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Strawberry Sweetness & Walnut Antioxidants

strawberry

strawberry

Good news for strawberry & walnut lovers! Researchers at UC Davis and the University of Florida are studying the strawberry genome in order to breed a strawberry that has both shelf life and that fresh-off-the-vine sweetness. At the University of Scranton, Joe Vinson, Ph.D., analyzed nine different types of nuts for antioxidant levels, and walnuts came out as the winners.
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Fruit Salad

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.

Photo Credits (flickr/jcoterhals)

Photo Credit: Jo Christian Oterhals (Flickr/jcoterhals)

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 [1]. 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. [2]

The "seeds" are actually achenes, and they are the true fruit of the strawberry plant (photo credits: flickr/MoHotta18)

The “seeds” are actually achenes, and they are the true fruit of the strawberry plant. Photo Credit: Dome Poon (Flickr/MoHotta18)

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 [3].

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 [3].

Both the endosperm and the endocarp visible here. Photo credits: (flickr/su-lin)

Both the endosperm and the endocarp visible here. Photo credit: Su-Lin (Flickr/su-lin)

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 [4]. 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 middle of these apple cores have begun to brown. Photo credits: (flickr/Stacy Spensly)

The middle of these apple cores have begun to brown. Photo credit: Stacy Spensly (Flickr/notahipster)

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 [4], 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.

References cited

  1. Lloyd, Robin. “Surprising Truths About Fruits and Vegetables.” Live Science. N.p., 22 July 2008. Web.
  2. “Aggregate Fruits.” Fruits Info. N.p., 2004. Web.
  3. “Is a Coconut a Fruit, Nut or Seed?” Library of Congress, 23 Aug. 2010. Web. 10 Aug. 2014.
  4. Helmenstine, Anne Marie. “Why Cut Apples Pears Bananas and Potatoes Turn Brown.” Chemistry.about.com. N.p., n.d. Web. 11 Aug. 2014.

Elsbeth SitesAbout the author: Elsbeth Sites is pursuing her B.S. in Biology at UCLA. Her addiction to the Food Network has developed into a love of learning about the science behind food.

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Fancy Chocolate Treats

Photo credit: Jesús Rodriguez (hezoos/Flickr)

Photo credit: Jesús Rodriguez (hezoos/Flickr)

Chocolate-covered strawberries have an innate beauty in their simplicity, making this snack both sweet and decadent. But this gourmet treat does not have to be expensive nor only savored at special events. Although it’s not quite as simple as dipping strawberries into soupy chocolate sauce, you can easily make chocolate-covered strawberries in your very own kitchen with a basket of strawberries, a bag of chocolate, and a little patience.

To perfect the crafting of chocolate-covered strawberries, it helps to first consider the composition of chocolate. Chocolate contains only a few ingredients: fat, sugars, proteins, and soy lecithin as emulsifier that holds everything together [1,2]. Cocoa butter, a fat that is derived from cocoa beans, makes up the majority of chocolate. Like many vegetable fats, cocoa butter is a mixture of fatty molecules called triacylglycerols. Different types of triacylglycerols—saturated, monounsaturated, polyunsaturated—have their own thermal and structural properties. Roughly 80% of cocoa butter are monounsaturated triacylglycerols [3]. The secret to chocolate perfection lies in the microscopic arrangement of these molecules. The texture (smooth vs. lumpy), appearance (glossy vs. dull), and melting temperature of chocolate (in your mouth at 98°F vs. in your hand at 82°F) all depend on how triacylglycerols pack together in the finished chocolate product.

Triacylglycerols are elongated, spindly molecules that can be packed together in different ways, sort of like long, skinny Legos. The three main ways that triacylglycerols can pack together are named α, β’, and β [3]. A pure mixture of triacylglycerols will form the most stable structure, β [4], and quality chocolate that is hard, smooth, and shiny will predominantly contain this β structure. Unfortunately, cocoa butter isn’t purely one type of triacylglycerol: while the 80% monounsaturated triacylglycerols will tend to pack together nicely into perfect β structures,  the other 20% of cocoa butter fat molecules can interfere and lead to less stable α or β′ structures. As shown in Table 1, chocolate can take on different combinations of α, β′, and β structures, categorized in order of increasing stability as crystals I-VI [2,3]. Crystal V possesses only the β structure, and so it boasts the most desirable chocolate characteristics, such as good sheen, satisfying snap, and melt-in-your-mouth smoothness.

Table 1. Properties of chocolate crystals (adapted from [2]).

Crystal Structure Melting Temp (°F) Chocolate Characteristics
I β′sub(α) 63 Dull, soft, crumbly, melts too easily
II α 70 Dull, soft, crumbly, melts too easily
III β′2 79 Dull, firm, poor snap, melts too easily
IV β′1 82 Dull, firm, poor snap, melts too easily
V β2 93 Glossy, firm, best snap, melts near body temp
VI β1 97 Hard, takes weeks to form

Unfortunately, getting chocolate to form the desired crystal type is easier said than done. When chocolate is melted and then left alone to re-harden on its own terms, uncontrolled crystallization occurs: any and all of the six crystal types will form at random. Chocolate that has been allowed to set this way ends up clumpy and chalky. To control crystallization and select for crystal V, the chocolate must be tempered. Through the tempering process, chocolate is first heated to 110-130°F to melt all the different crystal types. Most importantly, the temperature has to be higher than 82°F to melt the inferior crystals I-IV. Melted chocolate is then cooled down by adding “seeds” of chocolate that already contain only crystal V. These seeds are usually just pieces of chocolate that has already been tempered. Any piece of chocolate—chips, buttons, or chopped— can be used, as the majority of chocolate on the market has already been tempered. These seeds slowly cool the melted chocolate and act as a molecular template from which additional crystal V structures can grow [3]. As the chocolate cools, the stable crystal V will come together into a dense, even network, creating that lustrous, firm chocolate coating.

But beware: a drop of water can ruin all that hard work and perfectly tempered chocolate by causing it to seize. During the manufacturing process, water is removed from the chocolate, leaving behind a blend of fats and sugars. Introducing water to melted chocolate causes the sugar molecules to clump together in a process known as seizing [1]. These wet, sticky sugar clusters result in a grainy, thick batch of chocolate.

Seizing can happen when chocolate is melted in a double boiler, as water from the steam can get into the chocolate. It can also happen when pockets of chocolate are accidentally burnt. Burning is a chemical reaction that oxidizes the fats and sugars to produce carbon dioxide and water. Water that forms in the burnt pockets of chocolate will cause the rest of the batch to seize. But have no fear! Seized chocolate is not completely ruined: it can be saved by adding even more water or other liquids such as cream. Though it may seem counterintuitive, adding more water actually dissolves the sugar clumps, breaking them apart so that the chocolate can become smooth and creamy again [1]. Unfortunately, because there is now moisture in the chocolate, it will not dry and harden into a chocolate shell anymore. Chocolate rescued in this way can be used for hot chocolate, icings, fillings, or ganaches, which means you can still make an impressive chocolate treat even if the chocolate-covered strawberries don’t work out.


Chocolate-Covered Strawberries

1 lb. strawberries
16oz milk chocolate chips
Thermometer (optional, but would be helpful)

1. Melt half to two-thirds of the chocolate chips…

…In a double boiler: Stir constantly. Be sure steam doesn’t escape and sink into the chocolate. Do not cover.

…In the microwave: Heat on high 1 minute. Do not cover. Remove from the microwave and stir. If all the chocolate has not melted, heat again for 5-10 seconds. Repeat until completely melted
Note: If possible, avoid using a heat-retaining container like glass, which may burn the chocolate. Plastic is preferred.

2. Once completely melted, carefully continue heating until the temperature is 90-95°F.

3. Remove from heat, then add chocolate chips. Stir until the chips have melted and the chocolate is 82-88°F.

4. To test if the chocolate is ready, spread a thin layer on the back of a spoon or a piece of paper. It should harden in less than 3 minutes. If it doesn’t, stir in more chocolate chips.

5. When the chocolate is ready, carefully dip in strawberries. Make sure the strawberries are dry, before dipping. Allow dipped strawberries to dry on a sheet of parchment paper.


References Cited

  1. Corriher, S. Chocolate, Chocolate, Chocolate. American Chemical Society: The Elements of Chocolate. October 2007; <http://acselementsofchocolate.typepad.com/elements_of_chocolate/Chocolate.html>
  2. Loisel C, Keller G, Lecq G, Bourgaux C, Ollivon M. Phase Transitions and Polymorphism of Cocoa Butter. Journal of the American Oil Chemists’ Society. 1998;  75(4): 425-439.
  3. Rowat A, Hollar K, Stone H, Rosenberg D. The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation.  Journal of Chemical Education. January 2011; 88(1): 29-33.
  4. Weiss J, Decker E, McClements J, Kristbergsson K, Helgason T, Awad T. Solid Lipid Nanoparticles as Delivery Systems for Bioactive Food Components. Food Biophysics. June 2008; 3(2): 146-154

Alice PhungAbout the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.