Navigation Bar Back Button, text not center alligined

I am customizing the back button in the navigation bar using appearance proxy, also setting image and text attributes. Everything works fine but the text in back button is not center aligned.

Here is my code.

UIBarButtonItem *backButton = [[[UIBarButtonItem alloc] initWithTitle:title style:UIBarButtonItemStyleBordered target:nil action:nil] autorelease];
 UIImage *buttonBack32 = [[UIImage imageNamed:@"NavigationBackButton"] 
                         resizableImageWithCapInsets:UIEdgeInsetsMake(0, 10, 0, 5)];

[[UIBarButtonItem appearance] setBackButtonBackgroundImage:buttonBack32 forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
 [[UIBarButtonItem appearance] setTitleTextAttributes:
 [NSDictionary dictionaryWithObjectsAndKeys:
        [UIColor whiteColor], 
        UITextAttributeTextColor, 
        [UIColor colorWithRed:0.0 green:0.0 blue:0.0 alpha:0.8], 
        UITextAttributeTextShadowColor, 
        [NSValue valueWithUIOffset:UIOffsetMake(0, -1)], 
        UITextAttributeTextShadowOffset,
  [UIFont fontWithName:@"xxx" size:16.0], 
  UITextAttributeFont,
  nil] 
                                            forState:UIControlStateNormal]; 
self.navigationItem.backBarButtonItem = backButton;

If I set the font size "0.0" then text becomes too small in size and still not centered aligned.

Thanks in advance for any help.