While traditional oranges are available at your local supermarket all year long, the best time to enjoy the juicy, crimson flesh of blood oranges is during these winter months. So while you venture out for some delicious blood oranges, consider these fascinating tidbits. How do they get their characteristic color? How are they different from everyday oranges?
If pumpkins are on your menu this Thanksgiving, be thankful that hundreds of years of human domestication has turned this once super-bitter squash into a sweet dessert. Furthermore, human intervention may have prevented gourds and squashes from extinction. As for reasons to be thankful for cranberries, scientific research shows that a compound within cranberries could prevent urinary tract infections.
Now that summer is in full swing, what better way is there to use all the berries, apricots, plums, peaches, and nectarines in season than to make jam? Jam has a great texture that makes it the perfect spread for brioche toast and a sweet complement for porridge. The base ingredients of fruit, sugar, pectin, and acid are cooked until the jam reaches a spreadable consistency . Each ingredient plays an important role in the texture of the final product.
Sugar has many roles besides adding sweetness to the jam. When sugar is mixed with mashed fruit, it begins to dissolve and draw water out of the fruits through osmosis . This occurs because fruit has a lower concentration of sugars than the amount of sugar that is typically added. The hydrophilic groups on sugar make it miscible with other polar molecules like water. Sugar also acts as a preservative by forming bonds with water molecules, making fewer water molecules available to support the growth of various microorganisms that might cause spoilage, such as Aspergillus glaucus and Saccharomyces rouxii [2,3].
Pectin is soluble dietary fiber that naturally occurs in certain fruits such as apples, plums, and quinces. When heated and mixed with acid, this carbohydrate creates a thick gel that contributes to the consistency of jam. Acid from citrus increases the hydrogen ion concentration in the solution, which results in the pectin molecules losing some charge. With less electrostatic repulsion, the molecules can now aggregate to form a physical gel at a higher temperature of around 220ºF , resulting in watery fruit liquid dispersing itself within a web of pectin molecules .
Typically, under-ripe fruits have more pectin because fruit enzymes convert it to pectic acid during the ripening process . This means that high pectin content can often be a trade-off for lower flavor, so it is recommended to use two parts of ripe fruit for every part of under-ripe fruit for the best consistency and taste . Alternatively you can buy powdered pectin from the store to use with fruits that naturally have low levels of this carbohydrate, such as apricots, peaches, and raspberries.
Acid not only contributes to the texture of jam, but it also catalyzes the conversion of sucrose (from added sugar) into its constituent fructose and glucose molecules with the help of heat. This process is called sugar inversion, and it is necessary to prevent recrystallization during jam storage . However, this rarely occurs because finishing a jar of jam usually does not take too long. As an added bonus, acid also contributes to the flavor balance of the jam, preventing it from being too sweet.
Different fruits vary in acid and pectin content, so adjustments may be necessary to obtain the right texture and taste. You can also use a Brix test to measure the endogenous levels of sugars and dissolved nutrients. Basically if your jam contains riper and more nutrient dense fruits, the test will give a higher reading. Develop the right recipe, and you will want to eat out of the jam jar with a spoon!
- The Science of Jam and Jelly Making. University of Kentucky.
- Jam Making: Why all the sugar? Iufost.org
- Why does jam go mouldy, even in the fridge? University of Liverpool
- Fishman, M.L. & Jen, J.J. Chemistry and Function of Pectins. June 1986.
- Jam Making 101. Seriouseats.
- Inversion of Sucrose. Colby College.
About the author: Catherine Hu is pursuing her B.S. in Psychobiology at UCLA. When she is not writing about food science, she enjoys exploring the city and can often be found enduring long wait times to try new mouthwatering dishes.
Fruit salad can be made throughout the year, but nothing beats a crisp fruit medley on a hot summer afternoon. There are very few limits on what can be a fruit salad ingredient. If the object in question is fruit, it can go in. Segregating fruit from non-fruit seems simple, but from a botanical point of view, classifying these sweet and juicy plant products gets complicated. But, if armed with knowledge and lemon juice, anyone can achieve this delicious and vibrant potluck offering.
A fruit is the structure of a plant that bears the seeds. A plant’s flower houses the female reproductive parts, namely the ovary, in the flower’s center. When fertilized, parts of the ovary develop into seeds, and the rest becomes the fruit.
Berries or Not?
The average person defines a berry as anything whose name ends in the suffix, –berry. But to a botanist, a berry is a fruit containing multiple seeds in its interior, embedded in the flesh of the ovary. This includes blueberries, tomatoes, eggplants, grapes, bananas, persimmons, and chili peppers . A botanically-correct berry salad could be very savory; perhaps very spicy.
Blackberries, mulberries and raspberries all fall into the category of berry-imposters called aggregate fruits. Each little bump on a raspberry or blackberry is actually an individual fruit, as each is its own separate ovary, formed from one flower.
Botanically speaking, strawberries are actually not berries. Each pock on the fruit’s exterior is called an achene, and each achene is an individual fruit with a corresponding seed in the interior. The thing we call a strawberry is not a berry in the botanical sense, but rather an accessory tissue for an aggregate fruit (the achenes), formed from multiple ovaries of one flower. 
Neither Pine nor Apple, and Not a Nut
A pineapple is considered a multiple fruit. Whereas an aggregate fruit forms from one flower, a multiple fruit is the product of the fused ovaries of a cluster of flowers, thus each pineapple is one large composite fruit. Want to impress guests with a “multiple fruit” fruit salad? Your (somewhat limited) options include breadfruit, osage-orange, fig, and pineapple .
Coconut is Not a Nut
Technically speaking, a coconut is a fibrous one-seeded drupe. A drupe is a fruit with a seed enclosed by a hard stony shell, like a peach or olive. An unprocessed coconut has three layers. The smooth, green, outermost layer is called the exocarp. The next layer is the fibrous husk, or mesocarp, which surrounds the hard woody endocarp, which surrounds the seed. A supermarket coconut usually has been freed of its two outer layers. The part most likely found in a fruit salad are just shavings of the seed’s endosperm. This delicious white lining is meant to nourish the seedling coconut tree as it germinates .
Keeping Fruit Salad Colorful
Apples, pears, and bananas notoriously turn an unattractive brown after dicing. This is because they contain an enzyme called polyphenol oxidase . When the fruit is sliced, the enzyme is free to react with oxygen, as well as iron-containing phenols in the apple cells that had previously been kept separate. The products of these reactions are ugly, brown chemicals.
The key to preventing or slowing any enzymatic reaction is denaturing the enzyme. Heat will do the trick, as will reducing the fruit’s contact with oxygen by putting cut fruit under water or vacuum packing it. The simplest way to avoid browning is to apply lemon juice or another acidic substance on the cut surface. Enzymes can only function within a specific pH range, and acidic lemon juice will reduce the pH on the surface of the fruit. For those who fiercely oppose brown apples, try adding add sulfur dioxide , a chemical that acts as a preservative by binding to reactants in the fruit to interrupt the browning reaction. For anyone truly passionate about keeping their sliced fruits bright, upgrading to a sharper knife can help. A low quality steel knife may be corroded, and can make more iron salts available for the browning reaction.
- Lloyd, Robin. “Surprising Truths About Fruits and Vegetables.” Live Science. N.p., 22 July 2008. Web.
- “Aggregate Fruits.” Fruits Info. N.p., 2004. Web.
- “Is a Coconut a Fruit, Nut or Seed?” Library of Congress, 23 Aug. 2010. Web. 10 Aug. 2014.
- Helmenstine, Anne Marie. “Why Cut Apples Pears Bananas and Potatoes Turn Brown.” Chemistry.about.com. N.p., n.d. Web. 11 Aug. 2014.
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 . In plants, these circadian rhythms help control everything from the time a plant flowers to its ability to adapt to cold weather . Plants can even use their internal clocks to do arithmetic calculations to budget their energy supplies through the night .
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.
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  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.”
- McClung CR (2006) Plant Circadian Rhythms. PLANT CELL ONLINE 18: 792–803. doi:10.1105/tpc.106.040980.
- 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.
- 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.
- Bretzl H (1903) Botanische forschungen des Alexanderzuges. B. G. Teubner.
If you’ve ever wondered what 200 Calories look like on a plate, wiseGEEK has just the photo gallery for you! Meanwhile, scientists create a healthier chocolate by replacing fat with fruit juice. Read more
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.