Search Api custom field type
I think you need to follow following approach:

 /**
 * Implements hook_entity_property_info_alter().
 */
function modulename_property_info_alter(&amp;$info) {
  $info['node']['properties']['concat_field'] = array(
    'type' =&gt; 'text',
    'label' =&gt; t('Concatenated form of all the taxonomies that has been added to the node'),
    'sanitized' =&gt; TRUE,
    'getter callback' =&gt; 'modulename_search_api_property_concat_field_callback',
  );
}

/**
 * Getter callback for concatenate terms field.
 */
function modulename_search_api_property_concat_field_callback($item) {
  /** Do all your processing/functionality here. **/
  return &lt;resultant concatenated value&gt;;
}