the following pur- which may respond differently to time and temperature
poses: conditions, include color changes, softening, gelling of
starch and extraction of various soluble components.
1. Inactivation of enzymes prevents discoloration
or development of unpleasant taste during stor-
B. CANNING
age. Colors caused by the presence of chloro-
phyll or carotenoids are also protected from Canning is the major method of preserving vegetables,
enzymatic degradation. and is founded on the most effective method of destruc-
2. Proteins are forced to coagulate and shrink under tion of microorganisms. In canning process, the veg-
liberation of water. If this shrinkage happened etable is placed inside the container, the air is removed
during sterilization of canned products, they by vacuum, and the cans are hermetically sealed. The
would appear to be underweight. Also, starch cans are placed in the retort, and sterilized with steam.
that could otherwise cause a cloudy appearance The basic heat penetration characteristics are convection
can be removed. and conduction. The constancy of the rate of heat pene-
3. Oxidation during frozen storage will be reduced. tration for a given product in a given size container and
4. Many products obtain a clearer color. the ability to measure these constants with accuracy have
Vegetable: Horticulture and Processing 170-9
resulted in the remarkable safety of commercially pack pickling methods in which fermentation does not occur
canned vegetables. and the acid is added directly to the product in the final con-
tainer. A wide variety of vegetables are pickled commer-
C. FREEZING cially. Pickling is usually combined with some type of heat
treatment in the final container to prevent secondary fer-
There are three types of freezing processes. The individu-
mentation and subsequent spoilage. A recent development
ally quick-frozen vegetables are frozen by the fluidized
in the pickled vegetable industry has been the development
bed process and then packed in suitable containers. The
of the fresh pack or pasteurized cucumber and other veg-
second process involves placing the unfrozen prepared
etable pickles. For example,cucumbers are packed directly
vegetables in a container and then freezing it. The individ-
into jars, then covered in brine which is made of salt and
ual particles do not cohere and the final package contains
vinegar,preferably containing some lactic acid and season-
individual portions of the food rather than a solid block.
ings. The residual sugar in the cucumber can be fermented
Commercial freezing is conducted with moving air,termed
by the action of the lactic acid bacteria.
blast-freezing. Air is accelerated by fans and passed over
the vegetables. With advances in refrigeration engineering,
many plants are now freezing foods at (cid:2)40°C.
REFERENCES
The third freezing process is that of immersing the
vegetables in a liquid. Some of the liquids used for veg- 1. FAO. Food and Agriculture Organization Statistics. http://
etables freezing are liquid nitrogen,liquid carbon dioxide www.fao.org/. 2003.
and Refrigerant 12. Vegetables freeze extremely rapidly in 2. CW Vertucci. J. Seed Tech. 17:41–53,1993.
contact with such liquids. Refrigerant 12, a fluorinated 3. AC Leopold, CW Vertucci. In: Stanwood, P.C. and
hydrocarbon,can be adjusted to various temperatures. The MacDonald,M.B. (eds) Seed Moisture. Crop Sci. Soc.
Am.,Madison,1989,pp. 51–68.
use of such liquid nitrogen, liquid carbon dioxide and
4. RH Ellis, K Osei-Bonsu, EH Roberts. Annuals of
Refrigerant 12 meets food safety regulation.
Botany. 50:69–82,1982.
5. JD Bewley,M Black. Seeds:Physiology of Development
D. FERMENTATION and Germination, 2nd ed. Plenum Press, New York.
1994.
Fermented vegetables are the food products by the fermen-
6. AM Mayer, A Poljakoff-Mayber. The Germination of
tation of lactic acid bacteria,which are preserved primarily
Seeds,3rd ed. Pergamon Press,Oxford,1982.
by acidification. Olive products and sauerkraut are solely
7. HS Marks,JA Hilson,HC Leichtweis,GS Stoewsand. J.
produced by fermentation. Some cucumber pickles are pro-
Agric. Food Chem. 64:279–281,1992.
duced by fermentation, in which spices and salt are added
8. Ministry of Agriculture, Fisheries and Food. Food
before fermentation and fermentation occurs in the presence Surveillance paper No. 32,HMSO,London. 1992.
of these seasonings. This is called the genuine dill fermen- 9. DJ Lyons, GE Rayment, PE Nobbs, LE McCallum. J.
tation,but is rapidly being displayed by salt stock and fresh Agric. Food Chem. 64:279–281,1994.

171 Chemistry and Technology of Tofu
Making
K.C. Chang
Department of Cereal and Food Sciences, North Dakota State University
CONTENTS
I. Introduction ....................................................................................................................................................171-1
II. Tofu Processing Methods ..............................................................................................................................171-2
A. Soaking ....................................................................................................................................................171-3
B. Grinding ..................................................................................................................................................171-3
C. Heating ....................................................................................................................................................171-4
D. Separation of Soymilk ............................................................................................................................171-5
E. Coagulation of Soymilk ..........................................................................................................................171-5
1. Type of Coagulants ..........................................................................................................................171-5
2. Soymilk Concentration ....................................................................................................................171-6
3. Coagulant Concentration ..................................................................................................................171-6
4. Coagulation Temperature ..................................................................................................................171-6
5. Coagulation Methods ......................................................................................................................171-6
F. Pressing of Curd ......................................................................................................................................171-7
III. Roles of Soy Proteins in Tofu Making ............................................................................................................171-8
A. Storage Proteins of Soybeans ..................................................................................................................171-8
B. β-conglycinin (7S Protein) ......................................................................................................................171-8
C. Glycinin (11S Protein) ............................................................................................................................171-8
D. Gelation of Purified Soy Proteins ..........................................................................................................171-9
E. Soy Proteins in Tofu Making ................................................................................................................171-10
IV. Molecular Models for Tofu Curd Formation ................................................................................................171-11
A. Role of Protein Charges ........................................................................................................................171-11
B. Role of Lipids in Curd Formation ........................................................................................................171-11
C. Role of Phospholipids in Curd Formation ............................................................................................171-11
D. Hypothesized Molecular Model of Tofu Curd Formation ....................................................................171-11
V. Role of Phytic Acid in Tofu Making ............................................................................................................171-12
VI. Roles of Isoflavone and Saponins in Tofu Making ......................................................................................171-13
VII. Effect of Lipoxygenase on Tofu Making ....................................................................................................171-14
VIII. Effect of Soybean Storage on Tofu Making ..................................................................................................171-15
IX. Evaluation of Soybean Cultivars for Tofu Making ......................................................................................171-16
Acknowledgment ......................................................................................................................................................171-19
References ................................................................................................................................................................171-20
I. INTRODUCTION soybean began only after World War II. In the first part of
the 20th century,soybeans were known to the Westerners as
China is the birthplace of soybeans. In Oriental countries an oilseed and feed-stuff only. Aside from the image prob-
such as China,Korea and Japan,soy foods have been con- lem,the major obstacles in the utilization of whole soybean
sumed for thousands of years. In the1800s,soybeans were for foods in the Western society include the beany flavor
introduced to America, but large-scale production of and the flatulence factor. Substantial use of soybeans for
171-1
171-2 Handbook of Food Science, Technology, and Engineering, Volume 4
foods did not take place until recent decades,where a small Since its original invention,tofu manufacture process
part of defatted soy meal was used or processed further for has been improved greatly. Many manufacture and uti-
human foods. However, since 1980s, soybeans have been lization methods have been developed in various countries
used for making a variety of soy foods in America. Hence, and regions. Japan has taken the leadership role in the
some soybeans are known as vegetable legumes and soy advancement of the science and technology for tofu mak-
foods are popular vegetable foods. In very recent years, ing. It is well known that making tofu is not a difficult
because several soybean components have been discovered task. Many people can claim that they can make tofu.
to possess health benefits, hence, soy foods have become However,making excellent quality tofu consistently is not
health foods. Consumption of soy foods in America has an easy matter. The principles of tofu making are simple,
increased dramatically in the last few years. Soy foods are which consist of two main stages: (a) the preparation of
finding their ways in the main stream supermarkets. The soymilk and (b) the coagulation of soymilk to form bean
total retail value of soy foods has exceeded 4 billion US curd that is then made into various types of tofu. However,
dollars. The United States of America produces one half of many factors involve in the processing of tofu and raw
the world soybeans, which are estimated to be approxi- bean components affect substantially the quality of tofu.
mately 159 million metric tons (1). Only approximately 5% There have been several great books related to tofu mak-
of the US soybeans are used for making foods. However in ing,including the “The Science of Tofu”(2),the “Tofu &
the last three years, it was estimated that the annual soy Soymilk Production,the Book of Tofu,Volume 2”(3),and
food market had increased by 25% per year. Soymilk,tofu the “Soybeans: Chemistry, Technology and Utilization,”
and meat analogs are the three major soy food types in the (4). In this chapter, we focus on recent studies related to
USA. The increases in soymilk and tofu retail values in the soybean quality and tofu making and try to organize
USA are about 40–50% and 15–20%, respectively, each available information together to show how various fac-
year since 1997 (P. Golbitz, personal communication, tors affect tofu quality.
2000),while the overall growth of the US food industry has
been only 3% per year. It has been predicted by the United
II. TOFU PROCESSING METHODS
Soybean Board that by the year of 2010, soyfoods’retail
value will reach 100 billion dollars in the United States. Tofu manufacturing requires a series of unit operations.
Soy protein has long been known to have a good nutri- Generally, three steps are critical in determining product
tional quality. Recent discoveries of potential health ben- type:(a) soymilk extraction and solid content,(b) coagu-
efits of soy foods include reducing the risk of lation method (types of coagulants, breaking or not after
cardiovascular diseases, preventing certain cancers, curd formation), and (c) pressing or not. However, all
reducing postmenopausal syndromes,and increasing bone methods for making various tofu products begin with sim-
mass density all contribute to the recognition of soybean ilar steps for soymilk as shown in Figure 171.1. The tra-
as a health food. US Food and Drug Administration has ditional Chinese method separates raw soymilk from the
approved a health claim for processed foods containing okara (residue) before heating. In the Japanese process,
soy proteins that states ‘consumption of 25 gm soy pro- heating the slurry “go” prior to separation facilitates
teins per day in conjunction with a low cholesterol diet soymilk extraction and increases tofu yield. However,
would reduce the risk of heart disease.’All of these will Beddows and Wong (5) reported that yield and quality of
continue to enhance the consumption of this ancient silken tofu made by a laboratory scale with the slurry fil-
Oriental food crop in America and other parts of the tration prior to heating are better than that with heating
world. prior to residue separation. Both Chinese and Japanese
Tofu has found its history dated back to the China’s methods for extracting soymilk are known as the tradi-
Han Dynasty approximately 2000 years ago. Tofu has tional Oriental methods because of the presence of beany
been an integral part of the Chinese food culture; it is flavor in the final soymilk product. Regarding the beany
indispensable in the diets of Chinese and the people of flavor, several modern methods aimed at improving
several other East Asian countries, including Japan and soymilk taste and flavor have been developed in the past
Korea. Soybeans have contributed to the health of decades. We will discuss this in more detail in the lipoxy-
Chinese people in the history. We believe that a large- genase section. In tofu industry, many manufacturers
scale prolonged protein malnutrition has never occurred worldwide have adopted the Japanese process because of
in China might be attributed to the ready availability of higher tofu yield,available tofu manufacturing machines,
soy foods. Soy foods are not only nutritious but also very and a lower beany flavor than the Chinese process. Some
delicious, which have been included in thousands of industries wash the okara one or two times to extract
dishes of Chinese foods. Because of the functional prop- residual proteins/soluble solids and use the wash water to
erties and health benefits reported in recent years, soy grind the soybeans to improve yield.
foods are gaining acceptance increasingly in the Western After soymilk is produced, various steps are used to
society. manufacture different types of tofu. Tofu in the market is
Chemistry and Technology of Tofu Making 171-3
Soybeans Hot soymilk, 9−10 brix
Cleaning Adding coagulant
Curd formation
Washing
Curd breakdown
Soaking
15−20°C, 8−10 hr
10−15°C,12−16 hr
Pressing
Cutting into cubes
Wet grinding
(Water added)
Packing in water
Heating
(98−105°C, 2−5 min)
Sterilization/pasteurization
Separation Cooling
Okara
Momen tofu (firm, extra firm) product
Hot soymilk
FIGURE 171.2 Scheme for momen tofu production.
FIGURE 171.1 Initial steps in the preparation of soymilk for
tofu making.
Hot soymilk, 13 brix or higher
generally classified into soft, firm, and extra firm tofu,
based on water content and textural properties. Tofu is Adding coagulant
classified into momen (regular), kinugoshi (silken), soft,
packed silken, and aseptic tofu, depending on processing
Curd formation
methods. Figures 171.2 to 171.5 describe the methods
used in the tofu industry for the production of momen
tofu, silken (Kinugoshi) tofu, filled packed silken tofu, Cutting into cubes
and soft tofu,respectively.
Packing
A. SOAKING
After proper washing, soybeans are soaked in water to
soften their cellular structure for water grinding. Soaking Sterilization/pasteurization
time depends on water temperature, the soybean variety,
and the age of soybeans. Temperature is the main factor
Cooling
affecting the rate of water uptake,with higher rate associ-
ated with higher temperature (6). Generally, soaking in
ambient water takes 8–10 hours in summer and 16–18
Silken tofu product
hours in winter. After soaking, the beans weigh approxi-
mately 2.2–2.3 times of their initial weight (3). FIGURE 171.3 Scheme for Silken (Kinugoshi) tofu produc-
tion (no curd breaking and no pressing).
B. GRINDING
final products. For example, the water dosage for silken
After soaking,soybeans are ground with water into slurry tofu,soft tofu,and regular tofu is 5,7–8,and 10 times of
using a stone-mill or a stainless steel grinder. The amount raw soybean weight, respectively (2). Proper grinding
of water added during grinding depends on the type of gives appropriate small particle sizes in the slurry and
171-4 HHaannddbbooookk ooff FFoooodd SScciieennccee,, TTeecchhnnoollooggyy,, aanndd EEnnggiinneeeerriinngg,, VVoolluummee 44
increases between 0°C and 50°C (7). They found that the
Hot soymilk, 13 brix or higher
relationship is related to the content of sulfhydryl groups
(-SH) in soymilk. The decrease of –SH group may be
caused by a lipid oxidation activated by lipoxygenases in
Cooling soymilk
soybeans. Since water composes of 88–90% of tofu
weight,it plays an important role in determining the tofu
Adding coagulant taste. The source and quality of water are always an
important concern to tofu manufacturers. Water, contain-
ing a proper amount of minerals (about 100mg/L) includ-
Filling into container
ing calcium, magnesium, sodium, potassium, iron, and
manganese,provides a harmonious and mellow taste (2).
Heating to form curd and
sterilization C. HEATING
The step of heating is essential during the tofu processing
not only for killing microorganisms in the slurry,improv-
Cooling
ing nutritional quality by inactivating trypsin inhibitor
(TI), and reducing beany flavor, but also for denaturing
proteins so that they can coagulate into curd in the pres-
Filled silken tofu product
ence of a coagulant. Before heating, soy protein mole-
cules maintain their native globular structures in which
FIGURE 171.4 Scheme for manufacturing filled silken tofu
(cold filling and curd formed in container). the hydrophobic regions are wrapped inside. Upon heat-
ing, the soy proteins are denatured resulting in native
molecules unfolded,and their hydrophobic groups expose
Hot soymilk, 10−12 brix
to outside, consequently protein solubility decreases due
to aggregation. What extent of heat treatment is consid-
ered adequate for soymilk and tofu making? Hackler et al.
Adding coagulant (8) conducted a study of heat treatment on nutritive value
of soymilk protein fed to weaning rats,and found that heat
Curd formation treatment should be sufficient to inactivate 80–90% of
trypsin inhibitors for maximizing nutritive values. Trypsin
inhibitors are heat resistant. At 100°C,14 min are required
Pressing to inactivate 80% TI or 30 min to have 90% TI destruc-
tion. Wilson (9) suggested that the time/temperature
requirement for soymilk be based on 85% TI inactivation.
Cutting into cubes
Trypsin inhibitors are water-soluble proteins, a part of
which may be released in the whey during pressing of tofu
Packing in water making. Thus, the slurry used for tofu making requires
shorter heating time than that for soymilk as the final
product. Watanabe (2) recommends that boiling at 100°C
Sterilization/pasteruization
for 3–5 min is required for tofu making.
The optimum heating time of soymilk for making tofu
Cooling corresponds approximately to the maximum amount of
sulfhydryl groups. If heating is not adequate,soy proteins
do not dissociate into subunits; but in excessive heating,
Filled silken tofu product sulfhydryl groups are oxidized by air (10). Tofu prepared
with soymilk, which has been heated at 100°C up to 60
FIGURE 171.5 Scheme for manufacturing soft tofu (no curd
min, is softer than that from usual preparations (100°C,
breaking).
3 min) (11). This may be due to the oxidation of
facilitates the extraction of solid and nutrients into the sulfhydryl groups of soy protein during excessive heating,
soymilk. The smaller the particles, the better the extrac- resulting in the decreases of sulfhydryl group content and
tion,but okara (residues) becomes more difficult to sepa- tofu hardness. In the Japanese method,cooking the slurry
rate. The water temperature of grinding affects not only for about 7 to 14 min at 100°C gives the best soymilk
the flavor of soymilk, but also the texture of tofu. Tofu solid and protein recovery,especially for tofu (12). In the
firmness decreases as the water temperature of grinding Chinese method, the hardness of tofu increases slightly
Chemistry and Technology of Tofu Making 171-5
from 0 to 12 min of boiling soymilk, but decreased sig- and water. It consists primarily of MgCl ⋅6H O (92.3%)
2 2
nificantly after 30 min and 60 min of boiling (13). Amino plus all of the other salts and trace minerals in seawater
acid composition of soymilk has no significant changes (3.8% MgSO , 1.7% NaCl, 1.2% KCl, and 1% CaSO ).
4 4
when heating at 93°C,but the amount of cystine and tryp- Refined nigari is a relatively pure magnesium chloride
tophan decrease while heating at 121°C from 0 to 121 min (99.5%). Calcium chloride (CaCl ) is not found in seawa-
2
(14). Approximate 30% of cystine and methionine are ter, it is not a traditional nigari-type coagulant. However,
destroyed after 30 min of boiling soymilk (15). In some it gives tofu excellent flavor,almost identical to that made
tofu factories, the slurry is heated by a continuous pres- from natural nigari or magnesium chloride nigari.
sure cooker at various pressure increasing from 80°C to Moreover,it is the cheapest nigari-type coagulant and the
105°C for times ranging from 4–20 min. food-grade type produced in the U.S. and on the GRAS
(Generally Recognized as Safe) list. In Japan, most tofu
D. SEPARATION OF SOYMILK industries use nigari-type coagulants in combination with
calcium sulfate rather than alone. Nigari-type coagulants
In the Japanese process, soymilk is extracted from the
make the most delicious tofu, prized for its wonderful
slurry after heating. The separation of a small volume of
subtle sweet flavor and aroma. However, they have some
slurry can be done by filling the slurry in a cotton cloth
disadvantages compared with calcium sulfate and GDL.
bag then pressed by hands. Industrial processing can be
They react very rapidly with soymilk,so their use requires
done by drum pressing,screw pressing,centrifugation,or
more skill and attention and must be added slowly. Due to
shaker-filtration. The efficiency of soymilk separation
the extremely quick reaction of nigari with soymilk, the
depends on the extraction pressure and pressing time,the
coagulated bean curd is destroyed while agitation is still
pore size of the filter or screen,the particle size of slurry,
going on. The nigari-coagulated bean curd does not incor-
and whether the okara is re-washed or re-pressed. When
porate so much water as the sulfate type, consequently
the okara is re-washed and re-pressed to extract more pro-
produce tofu with lower yields and coarser texture than
tein and solids,the yield of tofu can increase by 15–20%.
that made with calcium sulfate. Nigari alone is not suit-
Okara contains 76–80% of moisture (24–20% solids) after
able for making silken tofu from hot soymilk,because the
being well pressed. About 29% of the solids and 17% of
high temperature and high solids content of soymilk make
the protein in the original soybeans are remained in the
it extremely difficult to solidify uniformly with nigari in
pressed okara that has not been washed (3). On dry weight
such a short reaction time. However,filled silken tofu can
basis, okara comprises 25–28% protein, 9–11% lipid,
be made with nigari if the soymilk is cooled to a low tem-
40–44% insoluble fiber,13–15% soluble fiber,and 4–5% perature (e.g. 4°C) prior to coagulant addition, and fol-
soluble carbohydrate (16). Most of total fiber in soybeans
lowed by heating to slowly coagulate the proteins.
is concentrated in okara.
Sulfate-type coagulants (especially calcium sulfate
known as gypsum) are the most widely used tofu coagu-
E. COAGULATION OF SOYMILK lants in the world. They have low water solubility (3.0 g/L
at room temperature), which is an important factor in
Coagulation is the most important and the most difficult
determining the speed of the coagulation reaction.
step in tofu making because it depends on the complex
Because of their low solubility,they react slowly with the
interrelationship of many variables, including soybean
soymilk; consequently allow the formation of bean curds
variety, soymilk concentration and pH, temperature, type
with a high water-holding capacity. Thus,they give 15 to
and amount of coagulant, and coagulation method. Hot
20% higher bulk yields than nigari. The resulting tofu has
soymilk is usually coagulated to form curd by adding a
a soft and smooth texture. Calcium sulfate can be used to
salt or an acid coagulant. Tofu coagulants are classified
make regular, firm, soft, silken, and even packed (pack-
into four basic types: (A) nigari-type or chloride-type
age-filled) tofu, whereas it is not easy to make the latter
coagulant, including magnesium chloride, calcium chlo-
two types with nigari. Calcium sulfate is easy to use even
ride, and sea water; (B) sulfate-type, including calcium
by relatively unskilled tofu makers. Even if agitation is
sulfate and magnesium sulfate; (C) glucono-delta-lactone
carried out slowly or the dosage varies slightly,the differ-
(GDL); (D) acidic coagulants including citrus juices,
ences in tofu yield and texture are not very large. Calcium
vinegar,and lactic acid (3). Each type of coagulant has its
sulfate as tofu coagulant produces tofu with a mild or
advantages and disadvantages.
bland flavor; however, the taste is slightly inferior to that
of nigari tofu.
1. Type of Coagulants
GDL is an oxidation product of glucose. It is manu-
Nigari or chloride-type coagulants include natural nigari, factured from corn starch by a fermentation process, and
refined nigari, calcium chloride, and seawater. Natural was first used for silken and packed tofu production during
nigari, known as ‘bittern’in the West, is extracted from the 1960s in Japan (3). GDL is fundamentally different
seawater by removing most or all of the table salt (NaCl) from nigari and gypsum-type coagulants,in which an acid
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rather a salt does the coagulation. Upon being dissolved in be remained. Several methods have been compared to
water, it is slowly hydrolyzed (about 2–3 hr) to gluconic determine the optimum amount of coagulant in tofu mak-
acid by water. The pH of 1% fresh aqueous solution at ing, including light transmittance of whey (%T), whey
room temperature is 3.5,it drops to 2.5 due to the conver- volume, pH of whey, and conductance of whey (19).
sion to gluconic acid within 2–3 hr. For packed and silken Among these methods, they found that whey transmit-
tofu, GDL is dissolved in previously cooled soymilk and tance and conductance correlated with coagulant concen-
forms gluconic acid gradually, then heat coagulates tration,and concluded that measuring the conductance of
soymilk with this acid to form homogeneous solidified the coagulating soymilk was faster and more reproducible
curd with rich water holding property in the container. The than obtaining pH and transmittance values of the whey.
GDL gives tofu a slightly acidic flavor and tender jello-like A rotational viscometer (viscograph) is applied by
texture. For better flavor and texture,GDL is often used in researchers to measure the optimum concentration of
combination with a calcium sulfate. coagulant for the coagulation of soymilk (20,21). Among
The acid-type coagulants, including lactic acid, acetic five coagulants studied,with an increase in coagulant con-
acid, and lemon juice, work well as natural coagulants. centration from the minimum required concentration
However, the yield of tofu is low, the texture is slightly (from 0.15% to 0.5% of soymilk volume depending on
crumbly, and the flavor of tofu is a little tart, when com- coagulant), there is an increase in whey volume and
pared with nigari and calcium sulfate as coagulants (17,18). decreases in the moisture of tofu (22). Sun and Breene
(23) found negative correlations between calcium sulfate
2. Soymilk Concentration concentration and both yield and protein recovery of tofu
from five soybean varieties.
The solid concentration of soymilk is related to “water-to-
bean ratio” that is defined as the total weight of water
4. Coagulation Temperature
added to the beans during soaking,grinding,and cooking
divided by the original weight of the soybeans (3). The The soymilk temperature for adding coagulant affects
ratio of water:beans can be very critical to affect protein coagulation rate as well as tofu quality. The yield and
extraction yield and the properties of tofu . In the range of moisture content of tofu decrease as the temperature of
9:1 to 14:1, 10:1 gave the best result in protein recovery coagulation increases,whereas the hardness and elasticity
(19). For making regular tofu,the best water-to-bean ratio increase (15). When soymilk is at high temperature, pro-
is about 10:1 which results in a soymilk with 6.0 to 6.3% teins possess high active energy that can lead to fast coag-
solids and 3.0% protein (12). However,5:1 to 7:1 ratios are ulation, resulting in the formation of curd with low water
required for making soft tofu or silken tofu (10). The holding capacity, consequently, tofu has hard texture and
amount of coagulant required to reach the optimum coag- low bulk yield. The hotter the soymilk at the time of coag-
ulation varies with the concentration of soymilk. Watanabe ulation, the less the amount of coagulant required. When
et al. (12) reported that more coagulant is required for tofu is coagulated at a high temperature, a small increase
more concentrated soymilk (in the range of 3% to 8%) to in the amount of coagulant may lead to a large decrease in
reach the same level of whey transparency,and the dosage yield. In the tofu industry, the temperature of coagulation
for calcium sulfate was about 10–20% more than that for varies from one factory to another,depending on the type
nigari. In our laboratory,we found that the coagulant con- of coagulant used. Generally, coagulation temperature
centration required to reach the optimum coagulation of ranges from 68°C to 95°C for those using nigari, while
silken tofu increases linearly with the soymilk solid con- those using calcium sulfate prefer the range from 70°C to
tent in the range from 6% to 11%, and the concentration 80°C in Japan. Beddows and Wong (5) reported that the
for magnesium chloride is about 13–15% more than that optimum coagulation temperature is 75–80°C for silken
for calcium chloride (unpublished data). tofu with gypsum as coagulant in a small bench scale. Shih
et al. (24) reported that the optimum coagulation tempera-
3. Coagulant Concentration ture is 85–91°C for soft tofu with modified nigari (mostly
CaSO ) in a medium scale. The operational temperature of
The amount of coagulant added into soymilk affects 4
coagulation also varies from one region to another. In
greatly the yield,texture,taste,and aroma of tofu. In gen-
America,tofu makers prefer a relatively high temperature,
eral,by observing curd formed and whey produced during
85°C,for using nigari and calcium sulfate,since less coag-
pressing, tofu makers could tell whether the amount of
ulant is required,the curd forms quickly,and the tofu has
coagulant is appropriate or not. When a proper amount of
firm and dense texture but no significant drop in yield.
coagulant is used, the whey is transparent and amber or
pale yellow. In the case of too much coagulant used, the
5. Coagulation Methods
whey is yellowish color and the curds have a coarse
or crumbly texture. If too little coagulant is added, the The addition method, the stirring speed at which the
whey is cloudy and some uncoagulated soymilk may coagulant is added,and the continuous stirring time after