ScrollViewDelegate.m

- (void)scrollViewDidZoom:(UIScrollView *)scrollView
{
    [self updateZoomableViewCenter:scrollView];
}

- (void)updateZoomableViewCenter:(UIScrollView *)scrollView
{
    CGSize scrollViewSize = scrollView.bounds.size;
    CGSize contentSize = scrollView.contentSize;

    CGFloat offsetX = (scrollViewSize.width > contentSize.width) ? (scrollViewSize.width - contentSize.width) / 2.f : 0.f;
    CGFloat offsetY = (scrollViewSize.height > contentSize.height) ? (scrollViewSize.height - contentSize.height) / 2.f : 0.f;

    UIView *zoomableView = [scrollView.delegate viewForZoomingInScrollView:scrollView];
    zoomableView.center = CGPointMake(contentSize.width / 2.f + offsetX, contentSize.height / 2.f + offsetY);
}