Microbiology of food preservation - Texas A&M University

Microbiology of FoodPreservationAlejandro CastilloTexas A&M UniversityMethods of food preservation• Cold storage• Heat processing• Reduced a w• Acidification• Atmosphere modification• Addition of chemicalpreservatives• Ionizing radiationsCold Storage•Freezing•Refrigeration

Freezing• Needs previousconditioning• Freezing is applied topreviously packagedfoodsFreezing• Vegetables– Selecting/sorting– Washing– Cutting (if any)– Blanching–Packaging• Meats, poultry, seafood, eggs etc.– Trimming–PackagingFreezing• Blanching• Enzyme inactivation• Fixing green color in some vegetables• Easing packaging of leafy vegetables(by wilting)• Displacing of air from plant tissues• Reduction of microbial counts

Freezing• Fast freezing– Lower temperature to -20°C within 30min– Blast freezing or immersion•Slow freezing• Lower temperature to -20╖C within3-72 h• Home freezersFreezing• Effects of freezing on microorganisms• Sudden mortality immediately onfreezing• Continued mortality rate decreasesgradually during storage• The lower the temperature the lowerthe bacterial declineRefrigeration• The lower the temperature thelower the growth rate• How can psychrotrophs grow atrefrigeration temperatures?

Psychrotrophs and psychrophiles• Greater content of unsaturated fatty acids• Slower metabolic rate• Better membrane transport– Still active at low temperatures• Production of larger cells– Additional RNA• More efficient flagella synthesis• Favored by aeration• Greater heat sensitivity• Usually do not grow above 35°C– Inactivation of respiratory enzymes–Leakage of intracellular constituentsHeat processing•Pasteurization•SterilizationPasteurizationTypes of milkpasteurizationLow temperaturelong timeAcronymLTLTTreatment63°C/30 minHigh temperatureshort timeUltrahightemperatureHTSTUHT72°C/15 s90°C/0.5 s94°C/0.1 s100°C/0.01 s

PasteurizationMilk pasteurization• Originally designed to targetMycobacterium tuberculosis• Coxiella burnetti is currently thetarget organismPasteurization• Thermoduric organisms–Organisms capable of survivingpasteurization–Examples: lactobacilli,micrococci, streptococci,lactococci–Pathogens are not thermoduricFactors affecting heat resistance ofmicroorganisms•Water•Fat•Salts– Salts that reduce a w (Na + , K + )more protective than those thatincrease a w Ca +2 , Mg +2 )• Carbohydrates•pH

Factors affecting heat resistance ofmicroorganisms (cont.)•Proteins• Numbers of microorganisms• Age or microorganisms– Old cells (stationary phase) moreresistant that younger cells (log phase)•Growth temperature• Inhibitory compounds• Time and temperature• Ultrasonic effectCanning• Processing of food products inhermetically sealed containers–Metal–Glass• Canned products are commerciallysterileSpoilage organisms associated withcanned productsMesophilic• Putrefactive anaerobes– Clostridium botulinum• Butyric anaerobes– C. butyricum•Aciduricflat sour– Bacillus polymyxa• Lactobacilli•Yeasts•Molds

Spoilage organisms associated withcanned products (cont.)Thermophilic• Anaerobic sulfide producers– Desulfotomaculum nigrificans•Flat-sour spores• B. stearothermophilus, B. coagulans• Anaerobic non-sulfide producers– Thermobacterium thermosaccharolyticumCanningpH categoryExamples of foodsSpoilageLow acid> 4.6Acid3.7-4.0 to 4.6Meats, seafood,milk, somevegetablesTomatoes, pears,figs•Thermophilic flatsourgroup•Sulfide spoilers•Gaseous spoilers•Mesophilic spoilers –PutrefactiveanaerobesThermophilic spoilersHigh acid

Foods of low and intermediatemoisture•Low moisture– Dry, dessicated–0.65 – 0.70.60-0.652 yearsNo microbialspoilageDrying•Freeze drying–lyophilization•Hot vacuum drying•Hot air drying•Sun dryingDrying depends on temperature andpressure conditions

Effect of the a w adjustmentmethodWays of adjusting the a w of a food• Adsorption– Adding water to dry foods to adjust a w• Freeze dried foods• Egg beaters•Desorption– Eliminate water from the food system• Hot vacuum dried or sun dried foods• At similar moisture content (% water)desorption tends to produce lower a w

Concentration• Adding solutes to the food system– Salting (dry fish)– Adding sugar (jellies, syrups)• Adding solutes and eliminating waterwater– Caramelized condensed milk (cajeta)• The solute plays an important role ina w reductionGrowth of Staphylocoucus aureus in mocha cakestored at 25°C as affected by moisturizing agent9Log CFU/g8765432120% Sucrose20% Glucose00 3 6 9 12 15 18Hours of storageAcidification•Low pH•Organic acids

Acidification•Fermented foods•Acidified foods–Acidic foods vs.acidified foodsAtmosphere modification•Controlled atmospheres•Modified atmospheresControlled atmospheres• Substitution of air by a gas mixture• Controlling gas concentration during storage• Usually includes CO 2– Greater inhibition of microorganisms by CO 2 atrefrigeration temperatures– Lower APC in steaks during storage in 100% CO 2compared to 100% N 2 or air– Shelf life of lean beef inoculated with PMA andstored at 5°C in 85% O 2 + 15% CO 2 was 9 dayslonger than beef stored in air.• Reducing O 2 concentration may affect foodquality–Red meats

Effect of CO 2 on microorganisms inmeat packs• Microbiota is shifted• Predominant PMA (aerobic, CO 2sensitive) to lactic acid bacteria (LAB,facultatively anaerobic, CO 2 resistant)• PMA are putrefactive, LAB arefermentative– Some sensorial change is still acceptedby consumersModified atmosphere packaging(MAP)• Substitution of air by a gas mixture• Vacuum packaging– 80% of fresh beef in the U.S. is vacuumpackaged– Not all O 2 is removed–The O 2 remaining is consumed by themicrobiota and the food–CO 2 is also produced by metabolic activity• Inhibitory for microorganisms• Careful selection of packaging filmSpoilage of vacuum packagedmeats• Long-term refrigeration spoilageassociated with Brocchothrixthermosphacta and lactobacilli• Clostridium laramie is also associatedwith gassy spoilage in vacuumpackaged meats

Food safety concerns• Some researchers believe that MAPresults in a selection of potentialpathogens by inhibiting competitivebiota– Aeromonas hydrophila– Listeria monocytogenes• A. hydrophila survives and growsbetter in meat under air packagingcompared to vacuumAddition of chemicalpreservativesGRAS substances• Chemical additives must be listed asGRAS by FDA regulations– GRAS = Generally Recognized as Safe• There is a maximum tolerance fortheir concentration in foods• Different preservatives target specificorganisms• The effect of some preservatives isaffected by food characteristics suchas pH

Benzoates and parabensHOOCCOOHEffect dependson pHEffect does notdepend on pHBenzoic acidHOCOOHp-Benzoic acidParabensParabens

Other organic acid derivatives• Sorbates– Used as sorbic acid or as calcium or potassiumsorbate (salts more soluble than acid)– Widely used as a preservative– One half the toxicity of NaCl– pH dependant– No effect above pH 6.0– May change the taste of foods if added inexcess (usually no more than 0.2%)– Effective against spoilage and pathogenicbacteria• Propionates– Lower tendency to dissociate– Effective in low-acid foodsSO 2 and sulfites• SO 2used as a gas or in liquid form• Used as preservative in wines, dried fruits,juices, molasses• Effect greatly favored by low pH• Bacteriostatic against Acetobacter and LAB– Juices and beverages• SO 2can change to sulfurous acid derivativesSO 3-2then to sulfites, bisulfites andmetabisultfites• Sulfites are used as preservatives in juices andbeverages– Also to prevent melanosis in shrimpNitrites and Nitrates• Used in curing formulas for meat• Stabilize red color of meat• Inhibit spoilage and pathogenicorganisms• Contribute to flavor development•NO 2 and NO 3 change to HNO 2then to NO

NO 2 and Clostridium botulinum• Prevents germination of sporesof C. botulinum• Also effective against S. aureus• Added to cheeses to preventgassy spoilage caused byclostridiaThe Perigo factor• Question:–Why is C. botulinum almost totally absent incured, canned and vacuum packaged meats?• Perigo and Roberts (1968) found that NO 2was 10 times more effective if addedbefore heat treatment– A chemical factor is thought to be producedduring heating that enhances the inhibitoryeffect of NO 2• Several researchers have reported that theantibotulinal effect is interdependent withNO 2 , with other curing ingredients, pH,heat treatment, and temperature/time ofstorageAntimicrobial mechanism•NO 2 inhibits C. botulinum byinterferring with iron-sulfur enzymesthus preventing ATP synthesis– Ferredoxin• Bacteria that lack ferredoxin areresistant to NO 2– LAB lack ferredoxin– Lactobacillus and other LAB spoilage incured meats is reported• Green ham (Sam I am)

Other antimicrobials• Ethylene oxide– Highly toxic, but no residues• Antibiotics and bacteriocins– Nisin, subtilin, tylosin• Antifungals– Natamycin, benomyl, thiabendazoleIonizing radiations•Electron beam•X-rays•Gamma raysFood irradiation“Food Irradiation is a process thatexposes food to a carefully controlledamount of ionizing energy todamage the DNA of parasites, fungi,and bacteria and eventually destroythese organisms.” (IFST, 1999)

Radioactivity vs. Irradiation Radioactivity:Spontaneous emission of particles,radiations or both Irradiation:Transmission of energy (in the formof waves) through spaceFood irradiation in the U.S.In the United States, ionizingirradiation of food is considered afood additive under current lawand thus must be approved byFDAIonizing radiations of interest infood preservation•Electrombeam•Gamma rays•X rays

Electromagnetic spectrumCharacteristics of different formsof radiation applicable in foodsElectron Beams:– Effective at converting electrons into high energybeams such as X-rays– Easily applicable on different types of foods,climatic conditions and harvest seasons– Can turn on and off with ease– Less penetrating than gamma rays– More energy than gamma rays– Electron beams generated by linear accelerator cangenerate 10 3 -10 6 Gy/seg, whereas the dose generated by60Co is 1-100 1-100 Gy/minCharacteristics of different formsof radiation applicable in foods Gamma rays:– Electromagnetic radiations emitted fromexcited nuclei ( 60 Co or 137 Cs )– Relatively inexpensive byproducts ofatomic fission– Greater penetration power– Half-life of 60 Co: 5.27 years– Half-life of 137 Cs: ~30 years

Characteristics of different formsof radiation applicable in foods X rays:– Produced by linear accelerator– Essentially the same as electron beambut after passing through a heavy metaltarget– Otherwise, essentially similar to gammaraysForms of radiation(from Jay, 1998)Types of treatments• Radappertization– 30 - 40 kGy– Commercial sterility• Radicidation–2.5-10 kGy– Pathogen reduction• Radurization– 0.75 – 2.5 kGy– Reduction of bacterial populations– Enhancement of food quality

Fundamentals of bacterialdestruction by irradiationGram positive bacteria more resistant than Gramnegative bacteriaMolds less resistant than bacteria (especially G+)– Some yeasts are highly resistant Types of microorganisms– Sporeformers are more resistant than non-sporeformers– Some highly resistant non-sporeforming bacteria– Deinococcus, Deinobacter, Rubrobacter, Acinetobacter Most highly resistant bacteria– Sporeformers– Paenibacillus larvae–type A Clostridium botulinum– Non-sporeformers– Enterococcus faecium R53– Homofermentative lactobacilliFundamentals of bacterialdestruction by irradiationFactors affecting irradiationperformance Number of microorganisms Composition of the food Presence or absence of oxygen Physical state of the food Age of microorganismsD-values (D 10 ) for selected bacteriaOrganismD 10 (kGy)C. botulinum, type E Beluga0.80C. botulinum type A, toxin production in meat 36.08Escherichia coli0.20Listeria monocytogenes0.42 - 0.55Salmonella typhimuriumStaphylococcus aureusYersinia enterocolitica, beef at 25°CYersinia enterocolitica, beef at -30°C0.500.160.1950.388Adapted from Jay, 1998

D 10 values for selected moldsOrganismAspergillus flavus sporesAspergillus flavusAspergillus nigerPenicillium citrinum, NRRL 5452Penicillium sp.D 10 (kGy)0.660.055-0.060.0420.880.42Adapted from Jay, 1998D 10 value for Salmonella Poona on cantaloupecubes treated by electron beam irradiation987Mean log 10 CFU/g6543210D 10 = 0.203 kGy00.050.10.150.20.250.30.350.40.450.50.550.60.650.70.750.80.850.90.9511.051.1Dose (kGy)D 10 values for selected virusesType of virusAdenovirusCoxsackievirusEchovirusHerpes simplexPoliovirusD 10 (kGy)4.1 - 4.94.1 - 5.04.4 - 5.14.34.1 - 5.4Adapted from Jay, 1998

Effect of irradiation onmicroorganismsAntimicrobial mechanismBreaking of DNA strandsIrradiation breaks 6 chemicalchains out of every 10 millionchains present in a cellUses in the United States• Control insects in wheat, wheat flour• Inhibit sprouting in white potatoes• "Sterilize" Trichinella in fresh pork (0.3-1 kGy)• Inhibit mold decay, control insects in freshfruits and vegetables (0.3-1 kGy)• Microbial disinfection of spices and herbs (upto 30 kGy)• Radicidation of fresh poultry• Recent approval for red meat, shell eggsFoods approved by WHO forirradiationType of foodPotatoesOatmeal flourDried fruitsPoultryFresh fruitsFresh meatsShrimpObjectiveSprout inhibitionInsect eliminationInsect eliminationRadicidationRadurizationRadurizationRadurizationDose (kGy)0.1-0.150.2-0.751.07.0 max.2.56.0-8.00.5-1.0Adapted from Jay, 1998

Effect of irradiation onqualityWater radiolisis3H 2 O H + OH + H 2 O 2 + H 2Consumer Acceptance• Most consumers have littleknowledge of the process or it’sadvantages.• Concern about foodborne illness hascreated interest in irradiated foods• After receiving educationalinformation market trial consumerswere more likely to purchaseirradiated foods

Packaging• Most foods are irradiated in theirpackaging• Irradiated foods are clearly marked–Radurasymbol– “irradiated for your food safety”• Packaging must be approved forirradiation use