LAB Flashcards
how is lactic acid produced in food fermentation?
from the break-down of hexoses such as glucose, mannose, fructose and galactose or disaccharides such as lactose, maltose, sucrose by lactic acid bacteria (LAB).
what gives LA its acidic properties?
the separation of hydrogen atoms from the rest of the molecule
what are the two classes of LAB? aka 2 primary hexose fermentation pathways that are used to classify LAB genera
- homofermentative
- heterofermentative
mechanism of homofermentative bacteria + which pathway do they use?
produce two moles of lactate from one mole of glucose (+2 ATP)
=> glycolytic pathway
mechanism of heterofermentative bacteria + which pathway do they use?
produce one mole of lactate from one mole of glucose, and produce carbon dioxide and acetic acid or ethanol as biproducts (+ 1 ATP)
pentose phosplhoketolase pathway
which medium constituents supply all the necessary nutrients for the growth of lactic bacteria?
casein enzymic hydrolysate, papaic digest of soybean meal and yeast extract
which fermentable carbohydrate is present in HHD agar?
fructose
what is the pH indicator of HHD agar?
bromo cresol green
homofermentative lactic acid bacteria produce lactic acid from …. and is indicated by …. colour formation
glucose/fructose
yellow
Homofermentative bacteria cultivated on HHD agar medium form …. colony on agar while heterofermentative bacteria form …. colonies on agar surface
bluish-green
white
example of foods using LAB fermentation
- cheese
- yogurt
- miso
- sour dough bread
=> LAB are involved in dairy, vegetable, and cereal fermentations
Lactic Acid Bacteria (LAB) are a group of ….. bacteria that produce lactic acid during the fermentation of carbohydrates
These bacteria are …. forming and can be both …. or ….
Gram-Positive
non-spore forming
rods or cocci
what are 5 genera of LAB?
Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Streptococcus
…. is often considered a LAB, however it is phylogenetically unrelated, and has a unique mode of sugar fermentation
Bifidobacterium
LAB are very adapted to environments that are rich in … and ….
nutrients and energy sources
what are some strategies Lab have to compete with other organisms?
acid production and acid tolerance
LAB are the most popular and widely available ….
probiotics
Only one Streptococcus species is associated with food: …
where is it use?
S. thermophilus
manufacture yogurt
which Lactococcus species is intimately associated with dairy products
Lactococcus lactis
which are the tetrad-forming LAB
Aerococcus,Pediococcus, and Tetragenococcus
… is not used in the food industry but is responsible for greening of cooked meat products
Aerococcus
… is a spoilage organism in the beer industry and is responsible for the buttery taste in beer
Pediococcus damnosus
… and … are both used as starter for sausage making and silage inoculants
Pediococcus acidilactici and Pediococcus pentosaceus
… are extremely salt tolerant (>18% NaCl) and are important in high-salt containing food like soy-sauce
Tetragenococcus
…, … and … are each coccoid LAB
Leuconostoc, Oenococcus, and Weisella
… are important for spontaneous vegetable fermentations like sauerkraut
Leuconostocs
…. species are associated with meat and can proliferate at low temperatures
Weissella
The genus … is the largest genera included in LAB
Lactobacillus
Lactobacillus is divided into three groups
Group 1 - Obligately homofermentative
Group 2 - facultatively heterofermentative
Group 3 - Obligately heterofermentative
Lactobacilli are the most … of the LAB, and will therefore be the final successors of many lactic acid fermentations
acid-tolerant
… mostly found in spontaneous sourdough bread reactions
Lb. sanfransiscensis
… (previously Lb. bulgaricus) is mostly yogurt associated
Lb. delbrueckii
…, …, and … can be found in many habitats
Lb. brevis, Lb. casei, and Lb. plantarum
LAB metabolism is characterized by efficient …
carbohydrate fermentation ending in lactic acid production
LAB metabolism mechanism
- A high-energy phosphate bond is required for sugar activation, before glycolysis
- Some species use the phosphoenolpyruvate : sugar phosphotransferase system (PTS), in which phosphoenolpyruvate (PEP) is the phosphoryl donor
- The main function of the PTS is to translocate sugar across the membrane with simultaneous phosphorylation
- Since there are two different molecular species outside (sugar) and inside (sugar phosphate) the membrane the translocation process does not involve any concentration gradients
LAB metabolism mechanism involving lactose
Lactose can be cleaved by B-galactosidase into glucose (glycolytic pathway) and galactose-6-phosphate and then enter one of several major metabolic pathways
LAB metabolism mechanism involving maltose
Maltose fermentation starts when maltose is cleaved into glucose and B-glucose-1-phosphate. The glucose is used in glycolysis, and the B-glucose-1-phosphate is a precursor to cell wall synthesis
LAB metabolism mechanism involving sucrose
Sucrose is cleaved into glucose and fructose which can then enter major pathways (glucose can be used in glycolysis, and fructose can be the terminal electron acceptor)
explain Obligately Homofermentative (group 1)
sugars are only fermented by glycolysis (group I Lactobacilli and some other species from other genera)
explain Obligately Heterofermentative (group 3)
only the 6-PG/PK pathway is available for fermentation (group III Lactobacilli, Oenococci, and Weissellas) the difference is that key enzymes of the glycolysis pathway are missing
explain Facultatively Heterofermentative (group 2)
use glycolysis for hexose fermentation, but some sugars induce a heterolactic fermentation reaction to take place (pentoses) (group II Lactobacilli, Enterococcus, Lactococcus, Pediococcus, Streptococcus, Tetragenococcus, and Vagococcus)
There is debate about if LAB should be classified as …, or as …
why?
aerotolerant anaerobes
facultative aerobes
because LAB can use oxygen as a terminal electron acceptor, but are not able to protect themselves against the toxic effects of oxygen the way genuine aerobic organisms would
Lb. brevis is unable to ferment glucose…, but will ferment it …
anaerobically
aerobically
When grown anaerobically several organic compounds can be used by … LAB as terminal electron acceptors. Which are they? (3)
=> contribute to flavour
heterofermentative
- citrate (cleaved into acetate and oxaloacetate, and oxaloacetate can be a terminal electron acceptor in several pathways)
- glycerol (terminal electron acceptor in anaerobic glucose fermentation)
- fructose (electron acceptor for heterofermentative LAB)
In the fermentation of plant materials (e.g. sauerkraut or kimchi) the main sugars are … (3). …. can be used as the energy source and … can be used as the terminal electron acceptor
glucose, fructose, and sucrose
Glucose
fructose
LAB have limited …
what does this entail
biosynthetic capability
LAB have a very limited ability to synthesize amino acids from nitrogen sources, and are therefore dependent on obtaining preformed amino acids from their environment
All dairy lactobacilli have …, which have been shown to be required for rapid growth in milk
proteolytic activity
LAB require … (2)
- nucleotides or nucleotide precursors
- several external vitamins
When a fermentation relies on active LAB that were added as a starter culture, …. can result in major commercial problems. This is a particular problem in the … industry
bacteriophage infection
dairy
Phage attacks on LAB can result in ….
unacceptably low LAB and flavor compounds produced along with decreased proteolysis
what do we call a complete failure of the starter culture due to phage attacks?
‘dead-vat’
what are 3 phage control plans?
- Characterization of the phage population involved (including genomic analysis)
- Analysis of “natural” and “intelligent” bacterial systems of phage defence
- Identification of phage counter-defense mechanisms
what are 4 naturally occurring phage defence mechanisms?
- Absorption Inhibition
- Blocking DNA Penetration
- Restriction Enzymes/ Modification Systems
- Abortive Infection Mechanisms (bacterial cell traps the phages from emerging)
what are 4 artificial phage-resistance mechanisms?
- Antisense RNA Strategies (Clone in antisense RNA which binds to phage DNA and stops viral replication)
- Cloned ORI (Clone the ORI for the phage into the bacterial genome, it competes with the phage one and slows viral growth)
- Clone in a Phage Repressor (Phage have a repressor for cell lysis, this can be constitutively expressed by the bacteria, which traps the phage)
- Phage Triggered Death (Bacterial suicide genes are placed under the control of a phage inducible promoter)
…. has been shown to have the greatest ability to remove mycotoxins from foods
Lactobacillus rhamnosus
about mesophilic LAB cultures
grow in temperatures of 10–48C, with the optimum around 38 C. Mesophilic starter cultures, composed of acid-forming lactococci, are often flavor producers, and are used in the production of many cheese varieties, fermented milk products, and ripened cream butter.
about thermophilic LAB cultures
have their optimum growth temperature between 48C and 58C. Thermophilic starters are used for yogurt and for cheese varieties with high cooking temperatures (e.g. Emmental or Gruyere).
how does LAB fermentation strongly influence the final organoleptic qualities of cheeses? (6)
- fermenting sugars, leading to a pH decrease important in the clotting phenomenon
- reduction or prevention of the growth of adventitious micro-flora
- protein hydrolysis which causes the texture and, partially, taste of cheese
- Synthesis of flavor compounds
- Synthesis of texturing agents, which may influence the consistency of the product
- Production of inhibitory components
Proteolytic activity of LAB in chess production
- LAB must get amino acids from their surroundings, therefore most have several proteinases and peptidases which provide the bacteria with free amino acids
- All milk proteins, including whey proteins, are available for hydrolysis at the start of the fermentation reaction
- The hydrolysis of milk proteins is responsible for bitter tastes in cheese
what are the 2 categories of flavour compounds produced by LAB
- Compounds produced by fermenting milk (lactic acid, acetic acid, acetaldehyde, diacetyl, and acetoin)
- Compounds produced during cheese maturation (LAB have less of a role here)
what are the inhibitory components of LAB? (2)
- Very few bacteria can grow at the pH that LAB produce
- LAB also produce inhibitory substances including: hydrogen peroxide, diacetyl, and bacteriocins
benefits of vegetable fermentation with LAB (5)
- High degree of hygienic safety from pathogenic bacteria
- Products are still “clean label”
- Interesting and appealing flavours
- Less energy input than other methods of preservation
- Storage without refrigeration
how is sauerkraut made? what is the science behind it and the LAB involved
Sauerkraut:
- The concentration of fermentable sugars in cabbage is between 3 and 9% in fresh matter
- Before processing the cabbage is shredded and salt is mixed in, brine begins to form immediately
- The containers are sealed and pressed with a weight to maintain anaerobic conditions
- The lactic acid fermentation is initiated by Leuconostoc mesenteroidesand followed by Lactobacillus brevis, Pediococcus pentosaceus , and finally by Lb. plantarum
- Ln. mesenteroides produces lactic and acetic acids and CO2, which rapidly lower the pH, thus limiting the activity of undesirable microorganisms and enzymes that might soften the shredded cabbage
- The CO2replaces air and creates an anaerobic atmosphere, which is important to prevent the oxidation of ascorbic acid and to avoid darkening the natural color of the cut cabbage
