Convenience food, by its very nature, is usually junk food.

If it's conveniently sitting on a shelf, chances are it's brimming with preservatives, sodium, and even artificial colors, all to give the appearance and taste of freshness.

There is the rare exception—convenience food that really is fresh. But get ready to pay through the nose for it:  If you've ever plunked down a dollar for a banana at Starbucks, knowing full well they're 70 cents a pound at the grocery store, then you're familiar with the feeling.

Odwalla and Naked Juice: Healthy and delicious, but pricey

Yet in that moment between errands, a dash in for essentials when you're already late, or that 20-minute lunch break, suddenly the lure of convenience food becomes irresistible.  The glow of the Odwalla or Naked juice cold case fills your vision, that fancy juice with real ingredients and not even enough preservatives to be stored at room temperature starts singing its siren song, and soon that five dollar bill is inching its way out of your pocket and into Coke and Pepsi's (you know that's who owns them, right?).

Before you know it, you're happily chugging that delicious green superjuice—twelve ounces of liquid afternoon pick-me-up.

When the moment's over and all that fruit sugar has burned away, I always feel like a sucker as I read over the ingredient list (not to mention my bank statement, littered with juice charges).  You see, the ingredients are so real, I wonder why I just didn't make this at home.

Right.  Because in the mad rush of my weekday morning routine, I have plenty of time to leisurely peel a quarter of a kiwi, a third of a mango, and an eighth of a peach to throw in my juice.  (As if I even have all that on hand.)  Don't you?

Recreating Naked Green Machine at home

Today's recipe is a convenient and cheaper solution.  Gather the exotic fruits only once, do all the prep work once, and enjoy green superjuice for a month.

The secret to convenience here is the frozen supercubes—like ice cubes, but made from pureed fruit and greens.  So all you need to keep stocked are apple juice and bananas; just pop them in the blender with the green supercubes when you want to make the juice.

Sure, 7-11 may start to miss you during the week, but your wallet will thank you.

This is also a nice way to incorporate greens or greens powder into your daily routine.  I used Amazing Grass Green Superfood Energy Powder here, which provides a boost of caffeine with yerba mate and green tea, though I could do without its artificial tasting lemon-lime flavor.  I recommend greens powder that includes lots of nutritious things like wheatgrass and chlorella, but if you don't have it,  you can use a half cup of frozen spinach along with a tablespoon or two of spirulina.

One warning: Just like in the real thing, all these fruits add up to a decent amount of sugar.  If you want to cut down, use unsweetened almond milk or water in place of the apple juice.

Homemade Green Superfood Juice

Super Cube Ingredients:

• 3 kiwi, peeled
• 1 mango, about 1 1/2 cups peeled and chopped
• 1/2 cup pineapple chunks
• 1/2 cup sliced peaches
• 1 teaspoon minced ginger
• 8 scoops of greens powder, or about 1/2 cup

Combine all ingredients in a food processor and pulse to combine.  Divide greens puree into ice cube trays and freeze.  Once frozen, empty cubes into a freezer bag and use within 3 months.

Makes about 2 cups of greens puree, or about 27 ice cubes to use in 9 smoothies.

Smoothie Ingredients:

• 3 green supercubes
• 1 cup apple juice
• 1 ripe banana

In a blender, combine all ingredients and blend until smooth.  Makes a 1 1/2 cups of thick juice.