Changes in the Profile of Genistein, Daidzein, and Their Conjugates
during Thermal Processing of Tofu
†
Ingolf U. Gru ¨ n,* Koushik Adhikari, Chunqin Li, Yong Li, Bin Lin, Jiuli Zhang, and
Lakdas N. Fernando
Department of Food Science, University of MissourisColumbia, Columbia, Missouri 65211
Profiles of genistein, daidzein, genistin, daidzin, and their acetyl- and malonyl--glycosides were
determined in tofu as affected by temperature and time. Tofu was heated in water at 80, 90, and
100 °C for 0 (control), 10, 20, 30, and 40 min, and the contents of the isoflavones of interest were
quantified using reversed-phase HPLC. Total isoflavone content decreased most likely due to leaching
of isoflavones into the water. Because the content of the isoflavones of the genistein series was
little affected by the treatments, the decrease in the total isoflavone content was almost exclusively
due to a decrease of the daidzein series. Changes in the profile of the daidzein series suggest little
decarboxylation of the malonylglycoside to the acetylglycoside, but considerable de-esterification of
the malonyl- and acetylglycoside to the -glucoside. Strongly temperature dependent decreases of
the aglycon suggest possible thermal degradation of daidzein in addition to losses due to leaching.
Keywords: Tofu; thermal processing; isoflavones; genistein; genistin; daidzein; daidzin; HPLC
INTRODUCTION
Soybeans are a rich source of isoflavones. The main
isoflavones found in soybeans are genistein, daidzein,
and glycitein, each of which exists in four chemical
forms, as an aglycon (genistein, daidzein, and glycitein),
a -glucoside (genistin, daidzin, and glycitin), an acetyl-
glucoside (6′′-O-acetylgenistin, 6′′-O-acetyldaidzin, and
6′′-O-acetylglycitin), and a malonylglucoside (6′′-O-ma-
lonylgenistin, 6′′-O-malonyldaidzin, and 6′′-O-malonyl-
glycitin) (1). Because there is indication that the agly-
cons might be more bioactive (2), knowledge of the effect
of processing on the exact composition of isoflavones and
their four forms is important (3). Isoflavones are the
most common estrogenic compounds found in plants and
have been shown to possess antimicrobial and insecti-
cidal properties (4) and to prevent and reduce the risks
of various cancers (5-7). Isoflavones, and genistein in
particular, inhibit the activity of protein tyrosine ki-
nases (8), which, together with a modulation of the
transforming growth factor (TGF) -1 signaling path-
ways, might be the major mechanisms of action by
which isoflavones reduce the risk of cancer (9). In
addition, isoflavones may have a role in decreasing the
risk of cardiovascular diseases (10) by reducing the level
of total cholesterol as well as low-density lipoprotein
(LDL) cholesterol (11). Other health benefit claims as
reviewed by Kurzer and Xu (1) include reduction in
postmenopausal symptoms and risks of osteoporosis in
women.
In raw soybeans the three families of genistein,
daidzein, and glycitein are found in a ratio of ap-
proximately 6:3:1, respectively (12). Unprocessed soy-
beans contain 1.2-4.2 mg of total isoflavones/g of
soybean, with large variation due to variety, crop year,
and growth location (13). Because soybeans are con-
sumed only after being processed into either fermented
products, such as soy sauce, or unfermented products
such as tofu, Wang and Murphy (14) determined losses
of isoflavones during processing of soybeans to manu-
facture tempeh, tofu, and soy protein isolate. They
observed that 61, 44, and 53% of total isoflavones were
lost in manufacturing these products, respectively.
Fukutake et al. (15) noted that the amount of genistein
was higher in fermented soy foods than in nonfermented
foods, which was attributed to the cleavage of the
-glycosyl bond in genistin by microorganisms during
fermentation to form genistein. However, the total
isoflavone content usually is higher in nonfermented soy
products than in fermented ones (12).
Tofu is one of the more accepted soy products in the
United States. The tofu analyzed in Wang and Murphy’s
(12) study contained 0.532 mg of total isoflavones/g of
tofu. In another study, total mean isoflavone content
in raw tofu was 0.297 mg/g, whereas cooked tofu
contained 0.258 mg/g (16). Coward et al. (17) reported
0.031 and 0.015 mg/g of genistein and 0.249 and 0.269
mg/g of genistin in tofu of two different brands, respec-
tively. The daidzein and daidzin contents of the first tofu
brand were 0.016 and 0.249 mg/g respectively, whereas
those of the second tofu brand were 0.015 and 0.269 mg/
g, respectively.
Heating causes a change in the conjugation profile of
the isoflavones in soy products. Coward et al. (3)
reported that baking and frying of isolated soy protein
and textured vegetable proteins did not alter the total
isoflavone contents but changed the profiles of indi-
vidual isoflavones due to conversion of the malonyl
conjugates. Moist heat increased the content of the
-glucoside conjugates, whereas dry heat caused an
* Address correspondence to this author at 256 Eckles Hall,
Stringer Wing, Department of Food Science, University of
MissourisColumbia, MO 65211 [telephone (573) 882-6746; fax
(573) 884-7964; e-mail GruenI@missouri.edu].
†
Contribution from the Missouri Agricultural Experiment
Station. Journal Series 12,924.
2839 J. Agric. Food Chem. 2001, 49, 2839-2843
10.1021/jf010028+ CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/23/2001