Determination of total Phenolic Content of Hazelnut  and Oil Coated Red Pepper: Method Validation
Dilara Bozkurt 1, Baran Onal-Ulusoy 2*, Mehmet Mutlu 1
1 Hacettepe University, Food Engineering Dept., Ankara, TURKEY.
2 Cankiri Karatekin University, Food Engineering Dept., Cankiri, TURKEY.

In the present study, the validation of total phenolic content analysis for dried and oil coated red pepper and raw hazelnut has been investigated. The common property of these two samples is they have high oil content. Red pepper and raw hazelnut have 45 % and 15 % of oil, respectively. For this purpose, the extraction methods given in the literature were compared. The total phenolic content  in red pepper and hazelnut  samples were extracted using 50 % methanol (two times) and then 70 % acetone(two times), and 80 % ethanol (3 times), respectively. Total phenolics were extracted from whole hazelnut, hazelnut defatted by soxhlet extraction and hazelnut defatted by cold hexane extraction. Hazelnut defatted by soxhlet extraction had the lowest total phenolic content among all treatment. Due to insignificant difference between other two treatments, it was decided to use whole hazelnut. 
The content of the extracts were determined according to the Folin-Ciocalteu reagent assay spectrophotometrically by using 7,5 % Na2CO3 and measured at 725 nm . The absorbance of the resulting blue color supernatant was measured using appropriate blanks. Gallic acid was used as the external calibration standard and the results were expressed as mg GAE/g sample. Total phenolic contents in red pepper and hazelnut were found 5,5 mg GAE/g sample (N=7) ve 2,9 mg GAE/g sample (N=6), respectively. The recovery tests for hazelnut and red pepper samples were tested at two levels. Recovery levels for hazelnut and red pepper were 80-90 %.  
During recovery tests, it was seen that recovery levels were reduced with increasing level of spiked gallic acid. Due to this unexpected result, gallic acids at different levels (5, 10, 15, 20, 30 mg) were extracted with the same procedure used for red pepper sample and reacted with Folin-Ciocalteu reagent. The recovery level was between (85-100 %) until the gallic acid level reaches to 15 mg. This indicated that using constant volume of 0.2 N Folin-Ciocalteu reagents during measurement was not sufficient for the samples containing more than 15 mg/g sample phenolic content.