What Is White Frost? The White Substance on Tea Cakes and Citrus Peel
Open tea you've kept for a few years, see something white on the surface, and your first thought is often, "Has it gone moldy?" Sometimes it has, sometimes not. The tea world calls a kind of white substance that looks like fine frost or powder "white frost." It may be constituents from the tea leaves or citrus peel slowly moving to the surface and crystallizing, or it may be mold hyphae. The two look a bit alike but are completely different things, so learning to tell them apart matters.
White Frost on Aged Tea Cakes
Raw and ripe cakes stored for many years in dry conditions occasionally show tiny white specks or white powder on the surface or between the leaves. The tea world has a few explanations for where it comes from:
- One holds that it's caffeine and other constituents in the tea gradually moving to the leaf surface with temperature changes and forming tiny crystals. Pure caffeine is naturally a white, needle-like crystal and sublimes easily, which gives this explanation some chemical basis.
- Another holds that much of what's called "white frost" on aged tea is actually traces of microbes left from storage, now dry and no longer active; such cases need to be distinguished from mold that's actively growing.
So far, there's been no systematic analysis of what white frost on tea cakes is made of. The safest attitude: when you see something white, don't jump to conclusions; look closely at its appearance and smell it.
White Frost on Xiao Qing Gan and Chenpi Pu'er
White frost is more common on the peel of xiao qing gan and big red mandarin. Citrus peel is rich in flavonoids such as hesperidin, as well as sugars and volatile oils. As the peel dries and is stored, its oil glands shrink and burst, the constituents inside move to the surface, and once dry they can crystallize into a layer of white frost.
There's no complete study focused directly on xiao qing gan, but we can look to "Huazhou pomelo peel" (Huajuhong), another citrus: researchers analyzing the white frost on its surface found it was mainly crystallized naringin, about 80% of the frost's mass, formed precisely when oil glands burst during drying and constituents migrated to the surface and crystallized. The same study compared samples with and without frost and found no clear difference in their microbial makeup. The main flavonoid in Xinhui mandarins is hesperidin, so it's generally inferred that the white frost on xiao qing gan is mainly crystals of hesperidin and similar constituents. Xiao qing gan is introduced in Xiao Qing Gan: Ripe Pu'er in a Citrus Peel and Big Red Mandarin and Chenpi Pu'er.
How to Tell White Frost from Mold
The most reliable way is to "look at the details and smell." Get a magnifying glass or a phone's macro lens and check the following:
| White frost (crystals) | White mold (hyphae) | |
|---|---|---|
| Shape | Fine powder, fine dots, or tiny grains, evenly covering | Fuzzy, cottony, or web-like, three-dimensional |
| Distribution | Mostly on raised spots, evenly spread over the surface | Often spreads from one point, concentrated in gaps, on the back, in damp spots |
| Touch | Wipes into powder, with the surface underneath intact | Often leaves a mark or discoloration after wiping |
| Smell | Normal tea or citrus aroma, or nothing in particular | Musty, damp, earthy, pungent |
| Over time | Changes little over a long time | Keeps spreading in damp conditions, and may show yellow, green, black, or other colors |
If what you see is fuzzy, spreading, or colored spots, or it smells clearly musty, consider it moldy and don't drink it. Detailed identification is in How to Identify Mold, and telling smells apart in Storage Taste, Aged Aroma, and Mold.
Can White Frost Be Wiped Off?
- When you're sure it's crystalline white frost: you can brush it off gently with a clean, dry soft brush or tissue; there's no need to wash it. When brewing, these constituents dissolve into the liquor. Some tea drinkers leave the white frost on xiao qing gan, considering it part of the peel.
- When you're not sure: set the tea apart somewhere ventilated and dry and watch it for a week or two. Crystals don't grow; if the white area spreads, turns fuzzy, or smells off, it's mold.
- When you're sure it's mold: don't wipe it off and keep drinking, and don't store it with other tea, so it doesn't affect them.
Less White Frost, No Mold
Crystalline white frost is related to storage time and temperature changes, and can't always be avoided, nor does it need to be. What you really need to keep out is mold:
- Keep the relative humidity of your storage below 70% as far as possible. See Temperature and Humidity for Storing Pu'er.
- For tea with citrus peel like xiao qing gan, keep the original packaging sealed, store it somewhere dry, and finish it soon after opening. See Storing Dragon Pearls, Loose Tea, Tea Bags, and Xiao Qing Gan.
- Don't put it in the fridge and then move it in and out repeatedly; the temperature difference makes moisture condense on the surface. See Can Pu'er Go in the Fridge?
FAQ
What is the white frost on aged tea cakes? One explanation is tiny crystals formed when caffeine and other constituents move to the surface; some white substances are actually dried traces of microbes. Its composition hasn't been systematically studied, so you need to tell by appearance and smell.
What is the white frost on xiao qing gan? It's generally inferred to be crystals of hesperidin and other flavonoids from the peel that form during drying; research on a similar citrus, Huazhou pomelo peel, found its white frost was about 80% crystallized naringin.
How do you tell white frost from mold? White frost is fine powder or crystals, even, doesn't spread, and has no musty smell; mold is fuzzy or cottony, spreads, may be colored, and smells musty.
Can white frost be wiped off? If you're sure it's crystalline white frost, brush it off gently with a dry soft brush; if it's mold, don't drink the tea.