I am looking for a good brand of tahini for hummus. I see a lot of Mid-East tahini in the Sunset. Are they good? (I saw one bad comment on the boards. I'd like to hear about other people's experiences.) Tahini is usually sold in big jars (and pretty expensive), so I don't want to buy a lousy one. Also, do I need to worry about tahini going rancid? How can I slow this process down, and how long does tahini last in the fridge?