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The Benefits of Well-Rested Produce

Cabbage - credit postbear

Beauty rest isn’t just for people—cabbages also benefit from a good night’s sleep. (photobear/Flickr)

In 400 BCE, the Greek admiral Androsthenes wrote* of a tree that

“opens together with the rising sun . . . and closes for the night. And the country-dwellers say that it goes to sleep.”

Over the next 2000 years, researchers discovered that the daily cycles first observed by Androsthenes fall into 24-hour periods similar to our own cycles of waking and sleeping [1]. In plants, these circadian rhythms help control everything from the time a plant flowers to its ability to adapt to cold weather [2]. Plants can even use their internal clocks to do arithmetic calculations to budget their energy supplies through the night [3].

But what happens when part of a plant is harvested for food? In a recent study, researchers at Rice University and UC Davis showed that cabbages can exhibit circadian rhythms as long as a week after harvest.

As with any plant, cabbages experience circadian rhythms while growing out in the field; however, cabbages stuck in the constant dark of a delivery truck or light of a 24-hour grocery store will inevitably lose their sense of time. Like travelers adjusting to a new time zone, cabbages deprived of cyclic light conditions suffer a severe bout of veggie jet lag. And just as travelers overcome jet lag by readjusting their sleep cycles, cabbages can “re-entrain” their circadian rhythms by being exposed to cyclic light conditions. This also works with spinach, zucchini, sweet potato, carrots, and blueberries, suggesting that post-harvest circadian rhythms are a general characteristic of many, if not all, fruits and vegetables.

The ability to re-entrain circadian rhythms in produce presents an intriguing new way to improve the palatability and even nutrition of our fruits and vegetables. In the wild, circadian rhythms can help plants defend themselves against hungry herbivores. The researchers showed that cabbages with re-entrained circadian rhythms use a similar mechanism to avoid becoming an afternoon snack for plant-eating larvae—with less damage from hungry larvae, re-entrained cabbages appear fresher and tastier than cabbages kept under constant light or dark conditions.

Circadian rhythms help protect produce from herbivores. Samples from cabbages kept in (A) cyclic “in phase” light, (B) constant light, or (C) constant dark conditions were fed to larvae. Cabbages kept in constant light or constant dark sustained the most damage.

Cabbages fight off larvae and other pests thanks to molecules called glucosinolates. Any cabbage can produce these molecules, but re-entrained cabbages produce glucosinolates in sync with their circadian rhythms. Because larvae also experience circadian rhythms, re-entrained cabbages get an extra boost of molecular larvae-fighting power just when they need it the most.

While glucosinolates are bad news for larvae, they have valuable anti-cancer properties when consumed by humans. In fact, the very molecules that plants create to defend themselves against their environment are often beneficial for our own health. Future research will show whether such phytonutrients in other types of produce can also be reconditioned to accumulate in predictable 24-hour cycles. Taking advantage of circadian rhythms in fresh produce could then give us more control over the way phytonutrients accumulate over time, helping us maximize the nutritional benefits of our fruits and vegetables. Improving the nutrition of our food could be as simple as giving our produce a good night’s sleep.

 

*The original Greek passage comes from Botanische forschungen des Alexanderzuges [4] with a very special thank you to Tovah Keynton for the English translation. The drawings (also from Botanische) depict the tree leaves transitioning into and then assuming their “sleeping position.”
TamarindTreeRhythms

References Cited

  1. McClung CR (2006) Plant Circadian Rhythms. PLANT CELL ONLINE 18: 792–803. doi:10.1105/tpc.106.040980.
  2. Kinmonth-Schultz HA, Golembeski GS, Imaizumi T (2013) Circadian clock-regulated physiological outputs: Dynamic responses in nature. Semin Cell Dev Biol 24: 407–413. doi:10.1016/j.semcdb.2013.02.006.
  3. Scialdone A, Mugford ST, Feike D, Skeffington A, Borrill P, et al. (2013) Arabidopsis plants perform arithmetic division to prevent starvation at night. eLife 2: e00669–e00669. doi:10.7554/eLife.00669.
  4. Bretzl H (1903) Botanische forschungen des Alexanderzuges. B. G. Teubner.

Liz Roth-JohnsonAbout the author: Liz Roth-Johnson is a Ph.D. candidate in Molecular Biology at UCLA. If she’s not in the lab, you can usually find her experimenting in the kitchen.

Read more by Liz Roth-Johnson


Reinventing the Egg

Even if you’re not watching your cholesterol, there are plenty of reasons to avoid eating eggs. Ethical issues aside, industrial eggs provide only about 20% of the energy it takes to produce them. And while some egg substitutes do exist, they often pale in comparison to the real thing. Josh Tetrick, the CEO of Hampton Creek Foods, thinks we can do better. Read more

How Bubble Wrap Explains Crisp and Mealy Apples

In our unit of pressure, we learned about the difference between a mealy and crisp fruit or vegetable. It turns out that bubble wrap is a good analogy. 

From Smith et al (2003) Postharvest physiology and pathology of vegetables.

We already know that water inside the vacuole of a plant cell and the cell wall work together to keep the cell firm and rigid. When cells are full of water, a force, such as your bite, will puncture the cells and break open individual cells. That is when you experience the release of juices that accompany biting down on a crisp apple. On the other hand, sometimes the network of polysaccharides between cells becomes weak, and when force is applied, the cells separate from each rather than are cleanly punctured through. This is when we encounter the classic mealy apple. With time, the polysaccharide “glue” that binds cells to each other begin to degrade. The cells also begin to lose water. This is why apples that have been stored incorrectly for long periods of time often turn mealy.

Take bubble wrap as an analogy. When the bubbles in bubble wrap are sealed and full of air, it is very easy to get that satisfying “Pop!” when you puncture the bubble. However, when the bubbles are not fully inflated or have already been punctured, then it is much harder to pierce another hole in the same bubble.

Plants under pressure

In our unit on pressure, we used plants as a model system. What makes lettuce crispy? How do you revive wilted lettuce? It’s really all about pressure- turgor pressure, to be exact.

TurgorPressure

We prepared tasting samples of dehydrated grapes (aka raisins) and kale chips to demonstrate the vital role that water and pressure play in plants. Under normal conditions, grapes are juicy and firm, and kale is hardy and stiff. We placed both in a dehydrator, which works as a low-temperature oven (~130 °F/54 °C). Water evaporates, and the cells lose turgor pressure and shrink. The grape becomes soft and mushy on the inside, and the kale, which is normally so tough and sturdy, shatters like a chip.

And for reviving that wilted lettuce? Soak it in cold water, of course.


RECIPES

Dehydrated Grapes

Grapes
Boiling water
Dehydrator

1) Wash grapes well.
2) Bring a pot of water to a boil. Blanch* grapes in boiling water for 30-60 s.
3) Pat grapes dry.
4) Places grapes on dehydrator racks. Turn on dehydrator. If it has a temperature setting, some recipes suggest 140 °F. Our dehydrator has only one temperature setting of ~130 °F, so we just went by touch. A wrinkled grape with a still-moist center takes 3-4 hours.

*Blanching dissolves the waxy cuticle on the surface of grapes. The wax is a natural defense mechanism against water evaporation.

Kale Chips

Kale
Olive oil
Salt
Pepper
Paprika, cumin, other seasonings

1) Rinse and dry kale leaves. Cut lengthwise in half, and again in thirds.
2) Toss kale with olive oil in bowl. Sprinkle with salt, pepper, and other seasons.
3) Arrange leaves in single layer on dehydrator racks. Turn dehydrator on. Let run for ~2 hours.

Gary Menes’ Veggie Platter

This week’s lecturer is Gary Menes. He is the chef at Le Comptoir, a pop-up restaurant at Tiara Café in LA. 

Gary Menes and sous-chef Wesley Avila weighed in on our topic of the week, “Pressure,” with their version of the veggie platter.  There were 20-odd vegetables and fruits present, including pickled onions, the season’s first cherries, pickled orange segments, Okinawan sweet potato, and quickly sautéed fava beans.

Gary packed cherries in a bag and used a cryovac machine to suck out all the air in the bag. The resultant vacuum compresses and bruises the cherries, changing their texture and flavor in the process.

Want to make quick-pickled onion petals? Slice the onion in quarters or halves and peel apart the layers to get petals. Heat up a quick-pickling solution of 3 parts water : 2 parts red wine vinegar : 1 part sugar. Once hot, submerge the petals and let rest for 30 minutes.



“It’s all about sugar” – Barbara Spencer

Barbara Spencer of Windrose Farm in Paso Robles was our lecturer on the topic of phase transitions. 

“Why are carrots harvested after winter particularly sweet?”

Plants use sugar as an internal antifreeze. This is an example of the concept of freezing point depression. When a solution freezes, the molecules into a crystalline structure. However, when impurities are introduced to the solution, they block the molecules from clustering together and freezing. In the case of carrots, sugar is the impurity, and it keeps the liquid inside the plants from freezing at 0 °C. This defense mechanism against frost translates into carrots that taste extra sweet to us. Even on a daily basis, Barbara always picks her carrots and melons before the sun comes up, because sugar levels increase at night. Strawberries ripen at night, too.

Image courtesy of farmstayus.com

Out where the farm is in Paso Robles, the valley makes for a very active microclimate. The difference between daily temperature highs and lows can be as large as 50 °F. Apples, which need a certain number of hours of frost, thrive well there.

Storage is just as important as growth, and Barbara invested in a refrigerated truck for transporting vegetables. Leafy greens always need water, and this is why supermarkets and farmers are always spraying or misting. There is a rule of thumb that every hour lost in not cooling freshly picked vegetables to the proper temperature equals one less day of shelf life.

We were surprised to learn about the storage capabilities of some apple varieties, such that you can keep them for months on end. They’ll be good right after picking, become very gnarly quickly, and then, if you wait long enough, they’ll taste fantastic again. Which varieties do you know are like that?

Jordan Kahn on the Molecules of Food

Jordan Kahn came in as our terrific pinch hitter when René Redzepi fell ill the night he was to fly over from Copenhagen for his lecture with Lars Williams. He was the first lecture of the class and provided an introduction to looking at food as molecules. Jordan is the chef/partner at Red Medicine in LA.

Jordan brought in an incredible bounty of wild plants, herbs, and flowers foraged just the day before in nearby Topanga Canyon.

He explained, and we tasted:

  • wood sorrel
  • chrysanthemums
  • radish pods
  • California bay leaf
  • young fennel
  • mustard
  • and more

Many wild plants have a much more intense and often completely different flavor than do their domestic counterpart. Why is this so? 1) Remember that from the botanical perspective, these ‘smelly’ molecules are defense mechanisms for the plant and help to repel predators. Wild plants face a greater selective pressure and therefore, perhaps evolution has selected for the most potent smelling plants. 2) Perhaps domestic herbs bought at the supermarket experience a dramatic degradation of flavor after harvesting. 3) Terroir affects taste, so that fennel foraged from Topanga Canyon tastes differently than does fennel from San Diego. Jordan gathered everything that he brought within a 30 sq. ft area. Herb farms, on the other hand, may be quite homogeneous in terrain as well as crop.