1. Concept of acidity • 1. Total acidity • 2. Effective acidity • 3. Volatile acid • 4. Acidity of milk
2. Significance of acidity • Organic acid affects color, aroma, taste and stability of foods. • Quality of foods can be judged by acidity. • Mature degree of some fruits can be judged by acidity.
3. Organic acid kinds and distribution of foods • There are organic acid and inorganic acid in foods, and mainly organic acid. • Familiar organic acids in foods: citric acid, malic acid, tartaric acid, oxalic acid, lactic acid and acetic acid.
Section 2. Determination of acidity
1. Determination of total acidity • 1. 1 Principle : RCOOH + Na. OH = RCOONa + H 2 O Indicator: phenolphthalein, PH 8. 2 • 1. 2 Applying range : Foods of slight color
2. Determination of p. H • p. H test paper • p. H meter
p. H meter
3. Determination of volatile acid • Volatile acid: mainly acetic acid, along with trace of formic acid, not including lactic acid, CO 2, SO 2, etc. • Content of volatile acid would increase because of fermentation of sugar. So content of volatile acid is an indicator of some foods’ qualities.
• Direct determination: Separating volatile acid by distillation or solvent extraction, and then titrate using standard alkali. • Indirect determination: removing volatile acid, and then titrate unvolatile acid.
Section 3. Separation and determination of organic acids in foods