9. Food microbiology Flashcards
-which type of microorgs are most often involved in foods? (3)
- describe the characteristics of the main one ish (gram WHAT, aero or anaerobic?, shape? –> 2 types of fermentation
- what 2 things are produced: why?
- lactic acid bacteria, variety of yeasts (ie saccharomyces stains) and molds (ie aspergillus stains)
- lactic acid bacteria: gram-positive, aerotolerant, rods or cocci
1) HOMOfermentative: fermentation yields primarily/ONLY lactic acid (pickles, sauerkraut).
2) HETERO fermentative: fermentation yields lactic acid AND a variety of flavorful compounds (waste product for bacteria) (fermented sausages such as salami, fermented milk products). - Lactic acid reduces food spoilage (by decreasing pH) and improves storage qualities (brings the pH down).
- Flavor compounds produced by different organisms give each product its distinctive
taste.
- does milk naturally contain lactic acid? what is usually done to make milk products?
- how to make yogurt? which bacterias? explain 3 steps ish
- how to make cheeses? which bacterias? 2 main steps
- yes! but usually pasteurized to remove potential pathogens (to have a more controlled product) –> desired microorgs are then added back to give the final product
YOGURT
- cultures of streptococcus thermophilus and lactobacillus bulgaris are added + incubated at 42°C
1) streptococci produces some acids (ie lactic acid) and favors growth of lactobacillus which produce most of the acid
2) acid denatures casein (by decrease pH) –> which results in thickening of milk
3) other microorgs are sometimes added (BL, bifidobacterium animalis)
CHEESE:
- hundreds of microorgs may be involved. raw milk cheese may contain pathogens
1) Curdling: lactic acid bacteria precipitate the proteins –> curd
2) Ripening: lactic acid bacteria ± other organisms –> flavor
what are the steps of cheese production? 7 steps
1) most milk will be pasteurized + standardized + filtered OR traditional cheese making –> raw milk
2) Starter cultures of lactic acid bacteria are added –> ferment lactose and produce lactic acid: promotes development of curd (precipitated proteins, fat and other suspended material) and whey (supernatant).
3) Usually, an enzyme, rennin, is added to the culture. Rennin cleaves casein which results in curd formation. (Rennin or lactic acid bacteria work, one or the other or both)
4) Whey is drained off: starting material for other processes, or discarded (high BOD, must be treated like sewage)
5) Curd is then heated (T° depending on final product): more whey is expelled, and the curd becomes more solid
6) Salt may be added: flavor, promotes extraction of water, inhibits growth of spoilage organisms.
7) Packaging (fresh cheese like cottage and cream cheese), or other processes: pressing, brining, ripening.
what is done with the whey produced during cheese making?
used to be thrown in the sewage but now: try to make it more sustainable and use it for other purposes: ie added to chips to make it cheese flavored
CHEESE RIPENING:
- ripening leads to changes in (3) –> these changes are due to what? (2 ish)
- explain process for cheddar, swiss (which bacteria) and gouda
- for how long is the ripening?
- the longer the ripening, the higher (2)
- oka: which bacteria?
- changes in texture, consistency, and flavor –> Due to the presence of specific microorganisms added
with starter culture or spread on the surface of the pressed curd + Some of the ripening is due to lactic acid bacteria, rennin or other enzymes present.
CHEDDAR, SWISS, GOUDA
- Wrapped (cheddar in paper, gouda in wax)
- Cheddar: ripening involves the lactic acid bacteria only (might get crunchy parts in your cheese –> crystals of lactic acid)
- Swiss cheese: propionibacteria produce propionic acid and acetic acid (flavor), and CO2 (bubbles).
- 3-12 months (or longer)
- The longer the ripening the higher the acidity and sharpness.
- Oka: pressed curd is brined, and surface is washed frequently. Growth of Geotrichum
CHEESE - ripening:
- blue and roquefort: what bacteria? explain. how long?
- why blue?
- camembert, brie: wrapped? what microorgs? what conditions for ripening? how long? why is the cheese soft?
BLUE/ROQUEFORT:
- Ripening is mainly due to the mold
Penicillium roquefortii.
- Spores from the mold are mixed with the milk (or the curd)
- Small holes are drilled in the curd before inoculating with bacteria + wrapping.
- 3-6 months.
- The mold grows in the holes (hyphii network) and produces spores (blue) + also produces proteases: affects texture of cheese
CAMEMBERT/BRIE:
- Unwrapped
- Mixture of mold (Penicillium) and bacteria (Brevibacterium) that are spread on the surface of the curd.
- Ripened in curing room (high humidity)
- 1-5 months.
- Proteases secreted by the microorganisms is responsible for softening of the cheese (break down protein so bacteria can have protein)
– Good sanitary practices are required so that the cheese does not get contaminated
*tends to smell like ammonia
WINE:
- difference between red and white wine?
- main steps?
- what is must?
- what can be added to wine? what does it do?
- which 2 main microorgs (bacteria, yeast or mold?) –> 2 good characteristics
- red: not filtered, everything is fermented including stalks, skin, etc. VS WHITE: only ferment juice (no stalks or skin)
- pressing + fermentation + aging + bottling
- MUST = crushed grapes
- metabisulfite (Na2S2O5) –> can be added to kill wild yeasts –> wild yeasts = hard to control + cannot tolerate high levels of alcohol
- commercial yeasts: S. cerevisiae and S. ellipsoideus:
resistant to Na2S2O5 + tolerate 12- 14% alcohol.
WINE:
- what may be added to final product (2)
- dry vs sweet wine?
- Sugar and organic acids may be added depending on the final product.
- DRY WINE: all the sugar is fermented (ie all becomes ethanol) –> use commercial yeast OR choose low sugar starting product
- SWEET WINE: some sugar is left or
added after fermentation –> not all is converted to ethanol
BEER:
- what is the main grain for making beer?
- can grains be digested by yeasts? solution?
- explain the steps to make beer: 9 steps ish
- Barley
- Grains contain starch that CANNOT be digested by yeasts –> Grains (BARLEY!) are allowed to germinate, producing amylase
1) MALTING: grains germinate (spray some water + right temp) to produce amylase
2) The grains are then dried (+ sometimes roasted) and crushed –> makes MALT
3) MASHING: Malt is then soaked in warm water (65°C) for 1-2h –> amylase degrades starch into fermentable sugars (glucose and maltose)
4) The mash is then filtered –> produces WORT
5) Hops is added to the wort. Wort is boiled for 1-2 hours (near sterilization, but not completely, some spores remain)
6) Commercial beer: filtered.
7) Cooled to 20°C
8) Yeast is added
9) Polishing: filtration, carbonation, pasteurization, etc.
- why is hops added to beer? (3 ish) added in which step?
- which microorgs (yeasts, mold, bacteria) are added to beer? 2 types + fermented at which temperature (important!) + top or bottom + produces what?
- how to make light beers?
- Hops gives flavor and bitterness. It also has antimicrobial properties and prevents lactic acid bacteria from growing in the final product
- added to wort before it’s boiled for near sterilization and before yeast is added
YEASTS!
- LAGER: Saccharomyces carlsbergensis (bottom yeasts, 6-12 °C) –> takes months to produce: only ethanol is produced
- ALE: Saccharomyces cerevisisae (top yeasts (so much CO2 produced that it floats on top of beer, carried up by CO2 during fermentation, 14- 23 °C) –> ethanol + flavors!
- Light beers: yeasts manipulated genetically so they can use all the sugars in the wort (just like dry wine!)
