Lecture 6: Protiens

Question 1

what is the role of protien in food?

Answer 1

• major mass of food,
• responsible for organoleptic properties
• functional and nutrional properties/values

Question 2

what are the two main groups of quantiative determination of protiens?

Answer 2

1. methods of determination of total nitrogen
2. methods that exploit chemical and physical properties of protiens

Question 3

what is the average conversion factor and how to find it

Answer 3

= 6.25

= 100/% protien

Question 4

Describe the three steps of the kjedlahl method

Answer 4

1. digestion: conversion of organic nitrigen to ammonia by adding sulphuric acid, heat and a catalyst
2. neutralization/distilation: neurtalzation by adding NaOH so ammonium sulphate is turned to ammonia to be distiled by boric acid
3. titration: adding a strong acid to restore intitial pH bufer

Question 5

what are the advantages and disadvantges of the kjedlahal method?

Answer 5

A: pffical method, highly accurate, good reproducibility

D: includes all nitrogen, long, 6.25 is an overestimation, risk with sulfuric acid at high temp

Question 6

Describe a second method based on the determination of tptal nitrogen

Answer 6

Dumas Method:

• combustion then reduction reaction
• organic material is put to intense heat and is made into water, carbon dioxide, nitorgen gas
• the CO2 is captured by a [] solution of alkalkine and then nitrogen gas is measured by guage or chromatography
• volume of gas=nitrogen content
• A: faster, no toxic chemicals, easy
• D: high cost to start, includes all nitrogen

Question 7

What are the five methods based on the chemical and physical properties of protiens

Answer 7

1. Biuret Method
2. Lowry Method
3. Bicinchoninic Acid Method
4. Spectroscopy in the UV region
5. Infrared Spectroscopy

Question 8

Describe the Biuret Method

Answer 8

• based on properties of peptide bonds
• reaction wtih copper ions in alkaline medoum and give a purple color
• reducing sugars, nucleic acids, bile pigments etc can interfere

Question 9

Describe Lowry Method

Answer 9

• peptide bonds
• reaction with phenolic groups of tyrosine/tryptophan using the Folin-Ciocalteau reagent
• happens in two parts 1. reduction of copper ions in presence of protiens 2. reduction of F-C reagent to blue colored complex

Question 10

Describe BCA method

Answer 10

• alternative to lowry
• based on structure, number of bonds, and presence of cys, trp, tyr
• easier and less interference
• 2 step rxn: 1. copper is reduceduced then 2. Cu+ reacts with BCA to change color from green to purple

Question 11

describe Spectroscopy in the UV region

Answer 11

• based on aromatic aa like tryptophan to absorb radation in UV region
• fast, direct, non-destructive, senstive to low amounts of protiens
• affected by aa compostion and presence of nucleic acids, interfeence so need a lot of sample prep, hard to extract from foods

Question 12

describee infrared spectroscopy

Answer 12

• rapid deterimination
• absorbtion of electromagneic radtion related to amount of protien in sample
• involve correction factors, accuracy depend on calibration of method

Question 13

How is the chemical index calculated?

Answer 13

from essential AA content of protien anyaluzed using a reference protien

CHemical index= aa pf protien studied/ aa of reference protien x 100