Lab 14 Yogurt Microbial Analysis Part 1 Flashcards
(39 cards)
1
Q
Describe yogurt production.
A
- Yogurt is a fermented dairy product.
- Uses starter cultures (~1:1)
- The ratio of bacteria affects yogurt quality (e.g., texture, acidity, flavour)
2
Q
What are the yogurt starter cultures?
A
-
Streptococcus salivarius subsp. thermophilus
(More commonly called Streptococcus thermophilus) -
Lactobacillus delbrueckii subsp. bulgaricus
(More commonly called Lactobacillus bulgaricus)
3
Q
Describe the mutually beneficial relationship between the two yogurt starter cultures.
A
- Lb has more proteases to release amino acids (St lacks the protealytic activity needed to degrade caseins)
- St produces formate, carbon dioxide, and folic acid (Lb needs these for purine synthesis)
- St makes lactic acid first (Lb has a lower optimal pH (5.5) than St (6.0 - 6.5)
- Together, they produce a better yogurt product than could be made by either one alone
4
Q
Which starter culture has more proteases to release amino acids?
A
Lactobacillus bulgaricus
Streptococcus thermophilus lacks the proteolytic activity needed to degrade caseins.
5
Q
Which starter culture produces formate, carbon dioxide, and folic acid?
A
Streptococcus thermophilus
Lactobacillus bulgaricus needs these substrates for purine synthesis.
6
Q
Which starter culture makes lactic acid first?
A
Streptococcus thermophilus
Lactobacillus bulgaricus has a lower optimal pH (5.5) than Streptococcus thermophilus (6.0 - 6.5)
7
Q
Which starter culture has a lower optimal pH?
A
Lactobacillus bulgaricus has a lower optimal pH (5.5) than Streptococcus thermophilus (6.0 - 6.5)
8
Q
Define: probiotic bacteria.
A
- Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host
9
Q
What does Health Canada require for products advertised as probiotic?
A
A minimum of 10^9 CFU/serving throughout the shelf life of the product.
10
Q
What are the two most common strains of probiotics used?
A
- Bifidobacterium and Lactobacillus
11
Q
What is selective media?
A
- Allows certain types of microbes to grow, while inhibiting the growth of other microbes.
12
Q
Give two examples of selective media mechanisms.
A
- Organisms that can utilize a specific sugar are easily screened by making that sugar the only carbon source in the medium
- Inhibition of other organisms by adding dyes, antibiotics, salts or specific inhibitors which affect the metabolism or enzyme systems of the organisms
13
Q
Describe MRS agar.
de Man, Rogosa, and Sharpe (MRS)
A
- Contains (1) polysorbate, (2) acetate, (3) magnesium, and (4) manganese which promote growth of lactobacilli
- (1) Sodium acetate and (2) ammonium citrate are used to inhibit many microbes
- Anaerobic incubation inhibits aerobic microbes
- Lactobacilli appear as large white colonies
14
Q
Describe M17 agar.
A
- Ascorbic acid stimulates the growth of Streptococcus spp.
- Disodium beta-glycerophosphate inhibits growth of lactobacilli (strong buffering capacity; keeps pH at ~6.8 which is too high for lactobacilli)
- Aerobic incubation inhibits anaerobic microbes
15
Q
Describe BSM agar.
A
- Bifidus Selective Media (BSM)
- Selective salts inhibit the growth of moulds, and gram-negative bacteria
- Three antibiotics also used to inhibit other bacteria.
- Bifidobacterium spp. can reduce a dye present in the medium (gives the colonies a pink-purple colouration
- Anaerobic incubation inhibits aerobic microbes
16
Q
What is gram-staining useful for?
A
- Helpful for confirming the ID of colonies on selective agar plates based on if cells stain purple (gram positive) or pink (gram-negative), as well as if they are cocci, bacilli, or spiral shaped, etc.
17
Q
Describe quantitative descriptive sensory analysis.
A
- Trained panelists detect differences among products and indicate which sensory attributes are different and to what extent.
- The intensity of various sensory attributes are scored on a line scale and then the resulting scores are compared among products
18
Q
What is the selective media for S. thermophilus?
A
M17
19
Q
What is the selective media for L. bulgaricus?
A
MRS
20
Q
What is the selective media for B. animalis?
A
BSM
21
Q
What are the incubation conditions for M17 media?
A
aerobic, 35C, 48 hours
22
Q
What are the incubation conditions for MRS media?
A
anaerobic, 35C, 48 hours
23
Q
What are the incubation conditions for BSM?
A
anaerobic, 35C, 72 hours
24
Q
How is line data from descriptive sensory analysis converted?
A
Measure the distance of the mark from the left end of the line.
25
How will *Bifidobacterium* colonies appear on the selective agar?
Red
26
How will *Lactobacillus* colonies appear on the selective agar?
large, white
27
During yogurt fermentation, which starter culture grows first? What are the ratios by the end of fermentation?
Both bacteria are typically added at a 1:1 ratio to pasteurized milk. During incubation, the *Streptococcus* initially grows more rapidly as the pH is too high for the *Lactobacillus*, but eventually they reach approximately equal levels by the end of fermentation.
28
How does *Streptococcus* appear on selective agar?
beige
29
Describe M17 agar.
* Contains a wide variety of nutrients needed for *S. thermophilus* to grow, including lactose.
* Since *S. thermophilus* produces large amounts of acid from lactose, disodium-β-glycerophosphate is added to the agar to provide a buffering system so that microbial growth is not inhibited over time due to the ever decreasing pH of the media.
* The glycerophosphate additionally suppresses the growth of *L. bulgaricus.*
* Ascorbic acid is also used in the agar to selectively stimulate *S. thermophilus* growth.
* The agar is light brown in colour and *Streptococcus* colonies appear beige.
* Note, this agar can also support the growth of *Lactococcus lactis* but incubating the plates at 35ᵒC as opposed to 30ᵒC can help suppress their growth.
30
Describe MRS agar.
* This agar media is designed to isolate *Lactobacillus* spp. but can be made more selective for *L. bulgaricus* by acidifying the pH to 5.4.
* The agar contains a variety of nutrients needed for *Lactobacillus* spp. to grow and contains ammonium citrate and sodium acetate to suppress the growth of *Streptococcus* spp..
* The agar is brown in colour and *Lactobacillus* colonies appear white.
31
Describe BSM agar.
* Used to isolate *Bifidobacterium* spp.
* This media contains selective salts and three antibiotics that inhibit moulds, enterococci, and other Gram-negative bacteria, and a compound that inhibits glycolysis and likewise, aerobic bacteria.
* *Bifidobacterium* spp. can reduce an azo compound in the media which gives the colonies their dark red/brown colouration.
* The media itself is light brown with a slight hint of yellow.
32
Yogurt is not subject to microbial spoilage.
True or False?
False.
Yogurt is still subject to spoilage by yeasts and moulds which are capable of growing at low pH (<4.6).
33
Yogurt is subject to microbial spoilage.
True or False?
True.
Yogurt is still subject to spoilage by yeasts and moulds which are capable of growing at low pH (<4.6).
34
List typical discriminative tests. [4]
* Triangle
* Duo-trio
* Paired comparison
* Multiple comparison
35
When are discriminative sensory analyses conducted? Describe panelist requirements.
* Tests if samples can be distinguished, before proceeding to descriptive analysis.
* Panelists require much less training than in descriptive tests, but many more panelists are required (typically many more than 20).
36
Describe descriptive analysis.
* In descriptive analyses, small groups of highly trained judges, who are knowledgeable about the commodity under study, develop adjectives to characterize the product and evaluate the intensity of each characteristic.
* In some studies, a variety of attributes are analyzed – appearance, aroma, texture, taste, aftertaste, among others. In other studies, judges may concentrate on one or a few sensory attributes.
* During descriptive analyses, an interval scale may be used to quantify the level of each attribute in the food product.
* This scale consists of a horizontal line with anchor points at equal intervals on the line. Each anchor point is labeled with a word or expression.
* A separate line is used for each sensory property to be evaluated.
* Each panelist records their evaluation by making a vertical line across the horizontal line at the point that best reflects their perception of the magnitude of that property.
* Once the panelists have completed their evaluations, the experimenter uses a ruler to measure the distance of each rating from the left end of the line to obtain a score; these scores are then tabulated and statistically analyzed.
37
*S. thermophilus* produces large amounts of acid from lactose. How do selective media combat this so its growth is not inhibited over time by the ever-decreasing pH?
M17 contains disodium-β-glycerophosphate as a buffering system
38
How does M17 agar select for *S. thermophilus*?
* Contains lactose and a wide variety of nutrients needed for *S. thermophilus*
* Ascorbic acid selectively stimulates its growth
39
How does M17 agar prevent growth of *L. bulgaricus* which can also use lactose for energy?
* It contains **glycerophosphate** which suppresses *L. bulgaricus growth*
