The air temperature influences the tanning process

Why do apple wedges turn brown?

14.07.2016

Know-it-all

Many people know that: For a healthy snack, an apple is cut into small pieces for the loved one. But before the fruit is packed into the snack box with other good things, the pieces of fruit are brown and unsightly. But why is that and what happens with it?

When the apple is cut open, the cell structures (cell walls, cytoplasm and organelles such as vacuoles) are broken up and air enters the injured cells. Vacuoles are cavities in the cell that are enclosed by a membrane and contain mainly water and phytochemicals. One of these substances is polyphenol, which is the starting point for the brown color. By destroying the vacuole, polyphenol comes into contact with atmospheric oxygen on the one hand and with the enzyme polyphenol oxidase (PPO) from the cell on the other. With the help of oxygen, the PPO can convert (oxidize) polyphenols. This creates quinone - a plant's natural defense against microorganisms. In contrast to the colorless polyphenol, quinone is more yellow in color. However, it can still be converted further, so that brown-colored melanin is produced. This is the substance that is also known from the brownish coloration of skin and hair.

Can the tanning process be slowed down or even stopped? Home remedies, on the other hand, are plentiful - advice includes dipping the apple pieces in water, refrigerating them, drizzling them with lemon, and mixing them with yogurt.

Freshly cut apple slices, image: Open Science - Life sciences in dialogue, CC BY-NC-SA 4.0

The know-it-alls take on the most common trick in this article: the use of lemon juice. To check the effect, we started a small kitchen experiment. We also wanted to test whether acid can generally prevent the tanning process or whether only humidification itself (by delaying contact with the air) helps. We also compared the effects of lemon juice with those of vinegar and water.

We used three Golden Delicous apples, cut out four wedges and placed them one at a time.

Three wedges, one from each apple, were sprinkled with the juice of freshly squeezed lemons. Three more columns were sprinkled with pure apple cider vinegar (5% acid) and again three more with tap water. Three columns were left untreated as controls.

The crevices were left in a shaded place at room temperature for two hours.

Results

After the test period, the cracks drizzled with lemon juice showed hardly any brown discoloration; with all other methods a darkening could be seen. The vinegar did not prevent the brown coloration either. The tan was even a little stronger than in the control group. The acidic environment does not help to slow down the tanning process. Neither did moistening with water produce the desired result.

Apple slices after two hours of testing, image: Open Science - Life sciences in dialogue, CC BY-NC-SA 4.0

The vitamin C (ascorbic acid) found in lemons is primarily responsible for the slower browning of the apple wedges sprinkled with lemon juice. Ascorbic acid prevents the oxidation reaction described by oxidizing itself and thus inhibiting the oxidation of the phenol. In addition, it helps to convert the yellowish quinone to colorless phenol - i.e. it reverses the oxidation reaction.

Due to its general antioxidant effect, ascorbic acid is known as an antioxidant and as such is added in industry as an additive E 300 to foods in order to increase their shelf life.

However, the effect of vitamin C is limited in time. It only lasts until the ascorbic acid present has been completely oxidized. If these reactions take place in a closed vessel (as in the case of airtight food), the oxygen can be used up beforehand, so that the oxidation is stopped.

Another note for everyone who wants to cook this experiment: Antioxidants are added to some vinegar, which also inhibits browning. This is not due to the acid, but to the additive.

Conclusion: If you want to prevent your freshly cut apple wedges from browning, you are well advised to use lemon juice because of the ascorbic acid it contains. And to be honest: it certainly tastes better than vinegar apples.

 

credentials

Dannenberg F .: Influence of different manufacturing processes on the color of cold-grated applesauce. Master's thesis at the University of Neubrandenburg. 2013

Long ago described by: Boswell J. G, Whiting G. C: Oxidase systems in the tissues of the higher plants. New Phytologist (1940) TOC VOL. 39, no. 3, 241-265

 

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8 thoughts on "why do apple wedges turn brown?"
  1. Martina Frank

    Hello - I just found your exciting blog online.

    Is he still active? Can I register anywhere online so I don't miss the new posts? Thank you very much!

    Greetings from Lake Zurich,

    Martina

    reply
    1. Know-it-all

      Dear Martina,

      please excuse the late reply. We are very pleased that you are interested in our blog.
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  2. Srila Sivathas

    The tanning reaction is a redox reaction. What does the oxidation, reduction and redox reaction look like?

    With kind regards

    reply
    1. The know-it-all

      Dear Srila Sivathas! The browning of cut apples is based on the following reaction: Under the influence of oxygen, polyphenol oxidases oxidize the polyphenol contained in apples to colored compounds, such as quinones. Their polymerisation creates even darker compounds. We hope this answers your question! Greetings, those who know better

       
  3. Wilke family

    We notice that old apple varieties from our tax orchards turn brown faster and more strongly than modern varieties from the supermarkets. Have these tanning ingredients been bred out or do they contain much more vitamin C, which prevents tanning?

    reply
    1. The know-it-all

      Dear Wilke family! You have observed that correctly, old apple varieties usually turn brown faster when sliced ‚Äč‚Äčthan the common varieties from the supermarket. This is because newly bred apple varieties often specifically contain fewer polyphenols, which are responsible for the tan creation. This is a shame because old apple varieties are often better tolerated by allergy sufferers than new varieties. Continue to enjoy the apples from your orchards! Greetings, those who know better

       
  4. Pingback: Bircher Muesli: The original Swiss recipe with all the tricks

    1. The know-it-all

      Dear Schlaraffenwelt team! Thank you for referring to our blog in your article on Bircher Muesli - which is great, by the way! Greetings, those who know better