The primary composition of foods includes carbohydrates, proteins, lipids, vitamins, and minerals in different combinations. Phenols, flavones, and colourants are often found as secondary components of food.⁴ These secondary components help to enhance the aroma and taste of a food by stimulating the sensory organs.^5,6 Similarly, the presence of bacteria can alter the metabolic, physiological, and behavioural conditions of humans. Recently, an oroneural connection was established in humans.^7,9 Different types of food are also known to improve the psychological condition.¹⁰ As a consequence, the determination of food composition, authentication, assessment, prediction, and microbial composition are essential for determining the quality of foods¹¹ (Figure 1.1).

In addition to nanostructure, other parameters are also used to detect the quality of food.¹³ One such parameter is flavonoids, which are low molecular weight polyphenolic substances having a heterocyclic ring (ring C). Flavonoids are resistant to heat, oxygen, humidity, and light. A hydroxyl group on C3 of ring C (flavanol) contributes to the photostability.¹⁴ The distribution of flavonoids is species specific for plants. Tea, red wine, fruits, and vegetables are enriched with flavonoids. Although quercetin is the main flavonoid, myricetin, kaempferol, apigenin, and luteolin are also found in foods. Humans need a minimum of 1 g per day of flavonoids for the body.¹⁵ The amount of flavanol can be determined from plasma and urine analyses, hence it is often used as a biomarker. External factors such as the season, ripeness, and food preparation and processing can change the amount of flavanol present. A minimum of 10 nM of flavanol is needed in our daily intake of food.¹⁶ The amount of quercetin depends on the types of bacteria present in the gut. Flavonoids are conjugated in the liver and are eliminated from the kidney. Flavonoids can be determined using many spectroscopic techniques.¹⁷ The matrix of the food is analysed in several industries such as the milk, meat, coffee, and wine trades.¹⁸ Although the quality of food can be roughly determined using the sensory organs, many sophisticated types of equipment are also used for this purpose.¹⁹

UV–visible spectroscopy uses electromagnetic radiation in the range 100–750 nm. The UV range covers 100–380 nm and the visible range covers 380–750 nm.³¹ It detects two different aspects: (1) colour and (2) fat oxidation. In oils, the greater the carotenoid (a pigment or chromophore such as chlorophyll, which most plants contain) content, the better is the antioxidant activity.^32,35 The presence of chlorophyll makes olive oil bitter.^36,37 To determine the fat oxidation, the p-anisidine value (AV) is measured.³⁸ This is the quantity of aldehyde produced during fat oxidation as a function of temperature, oxygen, and light.³⁹ In oils, an AV of >8 is not permissible.