FOOD ANALYSIS

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FOOD ANALYSIS LAB 4
LABORATORY EXERCISE 5 Determination of Ascorbic
Acid in Foods
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OVERALL OBJECTIVE

• To measure ascorbic acid levels in fresh and
  cooked refrigerated peas
• To use titration for this determination

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Specific Objectives

• To apply another aspect of titration to Food
  Analysis
• To standardize the titrant with a known
  concentration of ascorbic acid
• To extract the fresh and cooked samples for
  analysis
• To titrate the extracted samples and determine
  the total ascorbic acid content

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Specific Objectives

• To perform calculations for standardization of
  titrant and content of the samples
• To compare the values obtained for each sample
• To write a report using appropriate statistical
  techniques so that you can get that Sierra Nevada
  Pale Ale from mom

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• ASCORBIC ACID ANALYSIS
• Commonly referred to as a Redox Titration

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ASCORBIC ACID ANALYSIS

• Pretreatment is necessary to get ascorbic acid in
  a form for assaying
• Typical pretreatment is using m-phosphoric acid
• helps to get ascorbic into an consistent form for
  analysis (reduced form)
• minimizes oxidation of the vitamin and stabilizes
  it
• Uses a redox dye (2,6-dichlorophenol)

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Ascorbic Acid Analysis

• Extraction Method
• 10 g sample blended/pureed
• Blend for 2 min with 70 mL metaphosphoric
  acidacetic acid mixture in a blender
• Filter samples through cheesecloth/filter paper
• Bring to a volume of 100 mL with metaphosphoric
  acidacetic acid mixture
• Mix and take 2, 40-mL samples

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Ascorbic Acid Analysis

• Method of Detection
• The redox dye must react with ascorbic acid,
  however, we dont know what the relationship is
  between the two.
• STANDARDIZE THE DYE.
• Unlike the NaOH, we dont have a standard other
  then the ascorbic we are analyzing for. So, take
  a known amount of ascorbic acid and titrate to a
  faint pink color and compare against a blank,
  containing only the extraction solution.

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• Ascorbic Acid Analysis
• Subtract out the blank from the standardization
  values
• This determines the mg of ascorbic acid per mL
  of redox dye
• Now you can titrate your extracted peas
• From your standardization, calculate the ascorbic
  acid content (mg AA/ 100 g sample) of the two pea
  samples
• Compare your values between samples

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• Calculations
• For the Standard Solution
• 50 mg AA brought to volume in 50 mL vol flask.
  What is the AA?
• AA 1 mg/mL
• Take 4 mL of this solution titrate with the
  redox dye this averages out as 14.3 mL. The
  blank titrates to 0.2 mL. What is the
  standardizing relationship?
• 4 mL x 1 mg/mL 4 mg AA
• 14.3 - 0.2 14.1 mL dye
• Relationship 4 mg AA/14.1 mL dye

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• Calculations
• For the Peas
• If 40 mL of sample is titrated with 21.7 mL of
  redox dye what is the amount of AA in the sample?
• 4 mg AA/14.1 mL dye X mg/21.5 mL dye
• Cross multiply
• X 6.1 mg AA in the 40 mL sample
• Thus there is 6.1 mg AA/40 mL sample but we have
  a total of 100 mL sample
• (6.1 mg AA/40 mL sample)(100 mL sample)
• 15.25 mg in a 100 mL sample

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• Calculations
• Expressing the AA in peas
• We want to express out our results as mg/100 g
  of peas.
• Obviously, the 100 mL of sample extract
  contained the AA from the 10 g of peas
• (15.25 mg AA/10 g peas)(100 g peas)
• 152.5 mg in 100 g peas