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FNH 325 > Instructions > Flashcards

Instructions Flashcards

1
Q

Differentiate between a serological pipette and a Mohr pipette.

A
  • Mohr; some solution remains in after dispensing
  • Serological: all solution dispensed for accurate volume; markings all the way to bottom of tip
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2
Q

When pipetting, what volume has the greatest level of error?

A

Smaller volumes

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3
Q

What type of pipetting should be used for aqueous solutions?

e.g., buffers or diluted salt

A

Forward

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4
Q

What type of pipetting should be used for viscous solutions?

e.g., protein or glycerol

A

Reverse, users should pipet slowly to avoid bubbles and inaccuracies

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5
Q

What type of pipetting should be used for volatile compound solutions?

e.g., methanol; hexanes

A

Reverse

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6
Q

What type of pipetting should be used for nucleotide solutions?

e.g., DNA; PCR products

A

Forward

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7
Q

What type of pipetting should be used for acids and alkali solutions?

e.g., HCl or NaOH

A

Forward

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8
Q

What type of pipetting should be used for toxic sample solutions?

A

Reverse or forward

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9
Q

When should you never reverse pipette?

A

If the volume exceeds 75% of the maximum pipetting volume; extra liquid will flood the shaft resulting in damage to, or contamination of, the pipette.

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10
Q

Describe forward pipetting.

A
  • Depress to stop 1.
  • Aspirate
  • Depress to stop 1
  • Depress to stop 2 to completely empty tip.
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11
Q

Describe reverse pipetting.

A
  1. Depress to stop 2.
  2. Aspirate
  3. Depress to stop 1.
  4. Do not fully empty tip.
  5. Depress to stop 2 to return excess fluid to original sample or disposal container.
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12
Q

What happens if the reagent changes from green to purple before adding a protein sample?

Biuret reaction a.k.a. BCA protein assay

A

Glassware or pipette tips contaminated reagent (very sensitive to metal ions)

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13
Q

What are acids and bases according to the Bronsted-Lowry Theory of neutralization?
What defines pH?

A
  • Acids are capable of donating protons.
  • Bases are capable of accepting protons.
  • pH is the negative logarithm of the activity of hydrogen ions; pH = log (1/[H+]) or pH = -log [H+]
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14
Q

Why does pH change with temperature?

A
  • As temperature rises, molecular vibrations increase which results in the ability of water to ionize and form more hydrogen ions.
  • As a result, the pH decreases as temperature increases.
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15
Q

What is a potentiometer?

A
  • pH meter
  • Composed of 2 electrodes and a voltmeter
  • Hydrogen ion concentratoin (activity) is determined by the voltage that develops between the two electrodes.
  • The Nernst equation relates the electrode response to the activity.
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16
Q

How do you prepare BCA reagent from reagents A and B?

A

50 parts A to 1 part B

17
Q

How long is prepared BCA working reagent stable?

A

For a short duration in a closed container at room temperature

18
Q

What is required of the diluent in microbial enumeration?

A
  • osmolarity simiilar to the sample (otherwise microbes may lyse and die)
  • does not support microbial growth
19
Q

What are two important sources of error in preparing microbial serial dilutions?
How can they be avoided?

A
  1. Sampling error resulting from unequal distribution of bacteria in original sample or dilution tubes; avoid by ensuring representative sample and always mix well before removing an aliquot
  2. Inaccuracy in pipetting; avoid by always examining pipette to ensure correct use, and limit the number of transfers within reason since more transfers = greater chance of error.
20
Q

What are some factors that may result in a gram positive bacteria staining as if it were gram negative?

A
  • Overheating during heat fixation, over decolorization with alcohol, and even too much washing with water between steps may result in gram-positive bacteria losing the crystal violet-iodine complex
  • Cultures more than 24 hours old may lose their ability to retain the crystal violet-iodine complex
  • The organism itself; some gram-positive bacteria are better able to retain the crystal violet-iodine complex than others.

Therefore, use precise technique and interpret results with discretion.

21
Q

Which type of bacteria has a thick peptidoglycan layer?

A

Gram positive

(no outer membrane)

22
Q

Which type of bacteria has a thin peptidoglycan layer?

A

Gram negative

Outer membrane

23
Q

How is the gas flow rate of a Bunsen burner adjusted?

A

Gas adjustment knob.
Using the gas supply is poor technique since the adjustment is insensitive.

24
Q

Describe effective packaging material.

A
  • Nontoxic
  • Protects against microbial, insect, and other rodent contamination
  • Barrier to moisture, and oxygen
  • Protects against environmental toxicants
  • Filters out UV light
  • Provides resistance to physical damage.
25
Q

What are the main factors that determine the shelf-life of a food?

A
  • Rate of oxygen and moisture transfer through packaging
26
Q

Lactic acid is the major product of lactose fermentation.
What are the minor products?

A
  • Acetate
  • Ethanol
  • Volatile fatty acids
  • Contribute to aroma
27
Q

Why does yogurt thicken during fermentation?

A

Increasing acidity coagulates milk proteins

28
Q

What is the best quality indicator for foods that contain weak acids?

A

Titratable acidity, since it measures undissociated acids in a solution in addition to the [H+]

29
Q

Why is phenolphthalein referred to as a secondary indicator?

A

It changes from clear to pink ~8-9.6 pH. This is only close to the equivalent point where the moles of acid equals the moles of base and does not directly identify the equivalence point.

30
Q

Why is vacuum drying a more accurate method for determining % moisture in a food?

A

By applying low pressure, water can be removed using lower temperatures and shorter drying times than air-drying.

31
Q

What is ash?

A

Inorganic residue that remains from the incineration or complete oxidation of organic matter, and represents the total mineral content in foods.

32
Q

What may be influenced by manipulation of water activity in food?

A
  • Texture
  • Non-enzymatic browning reactions
  • Enzymatic activity
  • Lipid oxidation
  • Microbial growth
33
Q

What may colour of food indicate?

A
  • Nutrition
  • Flavour
  • Functional values
  • Consumers use colour when deciding what to purchase
34
Q

What foods are particularly sensitive to moisture and oxygen transfer through packaging?

A

Foods that are dehydrated or contain appreciable quantities of lipids or other oxygen-sensitive components.

FNH 325 (5 decks)

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