Raisins are dried grapes.  They are produced in many regions of the world.  Raisins may be eaten raw or used in cooking, baking and brewing.  In the United Kingdom, Ireland, Australia, and Canada the word “raisin” is reserved for the dried large dark grape, with “sultana” being a dried large white grape, and “currant” being a dried small Black Corinth grape.

Raisins are sweet due to their high concentration of sugars (about 30 grams of fructose and 28 grams of glucose in 100 grams of raisins).  The sugars can crystallize inside the fruit when stored after a long period, making the dry raisins gritty, but that does not affect their usability.  These sugar grains can be dissolved by blanching the fruit in hot water or other liquids.

Grades of Raisins in the USA

Grade A – The color is good, and the flavor is characteristic of raisins.  These raisins show development characteristics indicative of the fact that they are prepared from well-matured grapes (containing no less than 80% water by weight).  The processed raisins contain less than 19% moisture by weight.

Grade B – The color and flavor is reasonably good.  These raisins show development characteristics that highlight the fact that the raisins were prepared from reasonably well-matured grapes at least 70% water by weight.  These raisins also contain less than 19% moisture, by weight.

Grade C – The color and flavor are fairly good.  These raisins show development characteristics of raisins prepared from fairly well-matured grapes containing at least 55% water by weight.  These raisins also contain less than 19% moisture, by weight.

Substandard – Raisins that fail to meet the standards of Grade C.

Raisin varieties depend on the type of grape used, and are made in a variety of sizes and colors including green, black, blue, purple, and yellow.  Seedless varieties include the sultana (also known as Thompson Seedless in the USA) and Flame grapes.  Raisins are typically sun-dried, but may also be water-dipped, or dehydrated.  “Golden raisins” are made from sultanas, treated with sulfur dioxide (SO2), and flame-dried to give them their characteristic color.  A particular variety of seedless grape, the Black Corinth, is also sun-dried to produce Zante currants, miniature raisins that are much darker in color and have a tart, tangy flavor.  Several varieties of raisins are produced in Asia and, in the West, are only available at ethnic specialty grocers. Green raisins are produced in Iran.

Raisins range from about 67% to 72% sugars by weight, most of which is fructose and glucose.  They also contain about 3% protein and 3.5% dietary fiber.  Raisins, like prunes and apricots, are also high in certain antioxidants, but have lower vitamin C content than fresh grapes.  Raisins are low in sodium and contain no cholesterol.  New research has shown, despite having a high concentration of sugars, raisins fight bacteria in the mouth that cause cavities and gum disease.

Raisins can cause renal failure in dogs.  The cause of grape and raisin toxicity in dogs is not known.  New data suggest that, among individuals with mild increases in blood pressure, the routine consumption of raisins (three times a day) may significantly lower blood pressure, especially when compared to eating other common snacks, according to research presented today at the American College of Cardiology’s 61st Annual Scientific Session.

Antimicrobial Phytochemcals in Thompson Seedless Raisins (Vitis vinifera L.)

Inhibit Dental Plaque Bacteria
Topic: K12 Antibiotics and Other Secondary Metabolites
C. D. Wu, J. F. Rivero-Cruz, M. Zhu, B. Su, A. D. Kinghorn;
University of Illinois, Chicago, IL.
Presentation Number: O-068
Poster Board Number: 330

Keyword: oral pathogens, antimicrobial phytochemicals, raisins.
Raisins have been documented as a source of phytochemicals including flavonoids, stilbenoids and furan derivatives which may benefit overall human health.  Earlier reports the presence of antimicrobial compounds in the ethanol extract of Thompson seedless raisins (Vitis vinifera) against oral pathogens.

Objective: To further purify and identify the antibacterial components of Vitis vinifera active against oral pathogens.

Methods: Thompson seedless raisins (Vitis vinifera) were extracted by maceration using methanol as a solvent.  The fractionation process was guided by antimicrobial assay.  Active fractions were further purified using chromatographic methods.  The structures of pure active compounds were established by spectroscopic methods.  In an attempt to increase the resultant antimicrobial activity of oleanolic acid, a series of acylation and etherification reactions were performed on oleanolic acid.  The antimicrobial activity was evaluated against Streptococcus mutans and Porphyromonas gingivalis.

Results: Five known compounds were isolated from the hexane-soluble extract: oleanolic acid, oleanolic aldehyde, betulin, betulinic acid, and 5-(hydroxymethyl)-2-furfural.  Oleanolic acid and its derivatives inhibited growth of the cariogenic S. mutans and the periodontal pathogen P. gingivalis with minimum inhibitory concentration (MIC) values ranging from 7.8 to 1000 μg/ml and 3.9 to 1000 μg/ml, respectively. Oleanolic acid identified in the non-polar fraction exerted preferential growth inhibitory activity against P. gingivalis at 0.062 mg/ml, while at 0.031 mg/ml inhibited in vitro adherence of S. mutans biofilm.  The oleanolic acid sodium salt was the most active derivative synthesized. Raisin fractions did not suppress acid formation by S. mutans.

Conclusion: Raisins contain phytochemicals that may contribute to oral health by suppressing growth of selected oral pathogens or interfering with adherence of cariogenic bacteria.  Research supported by California Raisin Marketing Board, Fresno, CA.