7 - Colours Food Chemistry PDF

11/4/2024 7 - Colours NUT312 - Food Chemistry 1 Introduction  Color is very important in our appreciation of food ◦ Appearance is an important determinant of food quality ...

11/4/2024 7 - Colours NUT312 - Food Chemistry 1 Introduction  Color is very important in our appreciation of food ◦ Appearance is an important determinant of food quality  In food industry, it is important to maintain the colors of processed foods as close as possible to the ones of the original raw materials ◦ Some foods acquire their colors due to processing (brown cake and pink cured meat) ◦ Consumers are showing concern over the use of chemically synthesized food colorants 2 1  11/4/2024 Chlorophylls  Green pigments of: ◦ Leafy vegetables ◦ Green apple skin ◦ Unripe fruits  Functional pigments of photosynthesis in green plants ◦ They occur in the chloroplasts, the organelles which carry out photosynthesis in plant cells 3 Chlorophylls  Algae and photosynthetic bacteria contain different types of chlorophyll, but the higher plants that concern us contain only chlorophylls a and b, in the approximate ratio of 3 to 1 ◦ Chlorophylls a and b are porphyrins similar to hemoglobin except for the presence of Mg rather than Fe 4 2  11/4/2024 Chlorophylls  Chlorophyll is lost naturally from leaves during senescence due to chlorophyllase activity ◦ Carotenoids are more stable ◦ Red-orange leaves in autumn!  When green vegetables are heated (e.g. cooking, blanching prior to freezing, canning): ◦ Magnesium is lost to give brown pheophytins a and b 5 Chlorophylls  The acidity of plant cell vacuoles encourage pheophytin formation during heating ◦ Keep the cooking water alkaline by adding sodium bicarbonate  This is quite successful, but the alkaline conditions have an unhappy effect on the texture and flavour and losses of vitamin C are enhanced  In canning peas, chlorophyll loss is inevitable and artificial colours (tartrazine and Green S) are added 6 3  11/4/2024 Chlorophylls  Growing doubts about the safety of tartrazine are encouraging food chemists to examine the possible use of sodium copper chlorophyllin, a chlorophyll derivative, as food colour ◦ It is blue-green colour and survives the heating conditions involved in canning ◦ The amount of copper it contains is far too small to represent a toxicity hazard 7 Carotenoids  Responsible for most of the yellow and orange colours of fruits and vegetables ◦ Found in the chloroplasts of green plants alongside chlorophyll ◦ Classified as terpenoids 8 4  11/4/2024 Carotenoids  Two principal groups: 1. The carotenes, strictly hydrocarbons  The simplest carotene is lycopene 2. The xanthophylls, contain oxygen 9 Carotenoids  Xanthophylls arise by hydroxylation of carotenes  Xanthophylls are the dominant pigment in yellow tissues; whereas carotenes tend to give an orange colour 10 5  11/4/2024 Carotenoids  Carotenoids are popular as food colorants  Crocetin is the major pigment of the expensive spice saffron extracted from the crocus flower  Bixin is the major constituent of annatto in cheddar cheese 11 12 6  11/4/2024 Carotenoids  Although normally found in plants, carotenoids do find their way into some animal tissues (e.g. through animal feed) ◦ Egg yolk: lutein and zeaxanthin (xanthophylls) ◦ Salmon & crustaceans: astaxanthin  Carotenoids are labile to heat ◦ Heating: tendency for some trans double bonds of carotenes to isomerize to cis  More cis double bonds, lower colour intensity 13 Carotenoids  Epidemiological studies have suggested that fruit and vegetable consumption is associated with a reduced incidence of many different types of cancer ◦ α- and β-carotene and lutein are associated with reduced risk of lung cancer, and lycopene with a reduced risk of prostate cancer 14 7  11/4/2024 Anthocyanins  The pink, red, mauve, violet, and blue colours of flowers, fruit, and vegetables are caused by the presence of anthocyanins ◦ They are flavonoids located in the vacuoles of plant cells ◦ They occur in nature as glycosides  The sugar-free counterparts (aglycones) of anthocyanins are the anthocyanidins ◦ Cyanidin in red cabbage ◦ Pelargonidin in radishes and red beans ◦ Delphinidin in eggplants 15 Anthocyanins  Anthocyanins complex metal cations ◦ Occasional unusual color developments in canned fruit are due to interactions between the anthocyanins of the fruit and the metal of the can when the internal lacquering has failed  Anthocyanins react with SO2 ◦ SO2 is an antimicrobial preservative in wines (more in white) and fruit juices ◦ At high concentrations, it causes irreversible bleaching of anthocyanins ◦ Red wine spills on white tablecloths! 16 8  11/4/2024 Anthocyanins  Anthocyanins are popular as food colours, replacing the synthetic dyes in confectionary and soft drinks ◦ Good sources of anthocyanins  Skins of black grapes, byproducts of wine making  Red cabbage and elderberries extracts 17  When used in an application with a lower pH, like confections or beverages, anthocyanins will appear bright red to pink  But when that same anthocyanin is put in an application that has a higher pH, such as a cupcake. the anthocyanin will appear bluish-purple. Red cabbage dye in different pH 18 9  11/4/2024 Betalains  Red-purple pigments of beetroot  Two groups: ◦ Purplish red betacyanins (similar colour to the anthocyanins) ◦ Yellow betaxanthines (less common) 19 Betalains 20 10  11/4/2024 Betalains  Although they have a similar colour to the anthocyanins, betacyanines differ in that their colour is hardly affected by pH changes in the range normally encountered in foodstuffs  Concentrated beetroot are becoming popular as colorants for dairy and dessert products ◦ Their instability to heat makes them unsuitable for coloring cakes 21 Melanins  Undesirable brown pigments arising when plant tissues are damaged ◦ fruit and vegetables quickly turn brown if air is allowed access to a cut surface ◦ Enzymatic Browning (cut apples/potatoes/pears … etc)  The browning occurs when polyphenolic substances, which are usually contained within the vacuole of the plant cells, are oxidised by the enzyme phenolase (or polyphenol oxidase), which occurs in the cytoplasm of plant cells  Slicing / peeling / bruising/tissue damage bring enzyme (polyphenol oxidase) and substrates (chlorogenic acid) together 22 11  11/4/2024 Melanins  The most important substrate for phenolase in apples, pears, and potatoes is chlorogenic acid  The brown pigments that develop on the cut surface of fruit and vegetables may well be regarded as unappealing to consumers, but they do not reduce the flavour or nutritional value 23 Melanins  In animal tissues, the only substrate to polyphenol oxidase is tyrosine ◦ Tanned skin  During fruit and vegetable processing, we try to prevent phenolase action 1. Blanching the fruit and vegetables will reduce phenolase action to a minimum 2. Reducing contact with air by immersion in water 24 12  11/4/2024 Melanins 4. Adding chelating agents that inhibit phenolase by depriving the enzyme of the copper ion that is a component of active site 5. Immersing cut fruits in organic acids, such as citric acid and malic acid  Their acidity helps to keep the pH well below the optimum for phenolase action 6. Adding sulfur dioxide as it has a specific inhibitory action against phenolase  This has the additional benefit of preserving the ascorbic acid, but it bleaches anthocyanins and accelerates can corrosion 25 Tea  The enzymic oxidation of polyphenolic substances is highly desirable in the case of black tea ◦ The tea we drink is derived of the tea plant, Camellia sinensis, leaves that contain polyphenolic materials 26 13  11/4/2024 Tea  The leaves of the tea plant contain enormous quantities of polyphenolic materials - over 30% of the dry weight ◦ Caffeine, the stimulant found also in coffee and ‘cola’-based drinks, is some 3-4% of the dry weight of the leaf 27 Tea 1. Withering: freshly leaves are allowed to lose 20% of their water content, then macerated  Phenolase come into contact with the polyphenolic substances (catechin) in the cell vacuoles 2. Fermentation: macerated leaves are left at ambient temperatures for a few hours to ferment  Catechins are oxidised → polymeric brown thearubigins: principal pigments of black tea infusions. They contribute not only the bulk of the colour but also the astringency, acidity, and body. 3. Firing: leaves are dried out at 75oC to inactivate phenolase and to stop fermentation → black tea 28 14  11/4/2024 Tea  Green tea is manufactured by firing before the leaves are macerated so that the phenolase is inactivated before a significant amount of fermentation can occur ◦ An intermediate tea type ‘Oolong’ is fermented for only 30 minutes or so before firing  Catechin and thearubigins give the astringency to tea 29 Turmeric  Turmeric, the dried powdered roots of the Curcuma Zonga, is an important spice for food color and for dyeing textiles ◦ Essential component of curry powder ◦ Curcumin is the pigment 30 15  11/4/2024 Cochineal  Is the name given to a group of red pigments from various insects  Used as a food colour, as a dyestuff for textiles and leather and as a heart stimulant ◦ The principal component is carminic acid 31 Artificial Food Colorants  Chemically synthesized dyestuffs have many advantages over ‘natural’ colours ◦ They are much brighter, more stable, cheaper and offer a wide range of shades  Concern has been expressed in the media over the safety of synthetic food colours ◦ Food colours now permitted are as safe as any ‘natural’ components of our diet 32 16  11/4/2024 Artificial Food Colorants 33 Artificial Food Colorants  0.01-0.1% of the population may show intolerance towards tartrazine  The possibility of food colours being involved in children ‘hyperactivity’ has been dismissed by most medical authorities  The high costs of safety testing is making the appearance of new food colours extremely unlikely 34 17  11/4/2024 Artificial Food Colorants and US regulations  Synthetic colorants used commercially are also known as certified color additives: 1. FD&C dyes: water soluble compounds that produce color in solution 2. FD&C lakes: made by combining dyes with alumina to form water insoluble colorants ◦ They are used in oil-based and low water foods (e.g. chewing gum)  Pigments obtained from plants and animals are exempt from certification 35 18