foreach ($item->getCustomAttributes() as $attribute)

/**
 * @param SearchResultInterface $searchResult
 * @return array
 */
protected function searchResultToOutput(SearchResultInterface $searchResult)
{
    $arrItems = [];

    $arrItems['items'] = [];
    foreach ($searchResult->getItems() as $item) {
        $itemData = [];
        /* THROWS ERROR foreach ($item->getCustomAttributes() as $attribute) {
            $itemData[$attribute->getAttributeCode()] = $attribute->getValue();
        } */
        $arrItems['items'][] = $itemData;
    }

    $arrItems['totalRecords'] = $searchResult->getTotalCount();
    return $arrItems;

}