I am totally convinced it is down to LME. Especially if it is old or poor quality. Last kit I made was a Sundew which was nearing the expiry date and it had a distinct twang. I think it is more noticeable in light flavoured kits like the Sundew kits. There are other off-flavours which may be mistaken for twang such as chlorophenol tastes from chlorinated water, but I would say the classic homebrew twang is LME related. Ensure the kit is as fresh as possible, use a campden tab in the water you make it with and use quality yeast to ferment. If you have temperature control for the ferment then that is ideal. In my experience two can kits are generally better than one cans, but they have to be fresh. The Festival Golden Stag is definitely one of the better kits available. I did one a few years ago and it was one of the best kits I did.
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