Transcript fat map
Food Protection with Modified Atmospheres Hypobaric (Low Pressure) Storage Foods are stored in: low pressure air low temperature high humidity The hypobaric (Low Pressure) state results in: Reduced concentrations of O2 Reduced fat oxidation. Atmospheres have been found to be effective : 10 mm Hg for meats and seafoods; 10–80 mm Hg for fruits and vegetables (1 atm = 760 mm Hg). Vacuum Packaging By this method, air is evacuated from gasimpermeable pouches followed by sealing. Upon storage of a vacuum-packaged food product, an increase in CO2 occurs as a result of both tissue respiration and microbial respiration where O2 is consumed and CO2 is released in equal volumes. In the case of meats, up to 10–20% CO2 may develop within four hours and the concentration may ultimately reach 30% from respiratory activities of the aerobic biota. vacuum packaging minimizes : aerobic spoilage organisms product shrinkage fat oxidation discoloration Modified Atmosphere Packaging (MAP) Overall, MAP is a hyperbaric process that consists of altering the chamber or package atmosphere by flushing with varying mixtures of CO2, N2, and/or O2. The initial gas concentration cannot be readjusted during storage. Two types of MAP are recognized: 1. In high-O2 MAP, up to 70% of O2 along with about 20–30% CO2 and 0–20% N2 may be used. Growth of aerobes is slowed but not suppressed by the moderate concentration of CO2. This method is suitable for the packaging of red meats, as the high level of O2 will aid in maintenance of the red color. 2. In a low-O2 MAP system, O2 levels may be as high as 10% while CO2 is maintained in the 20– 30% range with N2 added as necessary. Controlled-Atmosphere Packaging or Storage Although controlled-atmosphere packaging or storage (CAP, CAS) is regarded by some as being different from MAP, it may be considered a form of MAP. While in a typical MAP the compositions may change upon storage, in CAP the gas compositions remain unchanged for the duration of the storage period. While low- and high-O2 MAP systems may be prepared with high-barrier plastic films, CAP requires aluminum foil laminates, metal or glass containers, since single plastic film is not entirely impervious to gases. PRIMARY EFFECTS OF CO2 ON MICROORGANISMS The following facts are well established following prolonged exposure of microorganisms to around10% and above of CO2. 1. The inhibitory activity increases as incubation or storage temperatures decrease. This is due in part to the: greater solubility of CO2 in water at the lower temperatures, in part to the additive effect of a less than optimal growth temperature. 2. Although concentrations from about 5–100% have been used, 20–30% seems optimal, with no additional benefits derived from higher levels. This is especially true for fresh meats, where 20% is about ideal. Higher levels can be used for seafoods. To maintain red meat color, they can be stored in a 20:80 mixture of CO2 + O2. 3. Inhibition increases as pH is decreased into the acid range. One practical effect of this is that CO2 is more effective for fresh red meats than for seafoods. 4. In general, the Gram-negative bacteria are more sensitive to CO2 inhibition than Gram positives, with pseudomonads being among the most sensitive and clostridia the most resistant. Upon prolonged storage of meats, CO2 affects a rather dramatic shift in biota from Gram negative to Gram positive. 5. Both lag and logrithmic phases of growth are delayed. Mode of Action of CO2 1affect a mass action on enzymatic decarboxylations 2-CO2 is dissolved in the form of carbonic acid (and thus can cause changes in cell permeability) 3- is an arginine antagonist (like Diacetyl) Food Products Fresh and Processed Meats In general, the self-life of red meats at −1◦C can be extended for up to 2 months if packaged in 75% O2 + 25% CO2. The high level of oxygen ensures that the red-meat color is maintained. Poultry Overall, the generally higher initial pH of fresh poultry meat is primarily responsible for this product’s not having the MAP self-life of products such as fresh beef. Seafoods Whereas the practical upper limit of CO2 for red meats is around 20%, higher concentrations can be used with fish because they contain lower levels of myoglobin. The concern over the use of MAP for fishery products has to do with the fact that nonproteolytic botulism strains are found in waters and they can grow at temperatures <4◦C coupled with the fact that the pH of seafoods in general is higher and more favorable to growth by pathogens. THE SAFETY OF MAP FOODS Clostridium botulinum As a general rule, foods that are to be subjected to MAP should possess one or more of the following antibotulinal hurdles: 1. have a water activity (aw) < 0.93 2. have a pH of 4.6 or less 3. cured with NaCl or NO2 4. contain high levels of nonpathogens (for raw meat, poultry, and the like) 5. maintained in frozen state 6. maintained at (4.4◦C) or below 7. have a definitive self-life (e.g., not to exceed 10 days) Listeria monocytogenes The fact that this bacterium can grow in the refrigerator temperature range raises concerns about its presence and potential for growth in MAP foods. L. monocytogenes numbers were reduced by the antilisterial effects of the lactobacillus due apparently to its production of lactic acid. Regarding the behavior of this organism on vacuum-packaged beef, it has been shown that critical factors are: Storage temperature (must be less than 4 0C) pH (must be acidic) Type of tissue (grew more on fat than lean beef) Other Pathogens When cooked bologna-type sausage was vacuum packaged, the growth of Yersinia enterocolitica and salmonellae was restricted but not that of Staphylococcus aureus. SPOILAGE OF MAP AND VACUUM-PACKAGED MEATS From the research of many groups, it is clear that when vacuum-packaged meats undergo long-term refrigerator spoilage, often the predominant organisms are: lactobacilli Brochothrix thermosphacta. Volatile Components of Vacuum-Packaged Meats and Poultry 1- Short-chain fatty acids: produced by both lactobacilli and Brochothrix thermosphacta produce sharp off-odors. 2-acetoin (more important) and diacetyl: have been found to be the most significant relative to spoiled meat odors. 3-di-methyl di-sulfide or di-methyl tri-sulfide, or methyl mercaptan. Some organisms (except B. thermosphacta) produced them. The volatile compounds produced by B. thermosphacta may be expected to vary between products with high and low glucose concentrations. The addition of 2% glucose to raw ground beef has been shown to decrease pH and delay off-odor. It would seem to be a way to shift the volatile components from short-chain fatty acids to acetoin (that derive from glucose.)