Is Watercore of Apples Really Sweet?

🍎Is “honey” apple really sweet? What makes watercored apple tasty?

🍎Watercore in apples is a phenomenon in which the center of the flesh turns translucent yellow. 🇯🇵In Japan, watercore is called “Mitsu (honey)” as watercored apples exhibit enhanced sweet and honey rich flavor. It has gained wide popularity with Japanese consumers. However, watercore is regarded as a physiological disorder in apples. This is because it is a sign of ripeness meaning that it cannot be stored for a long time.

Sorbitol is well known product of photosynthesis as the primary transport carbohydrate in apples. There is a close relationship between sorbitol accumulation in watercore formation. Water core was thought to be an accumulation of sorbitol and water taken into the cells and stored in the intercellular spaces. Several research groups have examined the sugar composition of apples with and without watercore. When the fruit is fully ripe, the conversion to sugar stops. Although factors such as a decrease in enzyme activity associated with maturation have been proposed as contributing factors to this phenomenon, there are still many details of the mechanism that remain to be elucidated.

🐝Though watercore is called “honey” it is actually not sweet. The amount of sorbitol, a less sweet sugar, is increased in watercored apples, but the concentration of other sugars is decreased. Thus, this “honey” is not as sweet as we expect. 🍏However, watercored apples are still delicious and I like to eat them. So, what is the secret behind its deliciousness?

🍏Several research groups detected that many volatile compounds including ethanol, ethyl esters, and aldehydes, are accumulated in the watercore. For example, methyl 2-methylbutyrate and ethyl 2-methylbutyrate are main components of pineapple aroma🍍🍍🍍. Ethyl hexanoate is known as a main component of the aroma of Japanese sake🍶🍶🍶.  These sweet and floral aroma would contribute to the high preference for watercored apples among Japanese consumers.

🍎In 2021, the research groups in Ehime Univ., Japan🇯🇵 reported further study about physical aspects of the watercore. They revealed the presence of spatial differences in turgor pressure and the water potential gradient between the watercore region and normal region. In relation to the reduction in turgor, fermentation and metabolism are markedly enhanced in the cells to afford these alcohols and esters. But it is still unclear whether turgor loss precedes metabolic changes in relation to watercore formation. 

🟡These compounds also physically contribute to the appearance with high tissue transparency. In the normal apple flesh, light reflection is randomly caused by the air layer remaining in the cell wall space. In the watercore, volatiles eliminate the air layer, resulting in a less diffuse reflection of light and a more transparent surface appearance.

📖Reference

1. “Profiles and Physiological Mechanisms of Sensory Attributes and Flavor Components in Watercored Apple” Nippon Shokuhin Kagaku Kogaku Kaishi, 2016, 63, 101.

2. “Direct evidence for dynamics of cell heterogeneity in watercored apples: turgor-associated metabolic modifications and within-fruit water potential gradient unveiled by single-cell analyses” Horticulture Research, 2021, 8, Article number: 187.

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