I have a small collection of flavors, mostly TFA. I mix it with 100% VG and 50/50 nicotine (heartland vapes). My final mixes come out to around 80%vg/20%pg. Anyway lately I have noticed that whatever flavors I make, whether single or complex, I get a bad aftertaste. This aftertaste is bitter and sour, reminding me of sour cream or plain unflavored yogurt. Very off-putting. Once again, it happens no matter what the flavor or combo thereof. Anyway, I keep my flavors and VG in a dark cupboard. The oldest are a little over 7 months old. Are they spoiled? Do they need to be refrigerated?