I managed to get the properties by making a reference to the contentservice. The returned object is of type Content. (Umbraco.Core.Models) Then i can loop through the properties:

    var contentService = Services.ContentService;  
    var contentItem = contentService.GetById(page.Id);

    foreach (var prop in contentItem.Properties)
                {
                   // prop.Alias , prop.Value
                    ...
                }

Strange that by using dynamic representation with UmbracoHelper you cant loop through this properties:

    var umbracoHelper = new Umbraco.Web.UmbracoHelper(UmbracoContext);
    var contentItem = umbracoHelper.Content(page.Id);

    foreach (var prop in contentItem.Properties)
                    {
                       // error

                    }