Searching for Numbers in Smart Search

I want to search blog posts hosted at Hubspot using the smart search. Each blog post has one or more topic IDs such as 4305550016. I created the custom index for it using the following code:

foreach (BlogPost blogPost in blogPosts)
{
    // Gets the text content of the blog post
    string text = blogPost.post_body;

    // Converts the text to lower case
    text = text.ToLowerCSafe();

    string topicIDs = "";
    foreach (object id in blogPost.topic_ids)
    {
        topicIDs += " ," + id;
    }

    text += topicIDs;

    // Removes diacritics
    text = TextHelper.RemoveDiacritics(text);
    System.DateTime publishDate = new DateTime(1970, 1, 1, 0, 0, 0, 0, System.DateTimeKind.Utc);
    publishDate = publishDate.AddMilliseconds(long.Parse(blogPost.publish_date.ToString())).ToLocalTime();
    // Creates a new Lucene.Net search document for the current text file
    SearchDocumentParameters documentParameters = new SearchDocumentParameters()
    {
        Index = srchInfo,
        Type = SearchHelper.CUSTOM_SEARCH_INDEX,
        Id = Guid.NewGuid().ToString(),
        Created = publishDate
    };
    ISearchDocument doc = SearchHelper.CreateDocument(documentParameters);
    // Adds a content field. This field is processed when the search looks for matching results.
    doc.AddGeneralField(SearchFieldsConstants.CONTENT, text, SearchHelper.StoreContentField, true);

    ... 
    iw.AddDocument(doc);
}

The index builds. However, when I search for a topic ID in the search preview on the index maintenance section of Kentico, I don't get any results. I appreciate any suggestions.