Summary: Food Ingredient Functionality: Proteins

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  • 2 Introduction: Proteins in foods

  • 2.1 Sources of proteins as ingredients

  • Which 3 roles can protein play?

    • Nutritional value --> essential amino acids
    • Bio-activity --> allergic reactions, reduction blood pressure
    • Physical/Technological functionality --> stabilise structures such as foam, emulsions, gels, in a food matrix
  • What are the drivers to study protein ingredient functionality?

    • Variability between variants/batches of 'the same' ingredient
    • Increase flexibility in applications of the ingredient
  • 2.2 Why we study: challenges in understanding functionality of protein ingredients

  • Why is the understanding of protein functionality different compared to polysaccharides?

    • Most proteins are quite similar in structure, there is no system to classify the food proteins from different sources and to give specific characteristic properties
    • For most proteins the properties of the final 'protein ingredient' is not only determined by the protein 'native' properties, but also very much by the way the ingredient was produced
  • The different steps in the production process of protein ingredients lead to two important consequences on their functionality:

    1. There can be a large variation in functionality within a set of similar protein ingredients (e.g. Whey proteins)
    2. Because of (1), the difference in functionality between proteins from different sources (e.g. Whey, soy) can be similar or smaller than the range of variation within a set of samples from each source separately

    The variation within a set of 'similar' protein ingredients (e.g. Whey protein concentrates) can be as large as variation between sets of different protein ingredients (e.g. Soy protein isolate versus whey protein isolate).
  • 3 Analytical techniques

  • What can be measured to identify different proteins?

    • Protein content 
    • Molecular weight distribution
    • Solubility
    • Degree of hydrolysis (based on free amino group content) 
  • 3.1 Protein content

  • What is a basic analytical method to identify different protein samples, if you research protein content?

    The nitrogen content using DUMAS or Kjedahl.
  • 3.2 Molecular weight distribution

  • What are 2 basic analytical methods to identify different protein samples, if you research molecular weight distribution?

    1. SDS-PAGE, protein composition
    2. Size-exclusion chromatography
  • What happens when researching the protein composition using SDS-PAGE?

    SDS is a surfactant that binds to the proteins, causes folded proteins to unfold, masks the protein's original charge and gives all proteins a similar charge-to-mass ratio. It also dissociates non-covalent complexes, such as the casein micelle into individual polypeptide chains. Only individual polypeptide chains are shown on the gel.
  • How are samples separated in size-exclusion chromatography?

    Separated based on their hydrodynamic volume, which is roughly related to the molecular weight. Note: 'native' conditions are used.
  • 3.3 Solubility

  • What is a basic analytical methods to identify different protein samples, if you research solubility?

    • Dissolve amount of protein (far from pI).
    • Protein solution divided over different containers, pH is adjusted
    • Samples centrifuged (to precipitate insoluble proteins)
    • Concentration of protein in supernatant determined with colorimetric assay (e.g. Bradford, Biuret, bichronic acid)
    • Solubility --> concentration of protein in supernatant relative to that in reference sample or to supernatant of sample where it was well dissolved

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