Roasting and browning vegetables as a method of heat treatment of raw materials
In the production of snacks, canned food, first and second lunch dishes, canned food for public catering, etc., in order to increase the nutritional value and impart certain taste qualities to the product, frying or browning eggplant, zucchini, beets, pumpkin, carrots, sweet peppers, onions, etc.
Roasting is the heat treatment of vegetables in fats until the weight of raw materials is reduced by more than 30% at a certain temperature regime.
Passing- roasting vegetables with a weight reduction of up to 30%.
Roasting or browning carried out in vegetable oil or animal fat in roasting ovens, on Krapivin stoves at relatively high temperatures. Vegetable oil or animal fat in this case not only performs technological functions, but is also an intermediate heat carrier that transfers heat from the heating surface of the oven to the product.
Duration of roasting and browning depends on many factors and primarily on the type of vegetables, the degree of grinding, the temperature of the active layer of the oil, the method of roasting, the initial and final moisture content of the product, etc., as well as the specific heating surface (the size of the heating surface per 1 m2 of the oven mirror) and makes for vegetables 5-16 minutes.
For each specific case, the duration of roasting is determined empirically. It must provide the true percentage of roast, established by the norms and requirements for the quality of the roasted product.
For frying and browning, refined vegetable oils are used - sunflower, corn, cottonseed and soybean oils, ghee, beef, lamb or bone fats, margarine, butter or ghee. When choosing a fat for a specific technological process, its biological value, organoleptic properties and physicochemical indicators are taken into account. The most important of these indicators are the melting and solidification points, refractive index, viscosity, specific gravity, acid, iodine, peroxide and acetyl numbers. Taste, odor, color, transparency, presence of sediment and consistency of fats are of great importance both for the quality of the finished product and for the correct implementation of the roasting and browning process.
The process of roasting and browning vegetables is a complex complex of physical, chemical, physicochemical and technological phenomena, complicated by heat and mass transfer and oil absorption.
Under the influence of heat, a number of interconnected physical and chemical processes occur in the product, as a result of which the release and removal of part of the moisture, oil absorption, volumetric shrinkage of the product, the release of gases, an increase in pressure inside the product, an increase in porosity, and a change in the density and heat capacity of the product.
In the process of frying, the proteins of the protoplasm of the cells coagulate, the cells shrink, the intercellular passages increase, the product decreases in volume 2-3 times. Carbohydrates also change: starch partially turns into dextrin, sugars are caramelized, protopectin turns into pectin, the product becomes soft and easily digestible. The tissue structure and density of vegetables change.
During frying from the surface of vegetables and root vegetables loaded in hot oil moisture evaporates... Since the concentration of moisture in the inner layers is higher than on the surface, the dry matter content in the surface layers is constantly increasing; due to the difference in concentration, moisture diffuses from the inner layers to the outer ones.
The temperature at which roasting and browning should be carried out is chosen so that the evaporation of moisture from the surface is somewhat ahead of its entry from the inner layers. Then, after some time, the surface layer is dehydrated, a golden crust forms and the product receives a specific taste and smell characteristic of fried. Crust formation occurs due to the initial stage of carbohydrates caramelization - sugars, starch, cellulose, pectin contained in the fried product.
This happens when the moisture content of the product in the surface layer drops so much that it allows the temperature to rise above 100 ° C.
Excessively high temperature moisture from the surface layers evaporates very quickly, the surface of the product begins to char, and the inner layers remain damp, since moisture from the inner layers does not have time to enter the evaporated place. At high temperatures, deep decomposition and caramelization of carbohydrates occur, which is associated with a deterioration in the color and taste of the product.
At the same time, oil deterioration processes are accelerated. Oil spoilage during frying largely depends on its initial properties, and in order to improve the quality, vegetable oils are refined, deodorized, hydrogenated (the content of unsaturated fatty acids in sunflower oil should be no more than 0.3-0.4%, in cottonseed oil - not more than 0.2-0.3%). The iodine color for sunflower oil is 10-12%, for cottonseed oil - 8-16%, iodine number - respectively 125-145, 104-116. For frying, refined sunflower or cottonseed oil of at least I grade must be used.
At a lower frying temperature the processes of evaporation and diffusion are balanced, the crust is formed very slowly or not at all. But the inner layers of the product are digested and become loose. The taste of such a product is low.
The issue of obtaining a fried product of proper quality, which would harmoniously combine such indicators as visible frying, mass fraction of dry matter, fat, taste, aroma, appearance, is rather complicated. Therefore, in general, the roasting regulations are established taking into account all the above factors and the readiness of the product is judged by the appearance and taste, as well as by the percentage of frying and the percentage of oil absorption * These indicators are standardized for each type of vegetables and type of product.
The percentage of roast is distinguished between visible and true.
The visible percentage of roasting shows the percentage reduction in the mass of raw materials when
To determine the visible percentage of frying, the required amount of raw material is weighed, loaded into a grid, fried, allowed to drain for 3 minutes, weighed again and the mass of the previously weighed container is subtracted.
The value of the visible percentage of roasting is used to control production, as well as in technological calculations to determine the consumption rates of raw materials per unit of finished product.
The term “visible” means that this change in the mass of the roasted raw material is visible when weighed on a scale, although this weight loss is not true.
The true percentage of roast shows the actual loss of moisture during frying as a percentage of the raw materials, i.e. it takes into account that some of the moisture is replaced by oil absorbed into the product during frying, therefore the true percentage of roast is always greater than the visible one.
Depending on the type and purpose of raw materials, the visible percentage of roasting ranges from 17 to 50, and the true percentage ranges from 24 to 64. Oil absorption (to the mass of the fried product) for most types of raw materials is 7-13%, in some cases these figures are much higher ( 27% for onions, 17.5% for a mixture of carrots, white roots and onions).
Frying vegetables in hot vegetable oil is carried out in several different ways. The most widespread method is deep frying, when the product is completely immersed in oil. Less often, they are fried in a thin layer, when only part of the product is immersed in oil.
The advantage of the deep layer frying method is the ability to easily mix and move the product, transfer the heat required for frying over the entire surface of the product piece, the disadvantage is the need for a large volume of oil, and less moisture evaporation rate compared to other methods.
Roasting of vegetables is carried out at a certain temperature, which is different for different types of vegetables. The maximum temperature for frying eggplants is 135-140 ° C, zucchini - 125-135, root crops - 120-125, onions - 140 ° C.
The duration of roasting depends on the type of raw material, the percentage of roasting, the temperature of the active layer of the oil, the specific heating surface of the oven, etc. and is 5-16 minutes for vegetables.
Frying raw materials in oil at a low temperature is not recommended, since this increases the duration of the process, decreases the productivity of the furnace, which leads to a decrease in the oil changeover rate and worsens the indicators characterizing the quality of oil and finished products.
Fresh vegetable oil always contains a small amount of water. Water is removed from the oil by calcining before loading the product into it and conducting the frying process in order to avoid foaming and ejection of oil from the oven. The sunflower oil is calcined at a temperature of 160-180 ° C, cottonseed oil at 180-190 ° C until foaming stops. The duration of calcination depends on the moisture content of the oil and generally does not exceed 1 hour. If this is not done, the bubbles of water vapor released during frying form a very stable foam due to the content of proteins, pectin and other foaming agents in the product. Calcining the oil is also mandatory for the sake of operational safety, rational use of oil, preservation of its quality, and proper conduct of the roasting process. Before use, the oil is filtered through a stainless steel sieve with a hole diameter of 0.8-1 mm.
After calcining, meshes with pre-prepared vegetables and roots are loaded into the oven. The roasting process is a complex technological process. As already described above, under the influence of heat in the product, a number of interrelated physical and chemical processes occur in raw materials and oil. The transfer of moisture and heat in a product is a single process associated with external heat and mass transfer. The quality of the roasted product and the rational consumption of oil depend on the correctness of the roasting process. Many years of experience have established that with an incorrect organization of the technological process, the quality of the oil rapidly deteriorates and after 3-4 days it becomes completely unsuitable for food purposes and must be transferred to technical needs. The deterioration of the quality of vegetable oil leads to a sharp decrease in the quality of the raw materials fried in it.
The quality of oil during frying changes under the influence of various factors: high temperature of water vapor emitted from raw materials during frying, air in contact with oil on a large surface, cutting quality of vegetables and root crops, continuous operation, full load of the oven with product, oil level in the oven, the level of the water cushion bordering on the oil and leading to the formation of an oil emulsion.