Pigments in fruits and vegetables Fat soluble/ lipophilic pigments Water soluble/ lipophobic pigments Chlorophyll Anthocyanin (red/blue/purple) Carotenoids (red, orange and yellow) Flavones & flavanols (yellow) Flavanals etc.
Classification based on chemical structure and property Tetrapyrrole = Chlorophyll Isoprenoid derivatives =Carotenoids Benzopyran = Anthocyanin and flavonoids
CHLOROPHYLL Found in green leaves & unripe fruits Required for photosynthesis Present in chloroplastids/chromaplastids 2 types of chlorophyll Chlorophyll a (blue green) Chlorophyll b (dull green-yellow)
Structure Tetrapyrrole pigments consists of four 5-membered pyrrole group joined together to form basic porphyrin ring A Mg atom coordinated in the centre and chelate by the 4 N of the pyrrole groups. Chlorophyll is metalloporphyrin A phytol group is attached at position 7. It is responsible for the solubility of the chlorophylls in organic fat solvents.
Degradation of chlorophyll 3 major reaction Loss of Mg Removal of phytol Oxidation & ring splitting (not in the syllabus) Chlorophyll molecules are sensitive to acid degradation, particularly carboxylic acids present in fruits and vege eg. Citric acid and malic acid. In fresh raw tissues, chlorophyll pigments and carboxylic acid are kept separate. Protected by lipoprotein in the chloroplasts Carboxylic acid do not have any effect on chlorophyll
Treatments such as cutting, blending, heat treatment can alter the permeability of cell membrane, allowing contact between acid and pigment This contact causes the Mg atom in the chlorophyll molecules to be displaced & is replaced by 2H atoms Chlorophyll Pheophytin (Mg free molecules) Chlorophyll a Pheophytin a ( dull-olive green) Chlorophyll b Pheophytin b (dull- yellow green)
Colour change is irreversible Chlorophyll a > sensitive than chlorophyll b Formation of pheophytin depends on; Presence of H+/ pH – low pH encourages formation Type of processing- high temp & prolonged time encourage formation Certain ion present (eg. Fe3+, Zn2+) can replace the colour lost to form stable complex colour
Hydrolysis of phytol group Catalyzed by enzyme chlorophyllase (an esterase) with an optimum activity at 77C Chlorophyllase catalyses hydrolysis of phytyl ester linkage at C7, forming methyl chlorophyllides Methyl chlorophyllide can lose its Mg2+ to form pheophorbides, a type of pheophytin, H2O- soluble, dull olive-green colour
The effect of acid and heat treatment on chlorophyll Chlorophyll chlorophyllase chlorophyllide +acid, heat -phytol +acid, heat - Mg -Mg Pheophytin Pheophorbide +heat + heat -CO2CH3 -CO2CH3 Pyropheophytin Pyropheophorbide *Murano, P.S.,Understanding Food Science & Technology, pg 160