Submitted to: Journal of Food Composition and Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2012
Publication Date: 8/9/2012
Citation: Lester, G.E., Lewers, K.S., Medina, M.B., Saftner, R.A. 2012. Compariative analysis of strawberry total phenolics via fast blue BB vs. folin-ciocalteu: assay interference by ascorbic acid. Journal of Food Composition and Analysis. 27:102-107.
Interpretive Summary: Strawberries are known as a good source of nutrients, in particular a group of nutrients called phenolics. The current laboratory assay used to measure phenolics provides an indirect measurement of phenolic content because it interacts with vitamin C and sugars which are abundant in strawberry. A new method directly reacts with phenolic groups forming a stable, measurable product. In comparing these two assays, we found that all previous studies of strawberry fruit that used the earlier assay have greatly underestimated phenolic content by more than half. This indirect assay should be replaced in future studies by the new Fast Blue BB assay. Our findings will benefit strawberry production researchers and postharvest physiologist/food scientists world-wide in more accurately quantifying total phenolics in foods.
Technical Abstract: Strawberry (Fragaria ×ananassa Duchesne ex Rozier) fruit were harvested from two field plots of five day-neutral cultivars: Albion, Monterey, Portola, San Andreas and Seacape. Marketable (salable) berries were harvested from each plot on 22 and 25 August, and unblemished fully ripe fruit were selected for further evaluation. All berries from each cultivar had similar average harvest weights (9.2 g/berry), and subjective marketability scores of seven, which was the minimum score for salability. Warmer weather events preceding the 22 August harvest (average 25 °C versus 20 °C preceding the 25 August harvest) significantly impacted ascorbic acid and total phenolic quality attributes but not soluble solids concentration. The warmer, more stressful environment prior to the 22 August harvest was associated with a lower berry free ascorbic acid:dehydroascorbic acid ratio (2.5:1.0 for the 22 August harvest versus 3.8:1.0 for the 25 August harvest), which likely resulted in the 22 August harvest having higher total phenolic concentrations (4.5 versus 2.2 gallic acid equiv. mg/g fresh weight, respectively). Phenolic concentrations were measured by the ‘new’ Fast Blue BB [4-benzoylamino-2,5-diethoxybenzenediazonium chloride hemi(zinc chloride) salt], (Fast Blue BB) a direct phenolic detection assay, which reported an average 2.9 fold greater total phenolic concentration then the traditional Folin-Ciocalteu assay, and the Fast Blue BB assay had no interaction with ascorbic acid or sugars. In contrast, Folin-Ciocalteu an indirect detection assay for total phenolics appeared to under report total phenolic concentrations and showed a positive linear interference response to ascorbic acid. Results from this study indicated that previous studies of strawberry fruit, using the standard indirect Folin-Ciocalteu assay, have greatly underestimated the total phenolics content, and that this assay should be replaced in future studies by the Fast Blue BB assay.