They do this because they need to supply large quantities of food to several schools at once. Doing so also enables them to trim costs and manpower. Whether they do so out of concern for speed or convenience, starting too early greatly increases the likelihood that preservatives will be added.
這是因為業者要同時供應多所學校，數量龐大，又為了節省成本精簡人力，為了擔心來不及或求方便，而提早把菜做好，因此添加防腐劑的可能性極高。