Heat Preservation

Introduction Cooking is the transfer of heat energy from some source to the food Top 3 ways to heat in kitchen stovetop conventional oven microwave oven Different methods of heat transfer

Conduction Heat transferred through direct contact Only the flat surface of the pan is hot enough to cook anything Pans are made of metals - conduct heat efficiently & do not melt on the stovetop Requires less time

Convection Heat transfer through a fluid Liquid or gas Oven is a confined area that gets hot by flames or electric coils Cooks the food from all directions Ovens heat foods from the outside in Requires more time

Radiation Transfer of heat using electromagnetic radiation Microwave oven

Microwave Oven – History http://www.ideafinder.com/history/inventions/microwave.htm

Microwave Oven Microwave ovens work by spinning water, fats, sugars & oils inside food Causes friction, which then heats the food & cooks it from the inside Fastest method

Microwave Acceptable dishes for the microwave: Paper, plastic, glass, microwaveable dishes Unacceptable dishes for the microwave: Metal, dishes with gold or silver on them Cooking Tips – see handout

In Review http://www.wisc-online.com/Objects/ViewObject.aspx?ID=SCE304

Degrees of Preservation & Thermal Death Curves

Heating Foods Kills some microorganisms, destroys most enzymes & improves shelf life Does not preserve a food indefinitely Creates 4 degrees of preservation – depending on product Sterilization Commercial Sterility Pasteurization Blanching

Overview of Heat Treatments Comparisons Mild Severe Aim Kill pathogens; reduce bacterial count Kill all bacteria; food will be commercially sterile Advantages Minimal damage to flavor, texture, nutritional quality Long shelf; no other preservation method is necessary Disadvantages Short shelf life; another preservation method must be used, such as refrigeration or freezing Food is overcooked; major changes in texture, flavor, nutritional quality Examples Pasteurization, blanching Canning

Sterilization Complete destruction of microorganisms At least temps of 250 F (121 C) for 15 mins

Commercial Sterility All pathogenic & toxin-forming organisms have been destroyed End products may contain viable spores, but they will not grow under normal conditions

Pasteurization Low-energy thermal process with two main goals 1. destroy all pathogenic microorganisms that might grow in a specific product 2. extension of shelf life by decreasing number of spoilage organisms present Product is not sterile & will spoil

Blanching Mild heat treatment Usually used on fruits & vegetables Primary objective – enzyme inactivation Lead to increase product shelf-life

Picking the Right Heat Treatment Think about these things: Time-temperature combination required to inactivate the most resistant microbe Heat penetration characteristics of the food & container

Clostridium botulinum (botulism) Most resistant microbe in canned foods Must kill this with right time-temp combination If food contamination is unknown processors assume C. botulinum to be present http://www.youtube.com/watch?v=RI7FVOBKTXM

In order to determine the time-temp combinations required to kill the most heat resistant pathogen and or spoilage organism in products you must understand thermal death curves!!!

Thermal Death Curves Plot the lethality of heat to microorganisms Two types thermal death time curves plot combinations of heats & times required to reduce a microbial population by ninety % thermal death rate curves plot the lethality of a given temperature to a microbial population over time Both specific to each microorganisms in given environmental conditions

Thermal Death Curves Heat kills bacteria logarithmically – example: if 90% are killed in 1st min at temp., then 90% of those remaining alive will die during 2nd min, & 90% of those remaining alive will die during 3rd min and so on

Thermal Death Curves Microbes are killed at a rate approximately proportional to their population the more of any given microbe one hopes to kill, the greater must be the application of heat Understanding Thermal Death Curves – see handout

More acidic (lower pH) foods generally take longer to kill all microorganisms