I heard you do and that you must only offer purified food?
It depends on what you actually mean to do when you say "offer to Kali".
Before performing a formal worship, following the methods laid down in scriptures, accompanied with the use of Mantras, Yantras, Mudras etc., one is definitely required to get initiated into the procedure.
Ramakrishna Paramhamsa was an Upanita (one who is invested with sacred thread) Brahmin priest already when he was offered the role of the main priest of the Dakshineswara Kali Temple. But since it is not at all advisable to embark upon Kali worship without being initiated into it, he took proper initiation from a proper Guru before becoming the chief priest.
This incident is mentioned in "A short life of Sri Ramakrishna Paramahamsa by Swami Tejasananda".
Sri Ramakrishna adapted himself to his new station and forgot everything else. Ramkumar, being troubled by Sri Ramakrishnaís love for solitude and growing indifference to the world, resolved to teach him the elaborate procedure of the worship of Kali, so that, later, he might assume the task quite efficiently. As it is not considered advisable to undertake the worship of Shakti, or Kali, without being properly initiated, Sri Ramakrishna, at the advice of his elder brother, got the necessary initiation from a Brahmin in Calcutta named Kenaram Bhattacharya who was noted for his devotion and experience. It is said that as soon as the sacred Mantra was uttered in his ears, Sri Ramakrishna, overwhelmed with religious fervour, gave a shout and plunged into deep concentration, which greatly astonished the Guru
If the plan is to simply do a devotional worship by offering flowers, food items etc. to an image of the Goddess, then that is permissible to do without initiation. But initiation is always the first step towards spiritual anything. So it is better to have it than not to have it.