Tannins are phenolic compounds that precipitate proteins. They are composed of a very diverse group of oligomers and polymers.All phenolic compounds (primary and secondary) are, in one way or another, formed via the shikimic acid pathway, also known as the phenylpropanoid pathway.Each HT molecule is usually composed of a core of D-glucose and 6 to 9 galloyl groups. At least 3 hydroxyl groups of the glucose must be esterified to exhibit a sufficiently strong binding capacity to be classified as a tannin. In nature, there is abundance of mono and di-galloyl esters of glucose (MW about 900). They are not considered to be tannins.Hydrolysable tannins is expected to get converted to gallic acid and ellagic acid on long term storage. Yes it happens during stoarge action of oxidative enzymes (i.e. polyphenoloxydases and peroxidases) increase and oxidative products are formed. hydrolysable tannins break into (ellagic acid + gallic acid + tertgallic acid + etc.

HTs are molecules with a polyol (generally D-glucose) as a central core. The hydroxyl groups of these carbohydrates are partially or totally esterified with phenolic groups like gallic acid (-->gallotannins) or ellagic acid (--> ellagitannins). HT are usually present in low amounts in plantsEllagitannins (ETs) belong to the so-called hydrolysable tannins found in many plants.Microbial metabolism and gastric digestion convert HTs into absorbable low molecular weight metabolites Tannins nor hydrolysable tannins are  positively influenced by elevated CO2, but in storage condition few of the oxidases and peroxidases remain active which break down tannins action of oxidative enzymes (i.e. polyphenoloxydases and peroxidases).ellagic acid + gallic acid + tertgallic acid + etc.

Thanks for your explanatory answer Dr.Ravi kant. Then what should be the storage condition? How to avoid this conversion?

Effect of different incubation periods: Optimization of incubation periods (3-14 days) was carried out in order to determine the best harvesting time for tannase from production culture. Maximum tannase activity (162.3 U/ml) was observed on the 7th day (168 hrs) of incubation.

Effect of different incubation temperatures : Maximum tannase activity (195.0 U/ml) was observed at an incubation temperature of 30°C although the organism had a capacity to grow over a wide temperature range (25°C to 35°C) (Fig. 3). Tannase production by A. niger can be evaluated at room temperature in our nation without additional efforts or costs. Similar optimum temperatures were reported for tannase from fungal isolates previously

http://pelagiaresearchlibrary.com/european-journal-of-experimental-biology/vol2-iss5/EJEB-2012-2-5-1430-1438.pdf

my opinion in storage conditions under Minus  40 degree  (- 40 degree)