The heat and mass transfer processes during the vacuum frying of potato chips

The heat and mass transfer processes during the vacuum frying of potato chips NguyễnThịMỹChinh Nong Lam University Author note Nguyễn Thị Mỹ Chinh, Food Technology, Nong Lam University Nguyễn Thị Mỹ Chinh is studying Heat and Mass transfer principles project, Nong Lam University This research was supported the Heat and Mass transfer principles project to know more heat and mass transfer processesduring the vacuum frying of potato chips. Contact: mylienanlene@gmail.com Abstract A heat and mass transfer were two processes which occurred during vacuum frying potato chips. The heat transfer process included the convection of heat from the surface to the product and the conduction of heat into the product. The mass transfer process was divided into two periods that are water loss and oil absorption. Oil absorption rate was related to the moisture loss rate during the vacuum frying process.From the heat and mass transfer processes, the increase in temperature of the product toward the boiling point of water (at the associated pressure) which followed by its steady increase toward the temperature of the oil was validated create the movement of water from the material and the osmotic of oil into the ingredients. These processes caused the modified physics and chemistry of potatoes and oil during frying. And the quality of potato chips also was affected by heat and mass transfer. Key words:heat transfer, mass transfer, vacuum frying, potato chips Introduction Potato chips fried which is one of the most common products that can be found in everywhere. However, thechemical is formed when frying potatoes at high temperatures may increase the risk of harm to human health. In other to demand the rising health concern and the trends toward healthier snacks, vacuum frying may be an option for production of fruits and vegetables with low oil content and the desired texture and flavor characteristics. Because deep-fried food occupy a large part of the food industry. The big problem with it is the high fat content that deep fried products have. Vacuum frying is a reasonably new technology which uses lower pressure and temperature rather than atmospheric deep-fat frying to improve the quality attributesof food products. That is the technology reduces the fat content without compromising the edible quality which hasa large industrial potential today. Purpose Vacuum frying is an alternative method to the deep frying that offers the health benefits of humans. Vacuum frying offers an alternative way to improve the quality of fried fruit and vegetables other than by atmospheric frying (Dueik and Bouchon, 2011a). The main factors that influence fried products are the frying time-temperature combination of the cooking process; the correct combination of which is necessary to produce a food product with acceptable physical attributes (Andres-Bello et al., 2011). Dueik and Bouchon (2011a) showed that vacuum frying significantly lowered the final oil content in comparison to atmospheric fried and it also slowed the rancidity of the oil. Most of the benefits of vacuum frying from the use of low temperatures and the minimal exposure to oxygen reduce the adverse effects on the oil quality (Shyu et al., 1998). The vacuum frying can reduce oil content in the fried product. Because the oil is absorbed less due to the difference between the pressure inside of product and the pressure vacuum frying can eliminate the oil sticks on the surface of the product in the final stages of frying.(Garayo& Moreira, 2002). The product can keep the natural color, flavors(Shyuand Hwang, 2001) and limit the loss of vitamins and minerals when fry in vacuum condition that are beneficial of the product due to the low temperature and oxygen content during the process (Da Silva và Moreira, 2008). Garayo and Moreira and Tychy (2004) demonstrated that vacuum frying could produce fried starchy products with a much lower acrylamide content than their counterparts fried at atmospheric pressure. Method The potato chips have to be processed through steps: Classification -> Washing ->Peeling ->Cutting ->Cleaning ->Blanching -> Dehydration->Vacuum frying -> Deoiling -> Cooling -> Mixing -> Packaging To frying potato slices in vacuum condition, we used the vacuum fried equipment.When boiling oil, the raw material was enclosed into a specialized pressure vessel containing a deep fryer. The vessel was connected to a vacuum pump which was the equipment used to generate vacuum. Vacuum pumps generally can be considered as compressors in which the discharge, rather than the intake, is at atmospheric pressure. The vacuum in a chamber is created by physically removing air molecules and exhausting them from the system. Removing air from the enclosed system progressively decreases air density within the confined space, thus causing the absolute pressure of the remaining gas to drop.A vacuum environment is created due to the absolute maximum pressure difference that can be produced is equal to atmospheric pressure. After this the vessel depressurized. The raw material was immersed into frying oil within the vessel, fried at 120-160 ° C for 5-20 minutes based on the size, thick slices of potato, fried furnace temperature and the amount of material.The potatoes completely submerged in oil andthe materials had to lower than the surface of the oil from 5-10 cm. During the frying process, we check the quality of the oil. If the index of acid is greater than four, we have to change the oil. If there are not burning smells, dark color, you can continue to fry, but we can not fry when the index of acid exceeds five. Fig.1. Schematic of the vacuum frying system To determine the time stop frying, we rely on appearance, taste and level frying of products. In assessing the level frying, we evaluated by two level frying: The apparent frying: the ratio between the decrease of material after frying and the material before frying. It is used to check or calculate the indicators, technical and economic norms and yield of equipment. A: the mass of material before frying B: the mass of material after frying The fact frying: the total of the apparent frying with the oil which is absorbed into the material. It is the evaporation of material after frying. M: the ratio of oil is absorbed into the material Finally the vessel is re-pressurized again. The repressurization should be done quickly. A centrifugation step can also be inserted for removal of excess frying oil after the frying step before the pressurization. The heat and mass transfer processes during the vacuum frying of potato chips The vacuum frying is the frying process that is carried out under pressures well below atmospheric levels, preferably below 50 Torr(6.65 kPa). Due to the pressure lowering, the boilingpoints both of the oil and the moisture in the foods arelowered (Garayo and Moreira, 2002). The heat and mass transfer processes during vacuum frying of potato chips was reported by Yamsaengsung et al. (2008). The heat transfer process included the convection of heat from the surface to the product and the conduction of heat into the product. The mass transfer process was divided into two periods that are water loss and oil absorption. The heat transfer process The convective heat of the oil and the product surface Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. The heat transfer of the oil and the product surface are forced convection due to the temperature increases in vacuum pressure condition. Under the impact oftemperature, heated oil expands and becomes more buoyant. Cooler, more dense oil near the surface descends and patterns of circulation can be formed, causing convection currents which transported energy. Therefore warmer oil rise, cooler oil drop and replaced the warmer oil. After oil is boiled at a certain temperature, potato slices are put into the oil. Heat is transferred by the direct collisions between the molecules on the surface of the potato slices and the motion-oriented moleculesof oil.The equation for the heat transfer per unit surface through convection can be expressed as: q = hc A ∆T         (1) where: q = heat transferred per unit time (W) A = heat transfer area of the surface (m2) hc= convective heat transfer coefficient of the process (W/(m2K) or W/(m2oC)) ∆T = temperature difference between the surface and the bulk fluid (K or oC) The conduction of heat into the product Conduction is the transfer of heat in the form of the collisionsbetween neighboring molecules. So after the heat transfers on the surface of potato slices, the heat continues to transfer by colliding between the molecules on the surface and molecules inside.Due to the conduction of heat transfer into the product, the temperature inside of the potato slices increases and the water of potatoes is evaporated. The loss water of potatoes caused loss heat, but it is not considerable.The equation for heat transfer inside potato slices is: q =Qt = kAt2-t1z q = heat transferred per unit time (W) k = thermal conductivity of product A = heat transfer area of the product T2 = the temperature of the product surface T1 = the temperature of the product The mass transfer process Water loss The water loss process of foods is generally characterized by three distinct periods. The first was an initial heat-up period during which the wet solid material absorbed heat from the surrounding media. The product was heated up from its initial temperature to a temperature where the moisture begins to evaporate from the food. In vacuum frying, this initial heat-up period is very short and therefore difficult to quantify. Frying temperature significantly affect the moisture content. One of major difference between potato slices fried under vacuum and atmospheric conditions is the surface structure of the potato chips form during the frying process. Because when the temperature at the potato's surface increase which reaches to evaporate and the moisture at the surface is out very strong. Due to the conduction of heat transfer into the product, the temperature inside of the potato slices increase which promotes the process of moisture transferring inside the material. The process of evaporation at the surface occurs simultaneously with the process which the moisture transfers from inside the potato to outside the surface. The water evaporation creates expansion of potato and some bubble on the surface of potato. The surface of a vacuum fried potato chips have less expansion and numerous small bubbles, as oppose to a potato chip fried under atmospheric pressure, which show more expansion and lesser but larger bubbles. The bubble formation at the potato's surface is the results of gas expansion inside the pores. For the vacuum fried chips, once the fryer is evacuated, the water vapor in the pores expands with little resistance even before the product is fried. During frying, little expansion may be produced by the superheated vapor trying to escape the pore space, therefore the vacuum fried chips lose more water than the chips fried under atmospheric conditions, and the water in the material usually gets out about 30-50%. Oil absorption The beginning of the frying period, the water in the potato chips evaporates quicker under vacuum conditions and it is less vigorous. The molecules oil will move into the materialabout 10-13% compared with the final product. This oil contributes to increase the temperature of the potato slices and decrease the hydrogen bonding of water with hydrophilic particles, allowing the separation of water occurred more easily. The vacuum pressure level affect the oil content of potato chips fried. Higher vacuum levels lead the pores on the surface of potatoes (due to loss of the characteristic hidrophilicity of raw potatoes) to develop rapidly. During this period, the capillary pressure is negligible (Moreira, CastellPerez, &Barrufet, 1999), so there is no driving force for the oil to flow into the chip’s pores. Vacuum pressure affects the speed at which moisture leaves the potato chips. The rate of evaporation and osmotic of oil will affect the shape and the size of the product. The rate of evaporation is increase, the volume of cell is increase, the material will be expands. After frying, the potato chips undergo the cooling period the surface oil that adhered to the surface of the potato chips into the pores of the chips (Moreira &Barrufet, 1995).If the absorption rate of oil is fast enough to occupy position where the water has evaporated, the final product will be porous and brittle. If the absorbed rate of oil is slow, the final product will reduce volume, the texture will be hard brittle and less porous. During vacuum frying, two process has approximately the same speed so the final product is often little change in size, good crispness and porous. The heat causes the cell volume increases, the water release will increase the concentration of dry material, the cell structure is destroyed, the oil absorbs into the cell. The modified quality of potatoes and oil during vacuum frying The frying process is the heating process which has the heat transfer substance is oil. The frying process occur heat transfer process and two mass transfer processes: the movement of water from the material and the osmotic of oil into the ingredients. The motivation of these two processes is the difference in temperature - humidity environment between oil and raw materials. The modify of raw materials Physical properties The change in state structure and volume due to the water loss and oil absorption are expressed through the phases: The first phase, the ingredients are put in the frying oil, the temperature of the potatoes is much lower than the temperature frying oil, so the potatoes have a temperature rise towards equilibrium temperature with the oil temperature. At this stage the state and the shape of the potatoes are not significant change. The evaporation of water decreases the humidity on the surface of potatoes rapidly and creates gas bubbles on the surface so that the potato pieces do not stick together. The rapid evaporation of moisture on the surface of potatoes also limits the penetration of oil into the potatoes. The second phase, the surface is dehydrated and swelled, the oil enter inside the product makes the water evaporate and leads change the structure of potatoes. The surface continues to be lost water leads to create the hard shell. The higher the temperature frying, the thinner and lighter the hard surfaces. The temperature of surface equals near oil temperature when potatoes ripe. Low humidity and high temperature catalytic reaction forms color and flavor in products and create a crust and dark shell. At the final state, after the frying process, oil is absorbed into porous structure of the product or in the gaps created in the potatoes due to condensation inside. The oil content in the potatoes increases, but most of them focus on the product's surface. The potatoes will become more porous when remove oil by centrifugal equipment. Chemical properties Proteins in the material are coagulated to form solid frame, the oil will occupy position of water in the frame. When we use centrifuge to separate the oil, the oil in the frame will be replaced by air to create the crisp, porous products. During vacuum frying, the water evaporates faster than the protein which is not entirely coagulated to form the frame, so the expansion possibility of raw material and the crisp of product are increase. And due to low oil pressure, so the oil boils at a low temperature. On the other hand, due to the large frame expander, the centrifugal process separates the oil easily. Therefore, the less fat fries, the less toxic. Lipids is modified, sugars and starches in the surface is caramel. Protopectin is hydrolyzed into soluble pectin, so the potatoes become soft (relative level). Maillard reaction creates color, odor, taste the products, some products which including acrylamide. Chlorophyll turned into pheophytin, and carotene less decomposed. It is more soluble in hot oil and make oil has orange color. The organic dissolve fat-soluble vitamins then transferred into the oil. Vitamin B1, B2 little damage, but vitamin C is destroyed 7-18%. The aromatic and ester also lost when fried. The modify of oil During frying, the oil is denatured by the effect of high temperature and long time, the effect of water release, the mixing of substances glucide, protid, lipids, forming emulsion, exposed on the open air and the heat transfer. The viscosity of oil increase due to the nutrients in raw material moving out, the color of oil is darker. The oil will be hydrolyzed, oxidation and polymerized: Hydrolysis process occurs quickly in high pressure and temperature conditions with the presence of steam. But in fact, the process takes place under normal pressure conditions, so the speed of hydrolysis is slower. Hydrolysis reaction form diglyceride, monoglyceride, glycerin and fatty acids. Glycerin continues to be hydrolyzed into acrolein and water. Oxidation process is the process occurs at high temperatures with the participation of air. It makes iodine index decreases due to the unsaturated oil decreases, the acid index increases, for some amino acids are released from the glycerides molecules. It also creates complex substances, which is volatile, and most of non-volatile polymer. The complex substances participate in the formation of taste or change the nutritional value of the products. But in fact, the oxidation is significantly reduced due to because the vacuum pump removes air from the enclosed system. The process occurs when heats the oil at high temperature in non air environment is polymerization process. So the triglycerides of oil will polymerize to form polymers. In fact polymerization occurs less due to the air is always in the process of frying. However, the potatoes which are fried in vacuum conditions exist less oxygen so the polymerization process is still taking place. The formation of polymers reduce iodine index of oil like oxidation process, they not only increase the taste but also reduce the oxidation. Conclusion The main purpose of using vacuum frying was a capacity for production of low oil content potato chips. Oil absorption rate during vacuum frying of potato chips was related to the moisture loss rate. The highest drying loss rate (thus the highest the oil absorption rate) was obtained with the highest oil temperature and lowest vacuum pressure. The faster the water loss rate, the higher the oil adhesion at the chips surface and then the higher oil absorption. In addition, as the percentage of free water was depleted in the product, less oil was absorbed. The de-oiling step played an important role in reducing the oil absorption during vacuum frying. Frying at vacuum conditions did not affect significantly the color, the texture characteristics and nutritional value of the chips at the end of frying. 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