Lab 14 Yogurt Microbial Analysis Part 1 Flashcards
Describe yogurt production.
- Yogurt is a fermented dairy product.
- Uses starter cultures (~1:1)
- The ratio of bacteria affects yogurt quality (e.g., texture, acidity, flavour)
What are the yogurt starter cultures?
-
Streptococcus salivarius subsp. thermophilus
(More commonly called Streptococcus thermophilus) -
Lactobacillus delbrueckii subsp. bulgaricus
(More commonly called Lactobacillus bulgaricus)
Describe the mutually beneficial relationship between the two yogurt starter cultures.
- 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
Which starter culture has more proteases to release amino acids?
Lactobacillus bulgaricus
Streptococcus thermophilus lacks the proteolytic activity needed to degrade caseins.
Which starter culture produces formate, carbon dioxide, and folic acid?
Streptococcus thermophilus
Lactobacillus bulgaricus needs these substrates for purine synthesis.
Which starter culture makes lactic acid first?
Streptococcus thermophilus
Lactobacillus bulgaricus has a lower optimal pH (5.5) than Streptococcus thermophilus (6.0 - 6.5)
Which starter culture has a lower optimal pH?
Lactobacillus bulgaricus has a lower optimal pH (5.5) than Streptococcus thermophilus (6.0 - 6.5)
Define: probiotic bacteria.
- Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host
What does Health Canada require for products advertised as probiotic?
A minimum of 10^9 CFU/serving throughout the shelf life of the product.
What are the two most common strains of probiotics used?
- Bifidobacterium and Lactobacillus
What is selective media?
- Allows certain types of microbes to grow, while inhibiting the growth of other microbes.
Give two examples of selective media mechanisms.
- 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
Describe MRS agar.
de Man, Rogosa, and Sharpe (MRS)
- 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
Describe M17 agar.
- 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
Describe BSM agar.
- 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
What is gram-staining useful for?
- 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.
Describe quantitative descriptive sensory analysis.
- 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
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.
