In this chapter, we discuss the influence of the processing methods on the content of phenolic compounds in fruits and vegetables. The intake of fruits and vegetables based?foods are associated with delayed aging and a decreased risk of chronic disease development. Fruits and vegetables can be consumed in natura, but the highest amounts are ingested after some processing methods, such as cooking procedures or sanitizing methods. These methods are directly methods are directly related to alteration on the phenolic content. In addition, the postharvest conditions may modify several phytochemical substances. Phenolic compounds are referred to as phytochemicals found in a large number of foods and beverages. The relative high diversity of these molecules produced by plants must be taken into account when methods of preparation are employed to obtain industrial or homemade products. Phenolic compounds comprise one (phenolic acids) or more (polyphenols) aromatic rings with attached hydroxyl groups in their structures. Their antioxidant capacities are related to these hydroxyl groups and phenolic rings. Despite the antioxidant activity, they have many other beneficial effects on human health. However, before attributing health benefits to these compounds, absorption, distribution, and metabolism of each phenolic compound in the body are important points that should be considered.
Most of the vegetables and some fruits are preferably consumed after some kind of processing, which can cause favorable or disadvantageous changes in the flavor and texture, increasing the food’s palatability and affecting the quantity and quality of bioactive compounds, such as phenolics. The biological, physical, and chemical modifications that occur during some processing methods, as the cooking, are predominantly related with sensorial, nutritional, and textural alterations, which may be beneficial or harmful to the human health. A high temperature can inactivate microorganisms, decreases anti?nutritional factors, increases the digestibility of foods, and modifies the bioavailability of the phenolics. In contrast, the thermal processing may have negative effects on these bioactive compounds. Furthermore, other processing methods have been adopted for fruits and vegetables, whether for domestic consumption or in the food industry, for example, fresh?cut, drying, blanching, pasteurization, use of electric fields and membranes, among others. In this chapter, we will address the influence of some processing methods in plant?based food based on the phenolic content, as well as on their bioavailability.
Phenolic compounds are a main class of secondary metabolites in plants and are divided into phenolic acids and polyphenols. These compounds are found combined with mono? and polysaccharides, linked to one or more phenolic group, or can occur as derivatives, such as ester or methyl esters . Among the several classes of phenolic compounds, the phenolic acids, flavonoids, and tannins are regarded as the main dietary phenolic compounds . Many studies have shown a strong and positive correlation (p ≤ 0.05) between the phenolic compound contents and the antioxidant potential of fruits and vegetables . This antioxidant mechanism, present in the plants, has an important role in the reduction of lipid oxidation in (plant and animal) tissues, because when incorporated in the human diet, not only it conserves the quality of the food, but it also reduces the risk of developing some diseases. Studies have shown that a diet rich in fruits and vegetables contributes to the delay of the aging process and to the decrease of the inflammation and oxidative stress risk, related with chronic diseases (e.g., cardiovascular diseases, arteriosclerosis, cancer, diabetes, cataract, disorders of the cognitive function, and neurological diseases).
The antioxidant activity of phenolic compounds is attributed to the capacity of scavenging free radicals, donating hydrogen atoms, electrons, or chelate metal cations . Molecular structures, particularly the number and positions of the hydroxyl groups, and the nature of substitutions on the aromatic rings, confers to phenolic compounds the capacity of inactivating free radicals, which is referred to as structure?activity relationship (SAR). The hydrogen atoms of the adjacent hydroxyl groups (o?diphenol), located in various positions of the rings A, B and C, the double bonds of the benzene ring, and the double bond of the oxo functional group (-C=O) of some flavonoids, provide these compounds their high antioxidant activity ). This characteristic can be observed in quercetin and catechin. Both compounds share a similar number of hydroxyl groups, at the same positions, however, quercetin also contains a 2,3?double bond in the C ring and the 4?oxo function . The advantage of this structure is an enhancement of the TEAC (Trolox equivalent antioxidant capacity) value, when compared to the saturated heterocyclic ring of catechin with approximately half the antioxidant activity.
The major sources of phenolics are fruits, vegetables, and beverages, such as coffee, tea, wine, and fresh?fruit juices. Besides its potential effects in protecting against several chronic diseases, it is essential to understand the modifications occurred to these compounds after food processing, and its bioavailability. Better knowledge of the modifications induced by different processing methods in phenolic compounds are essential to evaluate appropriately the bioavailability of these compounds.
The bioavailability of bioactive compounds is the absorptive process of these molecules across the intestine into the circulatory system, after food ingestion. In a review study, reported values of several polyphenols ingested pure (isolated compound) or in foods, ranged from 0.072 to 5 μM, when reached the plasma. The total intake of polyphenols, in the studies grouped for this review, ranged from 6.4 to 1000 mg/day . In elderly Japanese population, the consumption of polyphenols ranged from 183 to 4854 mg/day, with 665 to 1492 mg/day on an average. Beverages such as coffee and green tea were the largest source of these compounds .
Usually, from the total phenolics ingested, phenolic acids account for approximately one?third and flavonoids account for the two?thirds remaining. Phenolic and polyphenolic compounds, in isolate or associated to vitamins, such as carotenoids, vitamin E, and vitamin C, are reducing agents that protect human body’s specific tissues against oxidative stress. However, polyphenols are the most abundant antioxidants in diets based on fruits and vegetables. The most abundant benzoic acids ingested in human diet are gallic, ellagic, protocatechuic, and 4?hydroxybenzoic acids. Therefore, cinnamic acids are mainly represented by caffeic, ferulic, sinapic, and p?coumaric acids. Diets based on plant foods are a rich source of polyphenols that have health benefits and avoid the development of chronic diseases. However, food processing, such as blanching and thermal treatments, may influence its levels and induce its conversion to secondary compounds. In addition to molecular modifications occurring in phenolics during food processing, the absorption and metabolism of these compounds are triggered by enzymatic and nonenzymatic reactions . These molecules can also suffer conjugation reactions that may increase or decrease their bioavailability.
Processing methods has been associated with changes in the quantity and quality of (poly)phenols. The high diversity of these molecules produced by plants must be taken into account when processing methods of preparation are employed to obtain industrial or homemade products. There are innumerous studies comparing the biological actions and in vitro antioxidant activity of phenolic compounds in function of its content in plant?based foods and consequently, in humans. The phytochemical amount retained in fruits and vegetables after the processing, depends on the stability of these compounds during different food preparations. Molecular modifications induced by processing, and transformations that occur before the consumption, are mainly related to the sensibility of the compounds to oxidation and/or isomerization.
The physical and chemical transformations that occur during the thermal processing in each species and between different species can vary, depending on the processing method used, as well as on the temperature and time employed. In general, the thermal processing methods as the beverage pasteurization result in loss of the phenolic compound content, due to the high temperatures employed, as well as the cooking of vegetables in water, because it promotes the leaching of the phenolic compounds. Even though it is not possible to affirm that these effects are observed in all foods, the thermal processing methods such as microwave cooking, steam cooking, air frying, oil frying, and grilling induce alterations in the food matrix, promoting the extraction of these compounds and increasing its bioaccessibility.
Methods such as blanching can minimize the phenolic compounds oxidation by inactivating the enzymes (i.e., PPO and POD) responsible for the darkening of the vegetables and can be used as a preprocessing in order to avoid the loss of these compounds during the process of hot?air drying. The drying at microwave vacuum induces total phenolics retention at higher levels than in those fruits and vegetables that were air?dried. Regarding dehydration, the best method seems to be freeze?drying.
Beyond the thermal processing, the sanitizing methods such as the use of sodium hypochlorite and ozonization can also affect the amount of phenolic compounds, in a dependent manner of the food matrix, compound/method employed, concentration, and sanitation time. The use of ionizing and nonionizing radiation in the food sanitation, cause modifications in the profile of the phenolic compounds and the results vary according to the dose. However, the application of this technology usually induces the increase of the phenolic content, which may be related to the vegetable defense mechanism on reaction to the stress induced by the radiation application.
Better knowledge of the modifications induced by different processing methods in phenolic compounds is essential to properly evaluate the bioavailability of these compounds. The molecular modifications occurring in phenolics during food processing, the absorption and metabolism of these compounds are triggered by enzymatic and non?enzymatic reactions, and these molecules can suffer conjugation reactions that may increase or decrease their bioavailability and consequently, affect the beneficial effects to human health.