「ダブルタップでUIImageViewをズーム」ができません

ダブルタップでタップした箇所をズームする機能を実装したいのですが、scrollViewに乗せたUIImageViewをズームできません。
(参考サイトはこちらです。)
http://cocoadays.blogspot.jp/2010/09/3.html

imageViewをズームさせるにはどのようにすれば良いでしょうか？

- (void)viewDidLoad {
    scrollView = [[UIScrollView alloc] initWithFrame:self.view.bounds];
    scrollView.frame = self.view.bounds;
    scrollView.backgroundColor = [UIColor blueColor];
    scrollView.conteimgViewntSize = CGSizeMake(imgView.bounds.size.width+100, imgView.bounds.size.height);
    scrollView.frame = CGRectMake(0,50,320,500);
    scrollView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
    scrollView.pagingEnabled = YES;
    scrollView.bouncesZoom = YES;
    scrollView.minimumZoomScale = 1.0;
    scrollView.maximumZoomScale = 2.0;
    scrollView.showsHorizontalScrollIndicator = NO;
    scrollView.showsVerticalScrollIndicator = NO;
    [self.view addSubview:scrollView];

    img =[UIImage imageNamed:[NSString stringWithFormat:@"a.jpg"]];
    imgView = [[UIImageView alloc]initWithImage:img];
    imgView.frame = CGRectMake(0, 0, self.view.frame.size.width, 448);
    imgView.userInteractionEnabled = YES;
    [scrollView addSubview:imgView];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch* touch = [touches anyObject];
    if ([touch tapCount] == 2) {
        scrollView = (UIScrollView*)self.view;

        CGRect zoomRect;
        if (scrollView.zoomScale > 1.0) {
            zoomRect = scrollView.bounds;
        } else {
            zoomRect = [self zoomRectForScrollView:scrollView
                                             withScale:2.0
                                            withCenter:[touch locationInView:nil]];
        }
        [scrollView zoomToRect:zoomRect animated:YES];
    }        
}