Comparative study of frying to other cooking techniques influence on the nutritive value

Frying is one of the oldest methods of food preparation. It improves the sensory quality of food by formation of aroma compounds, attractive colour, crust and texture. Undesirable changes involved are loss of nutritive quality e.g. due to degradation of heat susceptible vitamins. The influence of common frying methods (frying in an oven, in a pan deep frying) on cooking time and nutritive value of vegetables, potatoes, meat, poultry and fish is described and compared to other cooking methods (boiling, steaming, stewing). Frying of vegetables, potatoes and breaded meat, poultry and fish, no matter whether in a pan or by deep frying, is associated with fat uptake (2 14 g per 100 g of raw food) while non breaded high fat food of animal origin loses fat during frying (2 30%). Data suggest that the fat quantity absorbed during frying increases up to a saturation limit which depends on the kind of food and on the amount of panade. Deep fried meat, poultry and fish usually absorb less fat than meat, poultry and fish fried in a pan. The kind of fat had no essential influence on fat uptake. After frying of vegetable food and of breaded meat, poultry and fish, the content of protein, carbohydrates and minerals was nearly fully retained while boiling and steaming reduced the mineral content by 25-50%. In the majority of cases frying including deep frying also retained the vitamins Bi, B2, Be and C better than boiling, steaming and stewing.


INTRODUCTION
Frying is one of the oldest methods of food preparation.It improves the sensory quality of food by formation of aroma compounds, attractive colour, crust and texture.Cooking, moreover, improves the hygienic quality of the food by inactivation of pathogenic microorganisms and enhances digestibility and bio-availability of nutrient in the digestive tract.Undesirable changes involved^ are loss of heat-and oxidation susceptible vitamins and of water soluble minerals.
The most important methods to fry food are: deep frying: cooking of food in enough hot fat or oil for the food to float on it shallow frying: frying of food in little fat or oil in a pan roasting: frying of mostly protein containing food with or without fat in an oven This paper will be devoted to the following subjects which will be discussed on the basis of own experimental results and literature data: -Influence of cooking medium and temperature on cooking time during frying of vegetable and animal food.-Influence of various frying methods, as compared to other cooking methods such as boiling, steaming, stewing and braising, on nutrient content (protein, fat, carbohydrates, minerals, vitamins).

Food
Potatoes and potato products, pork (fat and lean), hamburgers (mixture of pork and beef), meat loaves, fried sausage, chicken and.turkey breast meat, plaice fillets and pollack fingers were used, i.e. food which is frequently deep fried, roasted or fried in a pan, either breaded or unbreaded.
The food was prepared according to household recipes.Industrial convenience food including deep frozen French potatoes, potato pan cake and breaded fish were used as well.

Determination of the cooking endpoint
The cooking endpoint depends on the cooking degree of a food; it is usually characterized by the sensory quality attributes colour, shape, odour and texture.For the cooking endpoint, or cooking time, resp., of fried food, two items are essential: -formation of attractive and typical surface browning and crust, and of aroma compounds, and -a certain «inner» cooking degree, reflected by colour, taste and texture of the food, e.g.tender texture of meat and fish.
Both external appearance and «inner cooking degree» should be optimal when the cooking process is completed.
Various studies have shown that there is a relation between central temperature and colour during frying of beef and pork , Piekarski [23], Tilgner [24]).At a temperature of about 75 °C, the colour turns grey-beige, due to denaturation of the nneat pigments myoglobin, oximyoglobin and melhmyoglobin.Browning of the food surface and crust formation is described as consequence of many complex reactions including non-enzymatic browning reactions among amino groups of proteins and carbonyl groups of carbohydrates, pyrolysis and carmellization of protein and carbohydrates (Tilgner [24]).
The degree of browning, i.e. surface browning and crust formation of a food during frying, cannot be described by physical data alone.For deep frying e.g.there is only one certain point, i.e. a certain temperature of the heating medium at which both internal cooking and external browning are reached at one and the same time (Skjoldebrand [25]).Perfect cooking means to find out this optimal point.
In the present studies, browning during cooking was evaluated visually by five to seven trained assessors according to a ranking tests including five classes (Table I).The inner cooking degree was evaluated by measurement of the internal temperature of the food.partly browning, too thin crust optimal browning, golden-brownish, shiny as uniform as possible due to product ' components partly dark brown, a little bit non uniform, some brown-black parts burnt, too dark brown towards the black-brown colour

Evaluation of sensory quality
Sensory quality of fried foods was evaluated by means of the 9-point scale [1.1 ] by at least six trained test persons on the basis of colour shape odour, taste and texture (Bognár,Zacharias [9]).

Determination of nutrient content
The content of water (moisture), protein, fat, carbohydrates, minerals (ash), vitamin C, Bi, B2, Be were determined according to standard analytical methods ([1.2 -1.6], Bognár [3 -6, 12]).Changes in the weight and in nutrient content of cooked food were compared with the raw material by using following equations: X(i, g) nutrient loss in % = 100 -d(i,p) where: U(g) = weight of food or recipe (9) before cooking in g V(g) = weight of food or dishes after cooking in g X(i,p) = content of nutrient i per 100 g of raw food or recipe before cooking Z(i.p) = content of nutrient i per 100 gof cooked food or dish p after cooking Analytical results and literature dates were evaluated according to the usual statistical procedure.) a) potatoes, peeled; b) potato chips, fresh and deep frozen products, potato croquettes, fresh and dry products, potato pan cake, fresh and deep frozen products; c) beef, pork, chicken, turkey, small and large pieces (for roast only large pieces > 500 g); d) pork chop, nature and breaded, meat ball (from pork and beef), meat loaf, pork sausage, chicken breast chop, nature and breaded, turkey breast chop, nature and breaded; e) species not known (Bognár [11]); f) fillet of Florida pompano, grouper, mackerel, red snapper (Gall [16]); g) filet of plaice, fish fingers (Alaska pollack), breaded, deep frozen products; Comparative studies have shown that 190 °C are the maximum fat temperature necessary for both deep frying and frying in a pan, and that below 130 °C browning is insufficient.The temperature of the medium should be adapted to food size, i.e. the larger the diameter of the food to be fried, the lower the temperature.Otherwise surface browning is too fast, while the inner parts are still raw.Optimal temperature, depending on the food and calibration, are between 140 and 190 °G; the quantity of fat in relation to the quantity of food, and fryer capacity must be known (Bognár [7 -9], Piekarski [22], Tilgner [24]).
The results of sensory analyses listed in Figures 2  and 3 show that good to very good sensory quality can be achieved when potatoes, breaded meat, poultry and fish are deep fried or fried in a pan.Medium quality only is achieved from frying in a convection oven, even when the food surface was sprayed with fat; this was due to uneven surface browning and crust formation and to a less pronounced odour and taste.Not all foods are suitable for deep frying.The surface of deep fried chicken legs -unbreaded-is blistered and hard.Deep fried sausages of pork crack.Repeated use of fat for deep frying of potatoes, meat and fish is possible without loss in quality of the food fried; this is in agreement with findings of Moreiras-Várela [20,21] and Várela [25,26].Fat used 20 times did not influence the sensory quality of the potatoes significantly.

Changes of weight and nutrient content
Weight changes during cooking usually relate to the changes of water and fat content of food (Bergstrom [2], Gall [16], Moreiras-Varela [20,21], Várela [25,26]).The relation between weight yield and change of water and fat content for selected foods using different cooking methods are presented in Table II.

Grasas y Aceites
These values are always related to 100 g raw food.According to our experimental results and the literature data the following types of weight changes resulted from cooking: -water and carbohydrate losses in potatoes by boiling -water, fat and protein losses in meat and fish by boiling, braising and roasting -water and fat losses in meat and fish with high fat content (> 5 %) by deep frying and shallow frying -water losses and fat absorptions in potatoes, meat and fish with low fat content (< 5 %) and breaded meat and fish by deep frying and shallow frying.Figure 4 shows the mean weight losses during cooking of selected foods.The highest losses were found in potato chips after deep frying and meat after boiling and braising.The different frying processes had only a little influence on the weight yield in case of potato products and breaded fish, while significantly lower losses were noted for shallow fried breaded meat chops (pork, chicken and turkey breast).Boiling of meat occured significant lower weight yield than deep frying and shallow frying of small pieces of meat.[21], Várela [26], a) potatoes, peeled; b) potato chips, fresh (deep fry and deep fry + roast); c) potato chips, deep frozen products; d) fresh, dry and deep frozen products; e) small and large pieces; f) small pieces (deep fry, shallow fry, roast); g) small pieces, breaded; h) fillet of grouper, pompano, red snapper, mackerel; i) anchovy, hake, fillet of grouper, pompano, red snapper, mackerel (deep fry, shallow fry, roast); k) fish fillet (plaice) and fish fingers (Alaska pollack), breaded, (deep fry shallow fry, roast); Gall et al. [16] found that the mean weight yield for deep fried grouper, red snapper and pompano fillet were significantly lower than the yield with other cooking methods.No significant differences were found between the weight losses of mackerel fillets prepared by any of these cooking methods.The presented data suggest that the weight yield are related to the type and the dimension and also to the coating of food with flour or bread crumbs.
There was an apparent net increasing in the nutrient content of fried food, when compared to the raw food on the wet weight basis.However the content of these nutrients in all fried foods cooking by deep frying, shallow frying and roasting, was, with exception of fat, more or'less lower if it would have been calculated on the 100 g raw food basis.
The retention of protein in cooked food investigated varied from 90 % in boiled meat and 96 % to 100 % in deep fried potatoes, meat and fish (figure 5).The lower retention after boiling and roasting of meat and fish occured mainly due to leaching and dripping ).Our investigation shows results similar to those of Gall [16] and Moreiras-Varela [21].In the study conducted by Moreiras-Varela etal.[21] it was found that the deep frying of hake, beef, pork and sword fish did not change the digestibility of protein (Table III).Moreiras -Várela [21 ], Várela [26] a) potatoes, peeled; b) potato chips, fresh and deep frozen products, potato croquettes, fresh and dry products, potato pan cake, fresh and deep frozen products (deep fry, deep fry+roast); c) beef, pork, small and large pieces; d) beef, pork chop, chicken and turkey breast, small pieces; e) beef, pork chop, chicken and turkey breast, small pieces, breaded; f) fillet of grouper, pompano, red snapper and mackerel; g) anchovy, hake, fillet of grouper, pompano, red snapper and mackerel, fillet of plaice and fish fingers (Alaska pollack), breaded;

Table III Protein digestibility coeffizient of raw and deep fried food (iVIoreiras-Vareia
The change of carbohydrate content was investigated only in potatoes, potato products and breaded meat and fish.The results showed that the retention of carbohydrates varied from 95 % to 100 % depending on the kind of food.No influence of frying methods was found. The results of our investigations and of the literature reviewed about changes of mineral (ash) content in food during cooking are compiled in Figure 6.
Minerals are especially of water soluble potassium easily lost in cooking water by boiling of plant and animal food and dropping by roasting of meat (10)(11)).The losses of minerals (ash) in deep fried foods varied -as the figure 6 shows-from 1 % in potatoes and 26 % in beef and were significantly lower than in boiled foods from the same type.The mineral losses in breaded meat and fish varied after deep frying from 2 % and 8 % and were significantly lower than in deep fried meat and fish fillet without coating.The bread crumbs seems to be absorbed the minerals solved in gravy of meat.
Our findings and literature data about change of fat content in food during cooking are shown, that the fat in food may be lost, gained or remain unchanged after cooking (Figure 7 and 8).
No significant changes was found in boiled and steamed potatoes and vegetables (Bognár [9,14]).The quantity of fat absorption in deep fried potato chips, croquettes and potato pan cakes varied between 4.5 and lOg per 100 g of raw food.No fat absorption was occured during roasting of deep frozen potato chips.The highest fat absorption was found in shallow fried potato pan cakes.

boil, braise and roast (for large pieces of meat) deep try, deep fry + roast (for potatoes DF, DF/CX))
Figure 6 Effect of certain cooking methods on the mineral (ash) content of selected foods -mean values and variations- (Bognár,Zacharias [9], Bognár [11,14], Moreiras -Várela [21], Gall etal. [16]) a) potatoes, peeled; b) potato chips, fresh and deep frozen products, potato croquettes, fresh and dry products, potato pan cake, fresh and deep frozen products (deep fry, deep fry+roast); c) beef, pork, small and large pieces; d) beef, small pieces; e) pork chop, lean and fat, chicken and turkey breast, small pieces; f) pork chop, lean and fat, chicken and turkey breast, breaded, meat ball, meat loaf, pork sausage (deep fry, shallow fry, roast); g) sea fish (species not known); h) fresh water fish (trout, carp); i) anchovy, hake, grouper, pompano, red snapper, mackerel (deep fry, roast); k) fish fillet (plaice) and fish fingers (Alaska pollack), breaded (deep fry, shallow fry, roast); Frying of breaded meat, poultry and fish, no matter wether in a pan or deep fat fried was associated with a fat uptake of 2 to 13.3 g per 100 g of raw food, while meat, and fish, with a fat content of 5 to 20 g per 100 g raw food, lost during boiling or roasting 0.4 to 5 g fat per 100 g raw food (Figure 8).Significant amounts of fat was absorbed from frying fat by the chops of lean pork, chicken and turkey breast and by the fillets of fish with low fat content (grouper, red snapper and pompano).
Our findings and literature data (Gall [16] and Moreiras-Várela [21], Várela [26]) indicate that the gain or loss of lipid from meat and fish is related to the lipid content in raw food and to the quantity of the coating flour or bread crumbs on the food surface.Deep fried, breaded meat, poultry and fish usually absorb less fat than meat, poultry and fish fried in a pan.The kind of fat had no essential influence on fat uptake.
As known, cooked foods are important sources of vitamins in the diet.Research regarding losses during cooking of foods was concentrated especially on the retention of vitamin C and vitamin Bi.Ascorbic acid and thiamine are the nutrient most susceptible to the thermal degradation and leaching from food.
Experimental data concerning changes in the vitamin C-content of potatoes, potato products and green pepper during cooking by different methods have been reported by several authors; they are compiled in Figure 9.The most investigations of deep frying have been done with potatoes since for more than fifty years the preparation of french fried potatoes or chips is common praxis in most countries in the western world (Brubacher [15]).Cooking of potatoes under different conditions shows less vitamin C-loss for deep frying (5 -35 %) and deep frying + roasting followed by boiling (30 %) and stewing (76 %).Nearly the same relations could be shown in green pepper.The high variation of the vitamin C loss during deep frying of potato chips could be occured from the used analytical method.Our own investigations on potato dishes, prepared from different raw products like as fresh or deep frozen, let us conclude that the decomposition of ascorbic acid is depending also on the activity of the peroxida-seenzym in the raw product.Vitamin C-retention in potato chips, pan cakes and croquettes, prepared from deep frozen and dry products resp.was nearly 100 % since these products did not contain any peroxidase activity.
The losses of vitamin Bi during cooking of food ranged from 0 % to 10 % in potatoes and vegetables, 10 to 70 % in meat and fish (Figure 10).According to the cooking methods, meat and fish suffered quite several losses by boiling, braising and roasting, while after deep frying and shallow frying the losses were significantly lower in meat, breaded meat and fish respectively.Coating of meat and fish with bread crumbs effected also for thiamine a better retention during deep frying and shallow frying., Bognár [10,11,13,14], Brubacher [15], Miuccio [19]) a) potatoes, peeled; b) potato chips, (fresh and deep frozen products, potato croquettes (fresh and dry products), potato pan cake (fresh and deep frozen products); c) all varieties; d) green pepper, egg plants, veg.marrow; e) green pepper, egg plants, squash summer; f) small and large pieces; g) beef, beef ball, pork chop lean and fat; h) pork chop lean and fat, breaded, meat ball, meat loaf, pork sausage; i) chicken whole; k) chicken leg, chicken and turkey breast, small pieces; I) chicken leg, chicken and turkey breast, small pieces, breaded; m) mackerel, octopus, tinea, tuna; n) fish fillet (plaice) and fish fingers (Alaska pollack), breaded; B = boil; B/CO = boil + roast; DF/CO = deep fry + roast; SF = shallow fry; B/SF = boil + shallow fry;

Grasas y Aceites
The changes of vitamin B2 content during cooking of food ranged between 70 % losses and 240 % increases (Figure 11).The losses during boiling of potatoes, vegetables, meat and fish were occured mainly by leaching (10)(11)).The increases of riboflavine content during deep frying, shallow frying and roasting of potatoes, breaded meat and fish were significant.The gain of riboflavine is caused probably by generation of riboflavine from riboflavine precoursors during cooking.The kind of precoursors are not yet known and has to be elucidated in further research.Brubacher [15] found in sea pike also a gain of riboflavine during frying.He described it as probably sampling errors.a) potatoes, peeled; b) potato chips, potato pan cake (fresh and deepTfrozen products), potato croquettes (fresh); c) potato croquettes from dry product (enriched with 3-5 mg riboflavine per 100 g); d) all variety; e) green pepper, egg plants, squash summer; f) beef, pork, small and large pieces; g) beef, pork, lean and fat, chicken and turkey breast, small pieces; h) pork, lean and fat, chicken and turkey breast, small pieces, breaded, meat ball, meat loaf, pork sausage; i) species unknown; k) sardines (-34%), sea pike (-i-44%); I) fish fillet (plaice), fish fingers (Alaska pollack), breaded, balls of sea pike; B = boil; BR = braise; CO = fry in oven; DF = deep fry; DF/CO = deep fry + roast; SF = shallow fry; In natural food vitamin Be is present as pyridoxamine (PM), Pyridoxal (PL) and pyridoxin (PN) either free or bound to phosphate, proteins, aminoacids and glucosids (Bognár [12]).Data about the influence of cooking on the content of individual vitamin Be compound are rare.In the present work the effect of different cooking methods on the individual vitamin Be dérivâtes and the total vitamin Be in selected food were investigated by using a HPLC analytical method (Bognár [12,14]).
The total content of vitamin Be was on average 350|Lig in potatoes, 306 ¡ig in green pepper, 67 ¡xg in egg plants and 119 \xg in squash summer (Bognár [14]).Potatoes and vegetables contained besides PM and PL also a high level of pyridoxine.Cooking of potatoes and vegetables under different conditions have shown less vitamin Be-loss by deep frying and stewing followed by boiling (Figure 12).Our findings confirm with literature date (Zobel [30]) according to which leaching loss due to the high water solubility of the vitamin Be dérivâtes is high.Due to degradation, but also conversion into pyridoxamine, pyridoxal has been found to be the least stable vitamin Be compound in plant foods.
Raw pork contained on average 400 ¡xg vitamin Be per 100 g, chicken breast 790 \xg and turkey breast 640 |ig respectively.25 % to 50 % of total vitamin Be were PM, 40 % to 60 % were PL and 5% to 10 % PN.Boiling and braising reduced the content of vitamin Be by 50 % in pork and by 30 % in poultry and fish, while deep frying caused losses of 25 % in breaded pork and 25 % in breaded fish only.
Part of pyridoxal which was degraded nearly completely under the heat influence was transaminated into pyridoxamine.Of pyridoxine nearly 100 % were retained.A relation seems to be exist between the extent of the vitamin Be-loss and pyridoxal content of the raw food; the higher the PL content, the higher the thermal loss.

1
Cooking time and sensory qualityFrying time is influenced by -temperature of the heating medium -layer thickness and geometric dimensions of the food -initial temperature of the food -desired cooking degree Cooking time for selected foods and the cooking conditions are shown in Figure1.

2 Figure 3
Figure 2 Sensory quality of potato and meat dishes depending on the cooking methoa -mean values and variations-(Bognár, Zacharias [9]) a) fresh and deep frozen products; b) fresh and dry products; c) fresh and deep frozen products; d) lean and fat, small pieces;

Table II
Influence of certain cooking methods on the weight and on the content of water and fat of selected food (Bognár [9], Gall et aL, [16]) a) Weight of raw food -yield Content per 100 g of cooked food 100 c) Content per 100 g raw food -C [-= loss; + = absorption