MIDDLE EAST TECHNICAL UNIVERSITY
FOOD ENGINEERING DEPARTMENT
FDE 310
FOOD MATERIALS LABORATORY
Experiment 5
Protein Determination
Submitted to : Oya Nihan Uncu
Section 2/Group 2
Merve Aylin Aydoğdu
Büşra Karahan
Özge Önder
Elif Gökçen Sakar
Burcu Başak
INTRODUCTION
Kjeldahl method was used in laboratory to determine the amount of protein in Feta Cheese. Determination of protein content in feta cheese, there are three steps which are digestion, neutralization and titration. More than 90 % of protein containing samples and over 60 % of all other nitrogen containing samples can be analyzed using this basic procedure. (Persson,2008)
In the digestion step, the decomposition of nitrogen in organic samples utilizes a concentrated acid
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H2SO4
• Catalyst: potassium sulphate (K2SO4), copper (II) sulphate pentahydrate (CuSO4.5H2O)
• NaOH (40%)
• 0.1 N HCl solution
• Boric acid (4%)
• Indicator: methyl red (200 mg make up to 100 ml with %95 ethanol )
1 gram Feta Cheese was used as a sample in this experiment. First of all, the experiment was started with the digestion phase which was performed by mixing 10g potassium sulphate and 0.5 g CuSO4 solution that acts as catalyst, and added 20ml conc. H2SO4. The digestion unit temperature was set to 420±10°C and preheated for 15 minutes. Then, it was heated continuously at least 2 hours till a clear colorless solution was obtained. After finishing the heating process, the tubes which contained the mixture were left to cool and removed from the digestion unit. 50ml of distilled water was added to the digestion tube and 70ml of NaOH was added by the NAOH pump of the system. When the distillation was done, 50ml boric acid (4%) was added into the 250 ml flask. Steam distillation was applied and boric acid & ammonia were obtained. In order to determine the nitrogen content , titration was done with 0.1 N HCl, using a suitable indicator, two or three drops of methyl red, to determine the end-point of the reaction. The end point was indicated by the changing color to lilac or purple. Finally, the amount of HCl was recorded to calculate the percentage of
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However according to label of La vache qui rit cheese, it should contain 19 % protein content. We could not find the same result as in the literature however protein content of different feta cheeses taken from different regions varies. Besides, error may occur as a result of problems in laboratory or problems due to human error. These difference may be caused by sample size and type incompatibility with amounts and types of acid, salt or catalyst used; contaminated samples, leaks in apparatus connections, inadequate sodium hydroxide addition or sodium hydroxide carry over, and calculation errors.