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Newbie here.

We are a kombucha brand specialised in fermenting high end pure teas.
We have recently been developing a new product using an organic second flush black Darjeeling tea. An exquisite base tea and equally wonderful as a kombucha. This is the second season we have used this tea and we had minor issues with 2023 harvest and much more pronounced with recent June 2024 harvest.

The issue is with a high degree of mould spores present in the tea, which is evident within the first 48 hours of fermentation.
There is no visible mould in the tea itself or in the flavour when consumed as a straight tea.

We are 100 % confident the issue lies with the tea itself as we have zero issue utilising the exact same parameters of fermentation with 10 other pure teas.

We are based in India where the tea is grown and source directly from the tea estate.
We are currently in the process of microbiologically analysing the leaf to assess the presence of mould, which will likely further clarify our assessment.

In the meantime we need to try to address the issue by further heat treating the leaf to eliminate the mould spores. (We have 20kg and it is both a big loss financially as well as an issue for customers who love this particular kombucha).

Advice from a friendly tea grower/manufacturer here in India is that either the tea was processed with too much moisture +4.5% and/or that due to the hygroscopic nature of the leaf that it can absorb excess moisture in processing. His advice is to take the leaf and heat it at 100c for 5-7 minutes in a pan or the oven. My concern is a loss of flavour and aroma and he has informed that this will not be the case, however having never done this before I wanted to reach in to this network of fellow tea enthusiasts and garner some valuable feedback and insights from the community.

If anyone has any insight or recommendation we would be very happy to hear from you and if this isn't the right place for such technical inputs perhaps you can direct me elsewhere.

Cheers and happy brewing folks!

Sounds to me your friends assessment is right and that the tea has not been processed properly, some humidity got locked in. Further roasting can maybe fix that, but usually badly processed teas also have inferior taste – how do they taste when prepared as is?

Usually gentle roasting will enhance some flavour/change the profile, but if the base material has problems it might not be enough. You can try to roast, but it is a skill by itself, easy to overdo it. Also important to note that a roast usually requires a resting time afterwards, from a few months+ depending on roast level and tea kind. Not so easy.

All tea will contain yeast, mold spores, and bacteria on the surface - most of these are harmless and have little to no effect on the flavor while the moisture content is below ~6%; when the moisture content is higher, the mold spores can become active or the bacteria can begin digesting the leaf content (rotting) which is generally not what you want (outside of pu'er and other post fermentation teas).

It is possible that heat will kill any yeast or bacteria, but mold can form heat resistant spores that can survive long-periods of intense heat - far more than your tea will survive,
so this is not a certain fix, depending on what the actual contaminant is.

Two important questions for triaging this:

1) what does the infection look like and does it taste bad? - as a general rule, if a fermented product smells or tastes bad DO NOT CONSUME IT - humans are well evolved to reject harmful mis-fermentation; this isn't really a "preference" question like a strong smelling cheese - this will be very obvious "do not put this in your mouth" smell.

2) If it does not taste bad and is just unsightly - you can do what many other fermented products do and just scupper it from the top... certain beers (lambics), jams, wines (vin juan), etc all have a "toss the gross looking yeast cap" on top. There's nothing wrong with doing this for kombucha if it tastes good.

I used to make kombucha for the Penn State University Tea House (part of the former Tea Institute at Penn State); when we made kombucha out of Pu'er tea it would always develop a cap of kham yeast on top of the scoby. Kham yeast is usually white or grey, but because of the pu'er tea it would look disgustingly brown, almost black... (don't consume black mold is a good general rule). We had it tested with the food science department and it was fine - it even tasted great once the cap was removed.

If that tea is actively rotting - don't use it, just accept the loss. If the tea tastes fine brewed hot and the Kombucha smells fine, my guess is that you have a yeast problem, not a mold problem.

Source: me. I develop food products. This is not HACCP advice, but what I as a professional would do.