SOP: Aldehyde Content in Lemon and Lime Oils
GIFSQ — Global Institute for Food Safety & Quality
Principle
Aldehydes react with hydroxylamine hydrochloride to liberate acid; the liberated acid is neutralized with standard alcoholic KOH to a permanent alkaline colour of the indicator, the titration being repeated with a slightly larger volume of hydroxylamine solution against the first determination as colour standard, giving the aldehyde content as citral or decanal.
Apparatus required
- 30 ml glass-stoppered conical flask
Reagents
a. Bromophenol blue indicator: dissolve 0.1 g of the indicator in 5 ml of 0.05 N NaOH, and dilute to 100 ml with 60% alcohol.
b. Ethyl orange indicator: dissolve 0.05 g of the indicator in 60% alcohol, and dilute to 50 ml.
c. 0.5 N KOH standard solution: dissolve 28.06 g of KOH in 60% alcohol and standardize against standard HCl.
d. Hydroxylamine solution: dissolve 7 g of hydroxylamine hydrochloride (H₂NOH·HCl) in 175 ml of 60% alcohol; add either 0.3 ml of bromophenol blue indicator and enough 0.5 N KOH to give a permanent blue solution, or 0.3 ml of ethyl orange and enough 0.5 N KOH to give a permanent yellow solution; in either case, dilute the resulting solution to 200 ml with 60% alcohol.
Procedure
Weigh 5 g of lime oil or 10 ± 0.01 g of lemon oil into a 30 ml glass-stoppered conical flask
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Add 7.0 ml of hydroxylamine solution and 0.1 ml of indicator
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Shake and neutralize the liberated acid with 0.5 N KOH to a permanent full alkaline colour of the indicator used
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Continue shaking and neutralizing until the permanent alkaline colour remains in the lower layer after shaking the mixture vigorously for 2 min and allowing the layers to separate — the time required for the completion of the reaction is about 15 min
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Repeat the determination using a volume of hydroxylamine solution 1 to 2 ml more than the volume of 0.5 N KOH used in the first determination; use the liquid from the first determination as the colour standard for the end point of the titration liquid
Calculation
One ml of 0.5 N KOH is equal to 0.07813 g of decanal aldehyde or 0.07612 g of citral present in orange and lemon oils respectively.
Aldehyde % = (ml of 0.5 N KOH × factor × 100) / Wt of sample
Where,
- factor = 0.07813 (decanal) or 0.07612 (citral)
- Wt of sample = weight of the oil taken
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
- Winkler, W.O., J. Assoc. Offic. Anal. Chem., 36, 119 (1953).
- EOA Specifications and Standards, Essential Oil Association of USA, Inc.: Determinations 1-1.