Cooking 
The nutritional value of proteins is always influenced by cooking. When cooking takes place in acid ambient (in presence of lemon, vinegar, etc.) digestibleness increases. Prolonged cooking times, on the contrary, reduce digestibleness since proteins are less susceptible of being attacked by the gastric juices. 
Cooking modifies the nutritional composition of protein food. As a matter of fact, temperature, use of water or not during cooking, thickness of slices affect the stability of nutriments. At 50o/60o proteins tend to coagulate and to change colour, moreover prolonging cooking a fission into simplier molecules takes place and this brings to a better digestibleness of the food. However a too prolonged cooking can cause a decreased availability of some essential aminoacids as: cistein, tryptophan, metionine, lysin. Lysin, limiting aminoacid in cereals, is the most unstable of essential aminoacids. The complex starch-lysin, that produces the peculiar golden colour of bread crumble, is biologically not available. During stewing of meat it is possible to avoid the loss of nutritious elements by dipping meat into already boiling water. So doing, the quick coagulation of proteins, present into the superficial muscular fibres, prevents from nutriment loss. 
Also some enzymes, being proteins chemically, are inactivated by cooking. 
An excessive healing that cause the carbonisation of proteins brings to form several toxic compounds as aromatic polycyclic hydrocarbons with mutagenic and therefore cancerogenic effects. 
Fish, that contains very scanty quantities of connective tissue, needs very short cooking times which limit protein losses. 

Lipids 
Fats, or lipids, heterogeneous substances in nature spread both in the vegetable world or in the animal one, are energy-giving supplies and are constituents of cellular structures, they are also precursors of substances as hormones and prostaglandines. Moreover they are necessary carriers for liposoluble vitamin absorption. Lipids -because of their chemical composition- tend to form emulsions easily. According to their melting point they are: 
-fats, properly, (solids at room temperature) 
-oils (liquids at room temperature) 
The formers contain saturated fatty acids, while the liquid ones contain unsaturated fatty acids. 
Lipids are subdue to oxidation and consequently they grow rancid. This latter process is also helped by light and air and it can be prevented by some antioxidators as vitamin E. 
Lipids have a high energy power that greatly affects the energy supply of a food. They are necessary to the production and preparation of foodstuffs , thanks to their peculiar characteristics as melting, their nice creamy taste, their solvent power in certain flavours and odoriferous compounds. Therefore they are basic to obtain taste from proteins. 

Cooking 
Heating of lipids always brings to a changement of the nutritional properties of food and it depends on the presence of saturated-non saturated fatty acids. At mean-low temperatures solid fats melt to liquid fats but without chemical transformation. At higher temperatures fats resolve into glycerol and fatty acids; further degradation phenomena with oxidation products and polymerisation take place. Glycerol, for example, partly evaporates and partly changes into acrolein, a toxic product. 
Prolonging cooking brings to the so called “smoking point” with formation of dangerous substances. 
Other phenomena as growing rancid and auto-oxidation, which generally take place in presence of  atmospheric oxygen, are accelerated by cooking. Here there are changings in smell and taste (rancid) and also in colour. Food could be so dangerous for health again. 
Moreover by frying almost without fat food (vegetables as courgettes, potatoes, etc.) take great quantities of the fats where they are cooked (from 10% to 40%). 

Glucides 
Glucides, or sugars, are nutritional energy sources “par excellence”. They also have plastic function since they enter the constitution of the nucleic acids, cerebral lipids. 
Glucides give prompt available energy for oxidative metabolism, they help to keep glycaemia in homeostasis and good gastroenteric functions. 
Glucides also give the right energy to vital processes. Energy releases gradually during the metabolic processes that brings to the products, the released energy is stored as phosphoric links with high energy level (ATP). The organism uses just glucose as energy source. Therefore glucose and galactose can be used only after having been transformed into glucose. 
Glycaemic Index (G.I.) is important as indicator of the level of absorption of the different carbohydrates. Pasta, for instance, turns out to be less affecting the glycaemic level than bread, while potatoes cause a more evident hyperglycaemia. 
Variance in glycaemic index has to be related to several factors: for instance to the prevalence of  amylase- abounding into legumes- which are slower in absorption than amylopectines contained in cereals. It must also be considered the industrial processes on food, the way of cooking, the presence or not of fires, sodium,interatins of glucides with lipids and/or proteins. 
These factors would explain the lower glycaemic index of wheat pasta than that of wheat bread. 
Glucides are very spread in Nature, mainly in the vegetable world as cellulose (support material) and starch (supply material) and as glycogen in animals. Moreover they can be present as combined in protein, lipid substances, so forming compounds of primary metabolic importance. 
Glucides have an important function within sweeteners, gelationers, thickeners, they are also precursors of flavours and dyes obtained by chemical reactions. 
According to new researches, the role even energetic of polysaccharides as cellulose, emicellulose, pectins, rubber and phenolic polymers as lignin, considered “fibre” or “non starch polysaccharides”(apart from lignin) and so not counted in the caloric amount, should not be underestimated. 
As a matter of fact, fibre-even though it is not a nourishment- should be considered in the labelling of foods, since for not digested carbohydrates arriving at colon an energetic value of 2 cal/g is fixed. 
Fibre intake can cause problems of chelation of mineral salts or, anyway, loss and/or decrease in absorption of some nutrients contained inside fibre, so making its energetic contribution negligible. 

Cooking 
Starch cooked in a liquid absorbs the cooking water and de-structures so forming a high viscosity colloid; in cold it gelatinizes (starch water), this process is limited y adding water, acid substances as lemon and vinegar. 
Dry heat causes the change of starch into dextrines (dextrinization) which gives a peculiar flavour to bakery products, also increasing their digestibleness. 
At high temperatures (160o-180o) there are caramelising with water evaporation, browning and hardening in cold (barley-sugar). 
In presence of acid substances, as it happens when preparing jams, sugars change into simplier ones as fructose and glucose by cooking. 
Moreover it has to be considered that during cooking proteins and sugars can interact and form non usable products by the organism and this causes a reduction of the nutritional value of the so treated foods. 

Vitamins 
Vitamins are heterogeneous substances necessary to he lives of organisms that, not being able to synthesise them, take them in by food (but in certain circumstances some vitamins are synthesised by the intestinal “flora”). Vitamins enter the chemical structure of several coenzymes; their requirements are variable. 
Cooking 
Thermolabile vitamins denature, so losing their nutritional value, by cooking. 
For example, vitamin B1 (tiamin) degrades quite a lot, especially in meat, during that process. 
Decrease or destruction of vitamins, anyway, depends on the specific characteristics they have in presence of physical agents. Greater losses are in water- soluble vitamins. 
 

Minerals 
Minerals are necessary to several biological processes, they are taken by food. They are found as ions, salt ions and/or combined inside organic compounds, they enter the structure of cells and tissues. Minerals are divided into: 
-macroconstituents with mainly plastic function 
-microconstituents with essentially catalytic function. 
Minerals that are present in low quantities (milligrams or less) are called “oligoelements”, “trace elements” or micronutrients. 
Bioavailability of oligoelements mainly depends on the antagonism among ions and on presence of chelating elements into foods-which may improve or decrease absorption. 
Iron bioavailability depends on its being eme or non eme. 
Eme iron-present in meat and fish- is absorbed as it is, without any interaction with other components of the diet. 
Phytates and polyphenols (already contained in foods), but also calcium phosphate and ethylenediaminotetracetic (EDTA) that are added to foods during technological processes, inhibit the eme iron absorption. 
Eme iron constitutes 40/50% of the total iron present into animal source foods, while into vegetables it is possible to find non eme iron only. 

Cooking 
Cooking does not degrade mineral salts but it causes their dispersion into the cooking liquid. 

Water 
Water is the basic element where all processes necessary to life take place. One of its chemical characteristic is to be a dipole that makes it a solvent where several substances (micromolecules, macromolecules, eletrolytes, etc.) are dissolved. Water is therefore basic to metabolic processes of the different nutritious principles. 
It should be considered the kind of water taken and its effects also due to its “composition”. Hardness is determined by the content of calcium salts and magnesium. Water is classified “fresh”at 5 French degrees, “moderately hard” between 5 and 20 Fr.degrees, while it is considered “very hard” between 20 and 30 Fr.degrees (1 Fr.degree= gr.0,01 calcium carbonate/l). 
A too hard water is not suitable for cooking legumes and other vegetables since it hardens their cellulose whose digestion becomes difficult. 
Another value which determines the quality of water is the so called “fixed residue” (or “dry”) that is the quantity of minerals left after the complete elimination by evaporation of 1litre of water at 180o. 
  
By its minerals, water can compensate possible mineral deficiencies, anyway considering which of them are bioavailable and/or absorbable. It must be also considered that minerals present into water can interact with each other and with some components of  food as phytates and oxalates. 

Quality of  foods and their preservation 
Many factors influence the quality of foods. State of health of the animals-which edible elements come from- depends on the plants they eat. These last one get the effects of the quality of soil where they have grown up. The nutritious power of a food takes in account parameters as essentiality, toxicity, bioavailability. 
The general opinion that “natural foods” are “healthy” may prove itself  wrong. Some elements can contain molecules without nutritious value and they can interfere with some physiologic processes of the organism with negative consequences for health. 
Popular beliefs and new food trends, as some vegetarian diets, can lead the individual to make nutritious mistakes. 
Many Leguminosae as chickpeas, lentils, beans can cover a good deal of protein need. However unlike the animal ones, proteins of  Leguminosae have scanty quantities of sulphur aminoacids and so they are less bioavailable. Fish and seafood contain a “thiaminasis” that hydrolyses vit.B1 (thiamin). 
A frequent intake of such raw foods can cause a lack of thiamin and therefore neurological ailments and paralysis. 
Inside the pulp of some vegetables as courgettes, cucumbers, melons, carrots and peas there is abscorbic acid oxidase that oxidises vit.C into dehydroabscorbic acid which so loses its oxidoreducing power. A quick scalding of these vegetables avoids the process. 
With regard to preservation, methods based on heat, freezing, packing or other preservation techniques, produce a destruction of some elements naturally present into foods, but they grant destruction of the microbial charge. 
Deep-freezing instead allows to preserve food at fresh state since it stops the natural process of perishing and it keeps the original organoleptic characteristics unaltared. 
However to keep such features it is necessary to mantain the so called “refrigeration chain”, that is food must be always kept at optimum and needed temperatures during each preservation step (transport, storing, preservation). 
It is important to consider interaction among nutrients, chemical compounds and type of the container. A series of laws control packaging materials so as to guarantee wholesome foods. 
Materials should be appropriately inert so as not to be dangerous for health and not to cause changements in the organoleptic properties of foods. 
Some worries have been caused by food transparent films of PVC mixed with 20/30% of  plasticizers, whose particles may be transmitted by fats they are in contact with. These problems are just caused by those those plasticizers mixed with PVC to make this last one more elastic and adherent. Unlike the short chain plasticizers, the long chain plasticizers reduce the possibility of particle transmigration to food. A good alternative consists in using polyethylene films. But, for the same reason mentioned above, this last ones must not be used in contact with acid foods (tomato, lemon and vinegar) or with dishes as chicken or bass in salt. 

Toxicological aspects  
of some compounds 
Vegetables as cabbages,beets, spinach, rhubarb, oats, contain oxalic acid that interferes with calcium absorption and can predispose to kidney calculosis. 
Dried legumes, nuts, olives contain phytic acid which forms by chelation of different minerals as calcium, zinc, magnesium and iron, undissolvable compounds which the organism gets rid of. 
Some alkaloids, contained into tubers and certain exotic plants, may cause negative effects. Solanin, for instance,  forms in the peel of potatoes left in the sun for a long period, or in that of unripe potatoes. 
This alkaloid produces irritation of the gastro-intestinal tract and sometimes symptoms of 
central and peripherical cholinergic block. 
Caffeine, theobromine, theophylline are xanthines, that are alkaloids present into coffee, tea, cocoa, cola, etc. with stimulating effects on the central nervous system. However when strong coffee drinkers (more than five cups a day) abstain from drinking the beverage, show symptoms as unrest, tiredness, irritability which suggest a toxic effect of such substances. 
Recent discoveries have pointed out and explained scientifically the pharmacological properties of some vegetables that, sometimes, contain biologic elements as: hormones, antibiotics and chemical mediators. Such properties are called “extranutritious” and they can interfere with biochemistry and physiology, therefore with health. 

Conclusions  

The requisites that characterise food quality can be divided into biologic, technologic and trade requisites- apart from safety ones-. 
All these aspects are determined by raw materials, by the conditions of production, preservation and distribution of food. 
Moreover quality is not only influenced by chemical, microbiologic and physic characteristics, but also by any possible interference, contamination and transformation among nourishings.