Raisins

1. Sample description

Image 1: Dried Grape Skin
The outermost layer of the raisin, with the red pigment lost during the drying process, giving way to the predominant brown pigment. Some cells can still be observed.

Image 2: Fresh Grape Skin
The outermost layer of the grape skin, with many small cells and red pigments (anthocyanins). There are no stomata as in some other fruit skins.

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Image 3: Dried Grape Flesh
No cells can be observed, and there are fewer brown pigments.

Image 4: Fresh Grape Flesh
The flesh cells are large and round, and they do not contain pigments. Contains a lot of juice.
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Sample names: Dried grape skin, fresh grape skin, dried grape flesh, fresh grape flesh.

The samples were obtained from purple raisins and red grapes by using a knife to peel away the skin and thinly slice the flesh. The red colour on the fresh grape skin results from the presence of anthocyanin pigments. During the drying process (either through drying or sun exposure), anthocyanins degrade and the red pigment disappears, giving way to the dominant brown pigments in raisins. This brown colour is generated by caffeic acid, a type of acid produced after the hydrolysis of caftaric acid, which is naturally present in grapes, or generated through Maillard reactions as amino acids and sugars in the grapes are exposed to sunlight. Upon being dried, a significant amount of water is lost from the grapes, and with additional processing steps, the cells within the dried grapes are disrupted and no longer intact.

1. Background information

Classification:
Grapes have a botanical name as Vitis Vinifera, belonging to the genus Vitis, family Vitaceae, order Vitales, class Magnoliopsida, division Tracheophyta, and kingdom Plantae.

Grapes were first discovered in the Near East, with archaeological evidence showing that they were domesticated 6,000 - 8,000 years ago. Subsequently, grapes were quickly cultivated and traded throughout the Mediterranean region and became a popular food.


Biological characteristics:

Grapes are vines with long tendrils that allow them to climb horizontally and extend to 10 - 15 metres. The stems are small, greyish-brown with many cracks running along. The leaves are symmetrical, heart-shaped, and have long petioles. The leaf blades are deeply lobed with serrated edges. The flowers of the grapevine grow in clusters of white or pale yellow colour. Common pollination agents are insects, wind, and humans.

Grapes thrive in bright sunlight, temperate and sub-temperate climates with soil that are fertile, rich in minerals, and well-drained. The growth cycle of the grapevine is 12 - 18 months, with flowering occurring in spring and fruit ripening in summer and autumn.

Grapes are berries that grow in clusters of 15 - 300 fruits. The shape of the fruit can be elliptical or round, with a diverse range of colours, from light green to deep red, depending on the concentration of anthocyanins and flavonoids. They are two types of pigments found in the grape skin, and their concentration depends on the pH and temperature of the environment. The predominant sugar in grape juice is glucose, therefore, this simple sugar is also called "grape sugar".


From grapes to raisins:
To process fresh grapes into raisins, first, the grapes must be harvested at the ideal sweetness, typically from late August to the end of September. After undergoing processing and cleaning steps, the fresh grapes are dried, reducing the water content from 75% to 15% (water activity decreases from 0.9 to 0.5). The most common drying method is sun drying, followed by drying using ovens or shade drying. However, regardless of the methods used, the final raisins inevitably undergo colour changes. After reaching the appropriate level of dryness, the raisins go through several steps: shaking to remove impurities (stems, dirt, etc.), machine sorting to remove unripe fruits, treatment with specialised machines to remove seeds, and finally, coating with oil to prevent the raisins from sticking together.


Health benefits:

Both grapes and raisins contain various beneficial compounds such as fibre, potassium, iron, calcium, boron, antioxidants like polyphenols, antibacterial compounds like oleanolic acid, linoleic acid, linolenic acid, and tartaric acid, which aid in balancing gut bacteria.

However, it is advisable to consume raisins in moderation or as recommended by the manufacturers, as they are high in sugar and calorie content.

Other facts:

1. The information about the exact time and location where raisins’ discovery remains unknown. However, many archaeological studies and evidence indicate that they were first discovered in dried form on grapevines around the early 2nd millennium BCE in Mediterranean and Mesopotamia.

1. Ancient literature and folklores suggest that ancient people regarded raisins as a symbol of wealth, abundance, and fortune. Therefore, raisins were used as decorations, accessories, medicine, ceremonial offerings, and even as a form of tax payment.