How can I scrape hidden elements from HTML?

How can I perform the hidden data scraping in HTML. The data is there on the HTML head, but those are hidden meta tags with and the value I want to extract can be found on content property.

  <meta xmlns:og="http://opengraphprotocol.org/schema/" property="og:upc" content="2633" />
  <meta xmlns:og="http://opengraphprotocol.org/schema/" property="product:price:amount" content="419.99" />
  <meta xmlns:og="http://opengraphprotocol.org/schema/" property="product:price:currency" content="USD" />

I am able to get the meta tag with ‘meta’ selector but I am not able to scrape the content value within this tag. For example, if I want to get the UPC, I got an empty text in my scraping result.