Filter to category pre publishing

Hi all.
Trying to auto filter posts to categories based on content ie: if the content contains the word “apple” categorise as “fruit.
So far I have:

add_action( 'save_post', 'auto_catg', 10, 2 );

function auto_catg($post){
					$data = $post['post_content'];
					$data = strtolower($data);
								//Condition for FRUIT
		 if (
					strpos($data, 'apple')!==false
					or
					strpos($data, 'pear')!==false
					or
					strpos($data, 'grape')!==false
				) {
                       $post[post_category] = wp_set_object_terms( $post['ID'], 3, 'category', true );
					   $scam_cat = $post[post_category];						  }	

		  					//Condition for HERBS
		  elseif (
		  			strpos($data, 'mint')!==false 

				) {
         			   $post[post_category] = wp_set_object_terms( $post['ID'], 4, 'category', true );
				  }

		  		   return ($post);

}

Its very late so that may be a factor but I cannot see why this is not working?
Any help appreciated.