Home Tea Talk Tea Storage Keeping your tea fresh - Part 4 - Temperature

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Keeping your tea fresh - Part 4 - Temperature

To keep it fresh, should our good quality green tea be kept at room temperature, refrigerated, or frozen?  Occasionally , we hear that our customers store their tea in a food cabinet with spices and other dried foods.  What is the problem with this method?  Although dry storage is a common practice throughout America and many other parts of the world,  when it comes to freshness and effectiveness of green tea's antioxidants, there is a better place to keep your tea.  In this article we will look at the influence of temperature on freshness.

 

Even under low light and moisture conditions, if storage temperature is higher than 30C, 86F, the green color of the dry leaf starts to fade. Even with the presence of oxygen lower temperature delays oxidation, and thus deterioration.

When tea is stored without moisture, light and oxygen, at low temperature, it does not deteriorate readily but can mellow through aging.  Although aging of tea does not happen to all green teas and often depends on the growing, processessing and harvest type, (grade), low temperature storage can be significantly better compared to storing at room temperature.

 

 

Cold Storage vs. Room Temperature Storage of Green Tea

 

Above, the graph on the left shows the correlation between remaining vitamin C content (%) and noticeable difference in quality deterioration in Sencha Green Tea, (x axis: -1 slight, -2 a little , -3 somewhat, -4 significantly) (Yamanishi 1992), The graph on the right shows comparison of vitamin C contents in 4 tea storage categories 1. Nitrogen gas replaced 5°C, 2. Nitrogen gas replaced at room temp, 3. in a can with air at 5°C, and 4. in a can with air at room temp (Yamanishi 1992)

 

In this study, vitamin C is used as the indicator of green tea deterioration because vitamin C is readily oxidized and typically used to show the quality of green tea and antioxidants.  Generally, nitrogen gas is used to replace the existing oxygen in a bag since nitrogen has little effect on green tea deterioration.

 

What can you tell from the graphs?  Generally tea stored at 5°C as opposed to room temperature has higher residual vitamin C content, and especially when air is present.  Graph 1-2 (left) shows that when vitamin C content is 70% and above, the x axis is only 1.  This indicates that deterioration is minimal and in most cases, it is almost unnoticeable by our senses.

 

Also, in cold storage, If nitrogen gas replacement is used, tea stays fresher.

 

Kama'aina Green Tea test 

We also conducted simple experiment to compare our Kama'aina green tea stored at different temperature, room temperature (-20C), refrigerator (-5C), freezer (below 0C).  We did not use nitrogen gas, or vacuum packaging option to replicate more of what happens at household level.

Tea stored in a 1. freezer (left), 2. refrigerator (center), and 3. room temperature (right).

The tea was stored for 6 months sealed in a high-barrier bag and taken out of freezer and refrigerator 2 days before testing to ensure that there is no water condensation on dry leaf.

 

The result was not visibly noticeable, but there was clear distinction in smell and taste of the brew between cold storage vs. storage at room temperature.

 

Dry Leaf:

Smell: #1 and 2 retained fresh smell, while #3 had old stale smell.

Color: No noticeable difference

 

Brew:

  #1 Freezer #2 Refrigerator #3 Room Temp
Smell Fresh Fresh slightly oxidized (old)
Taste Fresh green Mildly Fresh green with oxidized after taste
Leaf color light green light green light green 
Liqour golden yellow golden yellow slightly amber yellow
Tea stayed fresh without much noticeable change Tea stayed fresh, but became milder over time through cold storage aging Tea did not lose color on the leaf, but smell and taste were affected significantly.

 

In conclusion, even our simple experiment demonstrated that storage at room temperature temperature aids in deterioration of green tea.  

 

Where should you store your tea?

Keep your tea in a refrigerator for the best result.  Freezer will keep your tea fresh too, but may affect your tea more once it's opened and moisture level of leaf changes.

 

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