Presentation on theme: "Frying Mass Transfer Water in a frying food migrates from the center to the surface. As water is removed at the surface due to heating, water is 'pumped'"— Presentation transcript:
Frying Mass Transfer Water in a frying food migrates from the center to the surface. As water is removed at the surface due to heating, water is 'pumped' to the surface. The rate of water loss and its ease of migration through the product are important to the final characteristics of the food. Heat Transfer Water evaporation from the surface of a frying food also removes heat from the surface and inhibits charring or burning at the surface. The heat of vaporization of water to steam removes much of the heat at the food/oil surface. Heat Removal As long as water is being removed at a sufficient rate, the surface of the food will not char. Subsurface water in the food will also conduct heat away from the surface and towards the center of the product.
Frying Interior Cooking The transfer of heat to the interior of the product by water will result in cooking of the interior of the food. Want enough heat to 'cook' the product, but not enough to cause damage - example -French fry Oil - Food Interactions Ideally the food products should have similar dimensions and thus, similar surface to volume ratios. Once an equilibrium is established all processes should be the same unless there are changes in equipment function or in oil composition. Oil The properties of oil change with frying. New oil has a high heat capacity that diminishes with use. Other factors such as viscosity may change dramatically with use
Food Oil Steam Food
Frying For a given product, there is an optimum amount of fat absorption, determined as the "best tasting product.“ Manufacturers of fried foods establish a desired value (or range) for absorbed fat (part of the finished product specifications), using some combination of tradition, recommendations, and sensory panel testing. The three main determinants of degree of fat absorption are: –· Frying temperature –· Concentration of surface-active agents –· Water binders in the food.
Frying Temperature. –If the fat temperature in the fryer is too low, frying time is often extended to achieve the desired depth of crust coloration. –However, this also allows more time for fat to penetrate into the interior of the product. –Also, the pressure of the steam being generated is less than optimum, which also makes it possible for more fat to penetrate. –We have all undoubtedly had the unhappy experience of being served pale, greasy french fries. This situation invariably means that the cook is not keeping good watch on the fryer thermostat (or that the fryer is being overloaded).
Frying Surface-active agents. –Emulsifiers promote breakdown of the steam barrier, and this allows the oil to wet the solid food surfaces. –Both of these effects increase the absorption of fat. –In some cases (e.g., cake doughnuts), a small amount of a wetting agent such as lecithin is added to the batter to help achieve the desired level of fat pickup. –Also, free fatty acids generated by fat hydrolysis and oxidized fat components (see below) are surface active and promote fat absorption.
Frying Water binders. –Soluble proteins, gelatinized starch, gums and other fibers, and similar water-binding ingredients decrease fat absorption by keeping more water in the system. –This property is used in formulating batters and breadings for coating foods that are to be deep-fried. –Studies have shown, for example, that the inclusion of 3% cellulose fiber in a doughnut formulation markedly decreases fat absorption.
Frying - Stages of oil Oil StageDescriptionTri Glycerides Polar fatPoly mers Free Fatty Acids (%) New fatNo surface-active materials >98< Break-inSome surface-active materials FreshBest for cake doughnuts OptimumBest for meats, french fries, vegetables, etc DegradedMuch surface-active material; is easily absorbed RunawayVery degraded
Frying - Stages of oil Break-in oil. White product, raw, ungelatinatized starch at center of fry; no cooked odors, no crisping of the surface, little oil pickup by the food. Fresh Oil Slight browning at edges of fry; partially cooked (gelatinization) centers; crisping of the surface; slightly more oil absorption. Optimum Oil Golden brown color; crisp, rigid surface; delicious potato and oil odors; fully cooked centers (rigid, ringing gel); optimal oil absorption. Degrading Oil Darkened and/or spotty surfaces; excess oil pickup; product moving towards limpness; case hardened surfaces. Runaway Oil Dark, case hardened surfaces; excessively oily product; surfaces collapsing inward; centers not fully cooked; off-odor and flavors (burned).
Frying - Quality of oil Indicators of frying oil quality: –Total polar compounds –Conjugated dienes –FFA –Dielectric constant –Color –pH –Sensory –Smoke point, Fire point, Flash point
Frying - Quality of oil Flavor Reversion –Special type of oxidative deterioration characterized by an objectionable flavor prior to the onset of true rancidity –May develop during exposure of fat to ultraviolet or visible light or heat –Reverted soybean oil described as “painty”, “beany”, “haylike” or “grassy” and in final stages as “fishy” –Linolenic acid most common source
Emulsifiers Hydrophillic-Lipophillic Balance (HLB) This is a concept for choosing emulsifiers. The value of HLB ranges from Low HLB emulsifiers are soluble in oil while high HLB emulsifiers are soluble in water. Bancroft's rule tells us that the type of emulsion (i.e. oil in water or water in oil) is dictated by the emulsifier and that the emulsifier should be soluble in the continuous phase. Low HLB emulsifier's are soluble in oil and give rise to water in oil emulsions.
Emulsifiers – HLB level SolubilityHLB Range No dispersability in water1-4 Poor Dispersion in water3-6 Milky appearance6-8 Stable milky appearance8-10 Translucent to clear dispersion10-13 Clear solution13+
If you decrease the size of fat globules then you need more emulsifier to fully coat the surface.
Stokes' Law Creaming or sedimentation is proportional to: –1. Diameter of the particle squared –2. Difference in density between the particle and the continuous phase And inversely proportional to: –3. Viscosity of the continuous phase Rate = [Diameter squared x density difference x g] / [16 x viscosity] How can we change 1? 2? 3?
Use of emulsifiers For Emulsions, If You Don't Have A Clue, Use At 5% Of The Fat. Use Unsaturated Emulsifiers With Unsaturated Fats. Mixtures Work Better Than A Single Emulsifier When Stabilizing Foams And Emulsions. Bancroft's Rule –Emulsion Stability Is Favored By Solubility In The Continuous Phase –i.e. High HLB----> oil/water –Low HLB-----> water/oil HLB and most other rules go out the window when protein and (sometimes) polysaccharides enter the system. Only saturated monoglycerides complex with starch. Emulsifier forms affect functionality. –flakes vs powder vs hydrates vs gels
Use of emulsifiers Many functions are due to affects on polymorphism. Emulsifier preparations frequently contain unsaturation and may be an important contributor to off-flavors. Emulsifier preparations are seldom pure and thus variation from manufacturer to manufacturer may be substantial. When you find a non-obvious usage of emulsifiers, the function is often related to interaction with starch or protein. Order of addition may be very important. Processing steps like homogenization may substantially change the function of emulsifiers.
Fat replacers Functions of Fats in Foods Plasticizer Mouthfeel Appearance Carrier for flavors Carrier for –Fat Soluble Vitamins –Carotenoids –Essential Fatty acids Heat Transfer
Fat replacers Water Carbohydrate Protein Modified fats Synthetic fats Reference ADA.1998.Position of the American Dietetic Association: Fat Replacers. J. Amer. Dietetics Assoc. 98(4):463-8
Fat replacers Water –Cheap –Label friendly –Usually free from regulatory constraints –Doesn’t effect calories from fat
Fat replacers Caprenin –Triglyceride containing: –Behenic (22:0) –Capric (10:0) –Caprylic (8:0) Used in the failed "new" Snickers Not successful because of a surprised increase in LDL
Fat replacers Salatrim aka Benefat –Short And Long Acyl Triglyceride Molecule Triglyceride containing –Stearic (18:0) and –Acetic (2:0) and/or –Propionic (3:0) and/or –Butyric (4:0) Salatrim – calories per gram –Reduction based on: –Short chain fatty acids –~50% absorption of stearate
Fat replacers Olestra –Sucrose polyester –6-8 fatty acids –Lipase sterically inhibited Olestra problems –Anal leakage –Gastric distress –Stripping of fat soluble vitamins Olestra Advantages –No calories –Can be used as frying medium –Now approved for snack foods
Fat replacers Use of fat replacers –Conclusion There are lots of tools Only sensory equivalent products will survive Potential to reduce calories, but it is likely that calories in the diet will be unaffected