Rough Lecture Outline for Midterm 4 material FST 10

Transcription

1 Rough Lecture Outline for Midterm 4 material FST 10 This is the kind of outline I would have made from the lectures and slides if I were taking the course. I can t promise that the notes contain everything I said, this quarter. But I didn t consciously leave anything out or insert any tricky material in the lectures. Food Safety Microbiological issues are the most important cause of food-borne illness Food-borne intoxications o The organism grows in the food, illness is caused by consuming a pre-formed toxin in the food o Famous examples Clostridium botulinum (in italics because it is derived from Latin, the Genus is capitalized, the species is not) Grows only in the absence of oxygen (air) Makes spores & is a gram positive bacterium (the spores part is important for the test) Secretes a heat-labile toxin ( Botox ) Causes botulism Is deadly (neurotoxin, death by paralysis/suffocation) Spores found in honey can cause floppy baby syndrome or SIDS Spores are found in dirt Home-canned, low-acid foods can be a danger Nitrite is added to cured meat to prevent C. botulinum growth, and also improves the meat s color Staphylococcus spp. (e.g. aureus) Grows in presence or absence of oxygen, oxygen aids its growth Is gram positive, but does not form spores Secretes a heat-stable toxin Disease (vomiting, GI distress) occurs soon after eating, lasts less than 24 hours Found on skin, and especially in the nasal passages (and fingers if you pick your nose) Makes toxin at temperatures between 50 and140 F, grows well between 70 and 105 F Bacillus cereus Grows in presence or absence of oxygen Makes spores & is a gram positive bacterium (the spores part is important for the test) Makes two toxins Normally associated with grains (cereals), but found in meat, as well Spores germinate in reheated rice Food-borne infections o The organism is transmitted by food, but grows in the victim. It may also grow in the food. Generally slower onset than intoxications, because the organism must grow in the body (whaich is pretty much the definition of infection ) o Famous examples

2 Salmonella spp. (e.g., enteritidis, but there are many types and the nomenclature is complicated just know Salmonella) The most common cause of death from food-borne illness Causes GI distress and fever, may last 5-7 days Rarely fatal, but is so common that the number of fatalities is large Typically in foods of animal origin, from fecal contamination Found in some raw eggs Also found on snakes and turtles Prevented by o Hygienic food handling o Heating (cooking, not warming) the food before consumption o Control storage temperature (cold or hot) Clostridium perfringens Gram-positive spore-forming obligate anerobe Makes a toxin while living in the intestine Self-limiting, not fatal Campylobacter jejuni Causes bloody diarrhea (often) Associated with poultry Cross contamination! Escherichia coli O157:H57 Rare, but often fatal Associated with undercooked hamburger (found on the outside of cuts of meat where it would be killed by cooking, but grinding meat contaminates the inside) Also found on fruits and vegetables, especially if they are picked up from the dirt Listeria monocytogenes Infections are rare, but occur in immuno-compromised individuals (e.g., pregnant women) Is about 25% fatal, second largest killer due to food-borne illness Found on meat, vegetables, your kitchen drain - everywhere Grows (slowly) in the refrigerator Parasites Much larger than bacteria (they are generally protozoa, not bacteria) Usually associated with water (except the sushi parasite, which is a nematode, not a protozoan) Barely mentioned Trichinella spiralis, the cause of trichinosis from eating infected pork or bear meat. Make sure your bear is well done. Viruses Genes (DNA or RNA) in a protein coat No metabolism Reproduce by using the host cell s machinery Transmitted (usually) by infected food handlers fecal-oral route Wash your hands, and don t eat at Poppy s 2

3 Famous ones: Hepatitis A, Rotavirus, Norovirus Prions just a protein that causes host proteins to misfold and make protein deposits (notably in the brain). Incurable, fatal Examples o Scrapie in sheep o Bovine spongeform encephalopathy (BSE) in cattle (Mad Cow Diesase) o Kuru in cannibals o Creutzfeldt-Jacob disease in humans (yeah, I know, cannibals are humans, too.) Preventing contamination of food HACCP o Hazard Assessment Critical Control Points o A procedure for evaluating dangers in a process involving food Preventing food-borne illness o HACCP o Avoid cross-contamination (hands, sponges, dishcloth, knives and food-preparation surfaces) in the kitchen o Keep the food hot or cold (not warm) o Wash your hands Preservation solving the conflict between annual harvest and daily meals Food Spoils by microbial action Main strategies o Control water content o Acidity o Temperature o Processing kill em all (or most of them) Water Content Water activity, a w, is the parameter that matters o How Drying Adding salt or brine Adding sugar o Why Produces high osmotic strength and draws the water out of microbial cells Bacteria can t grow Yeasts and molds are more resistant than bacteria Acidity ph is the important parameter o ph is the negative of the logarithm of the hydrogen ion concentration in moles/liter (whaaa?) ph 7 is neutral ph < 7 is acidic, termed high-acid foods ph > 7 is basic (or alkaline). Relatively rare in food systems o Bacteria (except some weird ones) do not grow at ph values of less than 4.6 High-acid (low ph) foods are generally safe Some molds do grow under acidic conditions 3

4 o Two common ways to decrease the ph Add acid (usually vinegar, which contains acetic acid) e.g., pickles made by infusion Use a fermentation add bacteria to convert sugars to acids Typically safe Lactic Acid bacteria (LAB) or yeast (wine & beer). Convert sugars to acid (usually lactic acid) to lower the ph Scavenge sugars so that the bad bugs don t grow Contribute flavor Some LAB are also salt tolerant, so you can add salt to suppress the bad bugs further Preserving food by the addition of acid by either method is a form of pickling Kimchee and sauerkraut are officially pickles Temperature o Freezing o Refrigeration (T < 40 F) Processing o Pasteurization o Filtration o Irradiation o Sterilization, such as in-can cooking (canning) o Aseptic packaging putting sterile product into a sterile container in a sterile environment o Ozone treatment is more commonly being used to sanitize equipment, and even food surfaces Dairy Products Dairying is ancient (6,500 BC) Historically, Northern Europeans drank fluid milk (Galaktopotes), but ate little cheese; Southern Europeans ate cheese, but did not drink fresh milk. Nowadays, Northern Europeans are also famous for cheese. Dairy processing was a commercial business in the US by 1850 Milk is secreted by the mammary glands into the udder after a calf is born o Protein and lactose go directly into the water phase of the milk o Milkfat is covered by a portion of the cow s cell membrane when it is transmitted to the milk (milkfat globule) o First milk is colostrum High in antibodies and minerals, relatively low in other things Usually reserved for the calf Compared to cows milk, human milk ( breast milk or Mother s milk ) is o Much lower in protein o Much higher in fat o Much lower in mineral content o Differences probably represent different nutritional needs between humans and cattle Milk composition varies with 4

5 o Species (cow, goat, sheep, human) o Breed (e.g., Holstein, Guernsey, Jersey cows) o Feed o Climate Milk proteins two groups o Caseins clotted by acid or the enzyme chymosin (formerly called rennin, found in the stomach of suckling calves) o Whey proteins the rest of the proteins remain in the Whey Caseins o Loosely-structured, easily-digestible proteins o α- β- and κ-caseins o Arranged in bundles called submicelles, with (part of ) κ-casein on the outside o Submicelles are held together in a casein micelle by calcium phosphate bridges o The calcium and phosphate would normally form a hard mineral, but the caseincalcium-phosphate structure keeps it suspended. o Calcium and phosphate form the mineral part of bone the rest of bone is protein (collagen, remember?) o Acid or chymosin destabilize the micelle structure, and the caseins form a curd Whey and whey proteins o Whey is the fluid left after caseins have been clotted and removed o Whey contains whey proteins, lactose and some other minerals (whey made by adding acid actually contains most of the calcium and phosphate from the casein micelle, but you don t have to know that.) o Whey proteins (in order of decreasing concentration) β-lactoglobulin most abundant in cows milk completely absent in human milk α-lactalbumin important in lactose synthesis Bovine serum albumin Generic blood serum and milk serum protein Good source of essential amino acids Immunoglobulins antibodies A few other proteins o Cream Milkfat (+ water & a little protein) Contained in the milkfat globule Rises to the top, since fat is less dense than water e.g., cream-line milk Can be removed efficiently using a centrifuge ( cold milk separator ) o Pasteurization (named after Louis Pasteur, who developed the concept for wine) The udder contains many bacteria The teat can become dirty (cattle with mastitis an actual infection can not be used) and can contaminate the milk Enzymes (e.g., lipases) may be present Because of bacterial action, or lipase action, fresh raw milk would have a short shelf-life 5

6 Pathogens could be present (most importantly causing tuberculosis (Mycobacterium tuberculosis), but Salmonella could also be present) Heat treatment inactivates enzymes, kills all pathogens and reduces the overall bacterial population. Traditional, vat-style pasteurization 144 F, 30 minutes High Temperature Short Time (HTST) 160 F, seconds Ultra High Temperature (UHT) F, 15 seconds o produces cooked flavor and slightly darker milk o shelf life of months o used for room-temperature-stored products of specialty products (whipping cream) that don t move quickly from the refrigeration case. Sterilization kills ALL microorganisms o Homogenization (developed by Fred Homogen (not really)) To prevent separation of the cream Force milk through small hole (or tube), and may hit a hard surface Breaks the milkfat globules into much smaller ones Smaller globules have more surface area, so new proteins stick to their surface, making them more dense and less likely to coalesce. Fat no longer rises o Removal of fat Changes the texture and appearance Add back non-fat milk solids to improve appearance & texture Removes fat-soluble vitamins (A & D in milk) Fortify the milk with Vitamins A & D Adding back the fat yields several products of controlled fat content Half and Half (~12.5 % fat, by weight) Whole milk (~3.5 % ) Reduced fat milk (2%...) Low fat milk (1%...) Nonfat milk ( skim milk ) no fat added back, <0.2% fat o The fat cream (high milkfat, some proteins, water) Whipped cream is a foam (dispersion of air in liquid) Foam is stabilized by milkfat globules, coated by proteins and water If the cream is whipped too much, or if the temperature is too high, the milkfat globules coalesce to form a continuous phase - butter o The fat butter A water-in-oil (W/O) emulsion (dispersion of a liquid in another liquid) containing some protein and some fat crystals Is about 16% water, by weight Spoils readily, so o Add salt to get salted butter o Remove the water to get anhydrous milkfat (= ghee) Since you WANT the fat to coalesce, churning is done at warmer temperature. 6

7 The water solution that was NOT entrapped in the butter can be called buttermilk, but it is not the cultured buttermilk sold in the US. o Margarine Butter substitute The first animal-fat-based margarine was developed in France for Napoleon s army Now made from partially hydrogenated vegetable oil (or mixtures of unhydrogenated and fully hydrogenated vegetable oils to avoid making trans fats) o Ice cream another foam, but stabilized by freezing the continuous phase (water) o Standards of Identity (SOI) Legal definitions for the composition and properties of a product Non-SOI products give us more choices, but you can t trust the name to tell you what you are getting. Is frozen yogurt made from yogurt? Probably not. Nonfat sour cream? How could that be? Fermented dairy products o Milks (sometimes health claims from the probiotic organism used in the fermentation) Kefir (cows milk, Southwest Asia, fermented with specific LAB) Koumis (mares milk, Southwest Asia, fermented with specific LAB) Acidophilus milk (fermented slightly with Lactobacillus acidophilus, not from milking bacteria) (Cultured) buttermilk (lowfat milk fermented with LAB (Streptococcus lactis and Leuconostoc citrovorum), contains the buttery flavor diacetyl) Drinking yogurts, e.g. Yakult. Typically fruit flavored (Go, Yakult Swallows!) o Yogurt whole or lowfat milk clotted by Lactobacillus bulgaricus and Streptococcus thermophilus (plus some others) Not stirred, cut or agitated during the fermentation, so that the whey does not separate (unlike cheese) o Sour cream and crème fraîche (higher fat products fermented with specific LAB) o Clotted Cream not actually fermented, just add lemon juice to cream to form a soft gel. Served a High Tea at Harrods and The Brown Hotel in London. Cheese clotted milk from which the whey has been separated (syneresis) o Two methods for producing the clot Fermentative bacterium introduces (lactic) acid very slowly Chymosin added to cut κ-casein and produce a curd Do both o Cheeses differ in Kind of milk used Method/conditions of clotting Method and length of time of ageing Microorganisms used during ageing ripening o Steps in making a typical cheese Heat (and cool) the milk Add starter culture and/or Rennet (an extract containing Chymosin) Cut and cook the curd 7

8 Drain the whey (pile up the curds, known as cheddaring ) Mill the curd Salt the curd, press it into the shape you want Ripen o Strategies for ripening The original starter culture (bacteria) Cheddar Swiss (Propionibacter shermanii make gas that forms the bubbles) Veins or mold inside the cheese Roquefort, or anything with blue in the name Surface molds (molds secrete enzymes, some of which are proteases) Camembert, Brie Surface bacteria Limburger o Sometimes, other flavors (garlic, herbs, wine, peppers, seeds) are added or infused into the cheese 8