Breadcrumbs in product always have category Magento app/code/core/Mage/Page/Block/Html/Breadcrumbs.php

Breadcrumbs.php

    protected function _toHtml() {             

       $cat_id = "";

       if (Mage::registry('current_product')) {
          $product_id = Mage::registry('current_product')->getId();
          $obj = Mage::getModel('catalog/product');
          $_product = $obj->load($product_id); // Enter your Product Id in $product_id

          if ($product_id) {
             $categoryIds = $_product->getCategoryIds();                          
             
             $primera_activa = false;
             foreach ($categoryIds as $key => $_tempid) {
                $_tempcat = Mage::getModel('catalog/category')->load($_tempid);
                
                if ($_tempcat->getIsActive()){
                    
                    //Cojo por defecto la primera categoría que está activa
                    if(!$primera_activa){
                        $cat_id = $_tempid;
                        $primera_activa = true;
                    }

                    if($_tempcat->getPrincipal())
                    {
                        $cat_id = $_tempid;
                        break;
                    }
                }
             }             
          }


          $category = Mage::getModel('catalog/category')->load($cat_id);
          $cat_name = $category->getName();
          $cat_url =  $this->getBaseUrl().$category->getUrlPath();
       }

       if (is_array($this->_crumbs)) {
          reset($this->_crumbs);
          $this->_crumbs[key($this->_crumbs)]['first'] = true;
          end($this->_crumbs);
          $this->_crumbs[key($this->_crumbs)]['last'] = true;
       }

       if($cat_id) {
          $this->_crumbs['category'.$cat_id] = array('label'=>$cat_name, 'title'=>'', 'link'=>$cat_url,'first'=>'','last'=>'','readonly'=>'');
          ksort($this->_crumbs);
          $home = $this->_crumbs['home'];
          unset($this->_crumbs['home']);
          array_unshift($this->_crumbs,$home);
       }

       $this->assign('crumbs', $this->_crumbs);
       return parent::_toHtml();
    }

crea_atributo_principal.php

<?php
error_reporting(E_ALL | E_STRICT);
require_once 'app/Mage.php';
Mage::app(); 

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

// below code will add text attribute
$setup->addAttribute('catalog_category', 'principal', array(
'type' => 'int',
'backend' => '',
'frontend' => '',
'label' => 'Principal',
'input' => 'select',
'class' => '',
'source' => 'eav/entity_attribute_source_boolean',
'global' => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_STORE,
'visible' => true,
'required' => false,
'user_defined' => false,
'default' => '',
'searchable' => false,
'filterable' => false,
'comparable' => false,
'visible_on_front' => false,
'unique' => false,
));