Heat tolerant bacteria, in particular Clostridium botulinum, which can be deadly, are a great concern in the canning indus- try. C. botulinum thrives in a low-acidic environment (pH 4.6 or above), which corresponds with the conditions of canned tea drinks. The spores of C. botulinum are, in effect, very heat tolerant. When the spores germinate to grow to vegetative bac- teria, they produce toxins. The botulinum toxin is deadly and kills half of 1 billion mice at a concentration of 1 mg of nitrogen in the toxin (109ip LD50mice/1mg-N, S toxin). Lethal dose for a human is considered to be a fewµg. Therefore, it is the re- sponsibility of the food canning industry to kill the spores of
TABLE1 Minimum Inhibitory Concentrations of Catechins and Theaflavins
Against Foodborne Pathogenic Bacteria
MIC (ppm)
Bacteria GTC EC ECg EGC EGCg CTFs TF1 TF2A TF2B TF3
Staphylococcus aureus IAM 1011 450 ⬎800 800 150 250 500 500 400 150 200
Vibrio fluvialis JCM 3752 200 800 300 300 200 400 500 500 400 500
V. parahaemolyticus IFO 12711 200 800 500 300 200 300 300 500 400 500
V. metschnikovii IAM 1039 500 ⬎1000 ⬎1000 500 1000 300 600 800 700 800
Clostridium perfringens JCM 3816 400 ⬎1000 400 1000 300 200 500 600 500 400
Bacillus cereus JCM 2152 600 ⬎1000 600 ⬎1000 600 500 600 1000 1000 800
Plesiomonas shigelloides IID No. 3 100 700 100 200 100 100 200 200 200 100
Aeromonas sobria JCM 2139 400 ⬎1000 700 400 300 300 600 600 400 500
GTC, green tea catechins CTFs, crude theaflavins
this bacteria. Billions of spores/ml are confirmed to be killed by heating at 121°C for 4 min. Over the last 10 years or so canned tea has been a growing market in Japan, with this popularity recently starting to spread to the United States and other countries. Present health regulations dictate strict rules for sterilization under which the spores of C. botulinum are without doubt eliminated. The product must, theoretically, be heated at 121°C for at least 4 min. However, this severe heat- ing process has a detrimental effect on the quality of the prod- uct, resulting in a poorer taste and aroma of the tea. A practi- cal alternative is to heat the canned product at 130°C or over for at least a few seconds, which is more than the equivalent energy expended in the case of treatment at 121°C for 4 min. Either way, the flavor of canned tea is inferior to a product that undergoes a more mild heat treatment.
Thus, if C. botilinum spores could be eliminated without employing such drastic heating for sterilization, it would be of great advantage to the industry. Accordingly, in view of tea’s antibacterial actions, some investigations were carried out into the fate of C. botulinum in canned tea beverages (2). Green tea, oolong tea, and two kinds of black teas—sweetened (8% sugar) and lightly sweetened (5% sugar)—were inocu- lated with C. botulinum spores (type A and type B, composed of a mixture of 5 strains each) to the concentration of about 500 spores/ml and incubated for 30 days at 30°C , without any heat treatment after inoculation. The viable countings were calculated by most probable number (MPN) method and pouch method. (MPN is a method for counting the number of spores in the solution by diluting the test solution decimally until a certain dilution does not kill the mice by intraperitoneal [ip] injection. From the number of this dilution the number of spores in the initial solution is determined. With the pouch method, the test solution is inoculated into the medium, the spores grow anaerobically, and the number of colonies are counted in the flat pouch.) In all of the tea drinks except green tea, there was a marked decrease in the viable cell count(Fig. 1).In the case of green tea, subsequent heating of the inocu-
FIG. 1 Changes in C. botulinum spore numbers after inoculation into vari- ous kinds of tea drinks without heat treatment.
FIG. 2 Changes in C. botulinum spore numbers after inoculation into canned green tea with treatment for 30 min at 85°C.
lated liquid for 30 minutes at 85°C decreased the viable spores (Fig. 2). In order to determine what components of the tea were responsible for the inhibition of spore proliferation, the polyphenolic and nonpolyphenolic fractions were isolated and each was incubated respectively with C. botulinum. The poly- phenolic fraction was confirmed to be effective as indicated by a decrease in the viable spore count, while the non-polypheno- lic fraction had no inhibitory influence on the spore count
(Fig. 3).
IV. ANTIBACTERIAL ACTIVITY OF TEA
POLYPHENOLS AGAINST C. botulinum AND OTHER HEAT TOLERANT BACTERIA
In the previous experiment, the fate of C. botulinum spores inoculated into tea drinks was examined. It was confirmed that the polyphenolic component of tea drinks killed the spores as well as the vegetative cells of C. botulinum. Pursu-
FIG. 3 Changes in C. botulinum spore numbers after inoculation into the polyphenolic fractions from black tea.
ant to the above, minimum inhibitory concentration (MIC) of each tea polyphenolic component against spores of C. botu- linum was determined (3). The constituents of the polypheno- lic fraction of green tea (GTC) and black tea (crude theaflav- ins) are described in Chapter 4. The MIC of these fractions
TABLE2 MIC of Tea Polyphenols Against C.
botulinum (ppm)
Spores Vegetative cells
GTC 300 ⬍100 EGC ⬎1000 300 EC ⬎1000 ⬎1000 EGCg 200 ⬍100 ECg 200 200 Crude theaflavins 200 200 TF1 250 150 TF2 (A&B) 150 250 TF2A 150 250 TF2B 150 150 TF3 100 200
TABLE3 MIC of Tea Polyphenols Against B. subtilis, B. stearothermophilus, and D. nigrificans (ppm)
B. subtilis B. stearothermophilus D. nigrificans
Spores Vegetative cells Spores Vegetative cells Spores Vegetative cells
GTC ⬎1000 ⬎800 300 200 ⬍100 ⬎1000 EGC ⬎1000 ⬎800 1000 300 500 ⬎1000 EC ⬎1000 ⬎800 ⬎1000 800 500 ⬎1000 EGCg 1000 ⬎800 200 200 200 ⬎1000 ECg 900 ⬎800 300 ⬍100 ⬍100 ⬎1000 Crude theaflavins 600 700 300 200 ⬍100 ⬎1000 TF1 ⬎500 ⬎1000 250 200 200 ⬎1000 TF2 (A&B) — — 200 — 100 — TF2A ⬎500 500 — 300 — ⬎1000 TF2B 500 450 — 300 — ⬎1000 TF3 400 400 150 200 100 ⬎1000
and their individual components against the spores and the vegetative cells of C. botulinum is shown inTable 2.Even the resistant spores were killed at concentrations of 200–300 ppm. These data support the antibotulinum potency of canned teas; in ordinary tea drinks, polyphenolic components are con- tained somewhere from 500–1,000 ppm. Furthermore, the MIC of these tea polyphenols was examined against such heat resistant bacteria as Bacillus subtilis, B. stearothermophilus,
and Desulfotomaculum nigrificans as shown inTable 3.These
data indicate that (a) tea polyphenols show almost the same antibacterial potency against the most representative ‘‘flat sour’’ bacteria B. stearothermophilus as against C. botulinum, (b) tea polyphenols did not work against B. subtilis as effec- tively as against B. stearothermophilus, and (c) tea polyphe- nols were effective against the spores but not against the vege- tative cells of D. nigrificans, a common putrefactive bacteria in fish cans. These results revealed that the antibacterial spec- trum of tea polyphenols is random and unpredictable.
REFERENCES
1. Y Hara, T Ishigami. Antibacterial activities of tea polyphenols against foodborne pathogenic bacteria (studies on antibacterial effects of tea polyphenols part III) [in Japanese]. Nippon Sho- kuhin Kogyo Gakkaishi 36:996–999, 1989.
2. Y Hara, M Watanabe. The fate of Clostridium botulinum spores inoculated into tea drinks (studies on antibacterial effects of tea polyphenols part I) [in Japanese]. Nippon Shokuhin Kogyo Gak- kaishi 36:375–379, 1989.
3. Y Hara, M Watanabe. Antibacterial activity of tea polyphenols against Clostridium botulinum (studies on antibacterial effects of tea polyphenols part II) [in Japanese]. Nippon Shokuhin Ko- gyo Gakkaishi 36:951–955, 1989.