问题描述：
[多选]
Maintaining reference standards is sometimes challenging 维持参考标准有时会有困难
A．difficult to prevent some drift in the product over time 难以防止产品随时间变质
B．short shelf life products need regular replacement 保质期短的产品需要定期更换
C．multiple references including both optimally stored (e.g. cold temperature) and fresh products may be needed 可能需要多个参考，包括最佳存储（如在低温下）和刚制产品
D．some products have desirable changes with age (e.g. cheese ripening) 有些产品会随着时间而产生变化（例如乳酪成熟）
E．frame of reference of the panel can drift or change seasonally 评估小组的记忆参考可随时变化
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