13 Questions
What is a characteristic of mold?
It is a strict aerobic microbe
Why may mold and yeast cause spoilage in food?
Because bacterial growth is restricted
What is the purpose of adding 0.1% buffered PW to the food sample?
To provide a uniform environment for microbial growth
What is the purpose of incubating the plates at 25 oC for 5 days?
To allow for the growth of yeast and mold
What is the function of the Sabaurude Agar (SA) or Potato Dextrose Agar (PDA) in the enumeration of yeast and mold?
To select for yeast and mold
What is the purpose of the catalase test in bacterial testing?
To detect the presence of catalase enzyme
Mold is an anaerobic microbe.
False
Yeast and mold can only cause spoilage in food when bacterial growth is unrestricted.
False
All mold strains are capable of producing mycotoxins.
False
The sample should be incubated at 37°C for 5 days.
False
PDA stands for Potato Dextrose Broth.
False
A positive catalase test is indicated by a lack of gas bubbles.
False
A green color in the O-F test indicates the production of mixed acids.
True
Study Notes
Mold and Yeast
- Mold is a strict aerobic microbe
- Both mold and yeast can cause spoilage in food when bacterial growth is restricted
- Some mold strains can produce mycotoxins, which can cause health problems in humans
Procedure for Enumerating Yeast and Mold
- Aseptically weigh 25g of the food sample in a stomacher bag
- Add 225ml of 0.1% buffered peptone water and stomach for 1 minute
- Perform 10-fold serial dilution of the sample (10^(-1) to 10^(-5))
- Spread plate onto Sabouraud Agar (SA) or Potato Dextrose Agar (PDA)
- Incubate the plates at 25°C for 5 days
Characteristics of Sabouraud Agar (SA) and Potato Dextrose Agar (PDA)
- SA and PDA are selective for yeast and mold
- PDA is a medium for mold and yeast growth
Bacterial Testing
Catalase Test
- Add H2O2 to the sample
- Presence of catalase enzyme breaks down H2O2 into H2O and O2, producing gas bubbles
- Positive result is indicated by the production of gas bubbles
Oxidase Test
- Use tetramethyl-p-phenylenediamine hydrochloride as a substrate
- Oxidase enzyme oxidizes the substrate, turning it deep purple
- Positive result is indicated by the deep purple color
- Pseudomonas is oxidase positive, while Lactobacillus and E. coli are oxidase negative
OF Test (Hugh and Leifson's Medium)
- Use bromothymol blue as an indicator
- Fermentation produces mixed acids, turning the indicator green
- Oxidation produces yellow color
- Pseudomonas and E. coli give positive results, while other bacteria give negative results
Mold and Yeast
- Mold is a strict aerobic microbe
- Both mold and yeast can cause spoilage in food when bacterial growth is restricted
- Some mold strains can produce mycotoxins, which can cause health problems in humans
Procedure for Enumerating Yeast and Mold
- Aseptically weigh 25g of the food sample in a stomacher bag
- Add 225ml of 0.1% buffered peptone water and stomach for 1 minute
- Perform 10-fold serial dilution of the sample (10^(-1) to 10^(-5))
- Spread plate onto Sabouraud Agar (SA) or Potato Dextrose Agar (PDA)
- Incubate the plates at 25°C for 5 days
Characteristics of Sabouraud Agar (SA) and Potato Dextrose Agar (PDA)
- SA and PDA are selective for yeast and mold
- PDA is a medium for mold and yeast growth
Bacterial Testing
Catalase Test
- Add H2O2 to the sample
- Presence of catalase enzyme breaks down H2O2 into H2O and O2, producing gas bubbles
- Positive result is indicated by the production of gas bubbles
Oxidase Test
- Use tetramethyl-p-phenylenediamine hydrochloride as a substrate
- Oxidase enzyme oxidizes the substrate, turning it deep purple
- Positive result is indicated by the deep purple color
- Pseudomonas is oxidase positive, while Lactobacillus and E. coli are oxidase negative
OF Test (Hugh and Leifson's Medium)
- Use bromothymol blue as an indicator
- Fermentation produces mixed acids, turning the indicator green
- Oxidation produces yellow color
- Pseudomonas and E. coli give positive results, while other bacteria give negative results
This quiz covers the procedure for enumerating yeast and mold in food samples, including preparation and serial dilution, as well as the importance of mold in food spoilage and human health.
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