Practice exam Flashcards
Why do we wait for items to cool down to room temperature before we weigh them?
- You must let the crucible cool before measuring because the heat from the crucible warms the surrounding air, which rises, then that air cools and falls.
- The rise and fall of the surrounding air is called a convection current and will give you an unsteady reading that is rising and falling.
- Water in the air can also condense onto hot objects.
What is the difference between pH and titratable acidity?
pH measures free protons (H+ ions) in solution (ability of an acid to dissociate)
Titratable acidity measures all H+ ions that can be neutralized by a base (useful with weak acids that do not fully dissociate)
What are the defining characteristics of products produced by air-drying and freeze-drying?
- Air-dried products have collapsed pores due to water moving through the product and evaporating form the product’s surface. Evaporation from the surface due to warm moving air also leads to case hardening. Combined, this results in a very dense and hard food product that does not rehydrate easily.
- Freeze-dried products have small pores as a result of water leaving the product via sublimation (as a gas) as opposed to moving through the product as a liquid. Since the water is not evaporated from the surface lke with air-drying, and due to the small pores remaining, freeze-dried products are brittle and not hard and dense. Small pores and lack of case hardening make freeze-dried products extremely hygroscopic.
- Additionally, air-dried products can be expected to be darker in colour due to use of higher temperatures that can accelerate chemical reactions, and the increased density of the food
What is the difference between accuracy and precision?
- Accuracy measures how close a measured value is to the true value.
- Precision measures how close together repeated measurements are, or in other words, the ability of an assay or equipment to produce the same measured value repeatedly.
What is the difference between reproducibility and repeatability?
- Repeatability refers to the ability to obtain the same result when an assay is performed by the same person, using the same equipment, but on different days.
- Reproducibility refers to the ability to obtain the same result when an assay is performed by different people, using different equipment.
Why should you average sample blank replicates before subtracting them from individual sample replicates?
- Blank technical replicates should always be averaged before subtracting them from individual sample technical replicates because individual technical blanks do not have any specific relationship to any particular sample technical replicate.
When is it beneficial to reverse pipette instead of forward pipette?
- Reverse pipetting is beneficial when you want to pipette a liquid that is dense or when you want to avoid creating bubbles in a solution. This is because reverse pipetting aspirates a volume that is larger than the set volume and therefore, if the liquid is dense and inaccurately aspirated into the pipette, the correct volume can still be dispensed.
- In contrast, forward pipetting will dispense the incorrect volume if an inaccurate volume is aspirated
- Additionally, since the last drop of liquid is not dispensed when reverse pipetting, air does not get pushed out into the solution, eliminating the formation of bubbles tha can interfere with results.
Why is it important not to touch items you are weighing, with your bare hands?
Hands contain natural oils, bacteria, and detachable skin cells that can add weight
Explain why Gram positive bacteria stain purple.
- When crystal violet is applied to bacterial cells, the purple dye passes through the outer membrane and binds to peptidoglycan in the cell wall.
- Iodine is then added to the cells and binds to the crystal violet, forming a larger molecule.
- In the next step, decolorizer is added, which dissolves the outer lipid membrane of gram negative cells and removes the crystal violet-iodine molecules bound in the thin peptidoglycan layer.
- In gram positive cells, the crystal violet-iodine molecules are trapped in the thick peptidoglycan cell-wall layer and cannot escape so the cell remains purple.
List 2 purposes of the pre-filter in the ISO-GRID FIltration apparatus.
- Filter out particles in solution
- Allow diluent to mix with the sample before being distributed across the hydrophobic grid
What would happen if you forgot to add solution 4 when quantifying sugar levels in different soft drinks? Be specific.
What about solution 2?
If solution 4 (phosphoglucose isomerase) was not added, F-6-P would not get converted into G-6-P and subsequently NADPH, and thus fructose levels in the solution would not be quantified, only glucose levels.
If solution 2 was not added, there would be no NADP+ or ATP to fuel the reactions and therefore neither glucose nor fructose would be quantified.
A farmer wants to determine the shelf-life of his lettuce at 95% relative humidity and 4 degrees C. Food products are considered spoiled when aerobic bacterial counts reach > 10^7 CFU/g.
How would you analyze the lettuce?
Would you use spread or pour plating?
Explain.
- Goal is to determine if product contains > 10^7 CFU/g, so that is target to enumerate
- Should plate 10^-3 to 10^-5 dilutions.
- Spread plating is best because we want to quantify aerobic bacteria and pour plating provides a more anaerobic environment. Also, we are assuming there is a high microbial load so having a large sample size (1 mL) is not necessary.
You discovered the casein solution you used to construct a standard curve was actually 1.7 mg/mL, not 2 mg/mL. How will this affect the protein concentration you determine?
Overestimation
Determine which means are different.
.
Gatorade and Rockstar are tied with 2% CV.
If the D-values for yeast in apple juice at 50 and 60 degrees C were 18.5 and 4.3, respectively, what is the Z-value?
What is the definition of the Z value?
Z value refers to the thermal resistance of a microbe. It indicates the change in temperature needed for a one-log reduction in the D-value.
- Pan has not be pre-dried in oven
- Triplicates should have been performed
- Weight not recorded to 4 decimal places
- Pans need to cool to room temperature before weighing
Assume spread plating.
Volume of 0.1mL plated in spread plating.
