Protein–polysaccharide interactions and aggregates in food formulations

Abstract

The protein–polysaccharide combinations that lead to electrostatic complex and coacervate formation are the object of extensive research using both layer-by-layer and mixed emulsion approaches. The protein–polysaccharide conjugates demonstrated interesting physicochemical properties as stabilizers and emulsifiers, as well as texture modifiers in food products. Furthermore, they are potential optimal nutrient delivery systems. Their complex behavior due to several factors such as pH, ionic strength, concentration, heat, and mechanical treatments is the main reason behind the continuous growth of the research field. The review is reporting some recent advances on the topic, along with an overview of the possible interactions between protein and polysaccharide, from Maillard reaction to enzymatic cross-linking passing through coacervates.

Introduction

The organization of food constituents at multiple spatial scales and their interactions is the so-called food structure [1, 2, 3]. The food structure is a function of the food ingredients and production process. The composition of the food and its structure determines the texture, the perceived attribute, and the mechanical properties [4]. Food goes through several steps during consumption. The initial mechanical breakdown of the food structure starts with the mastication and mixing using the tongue. The second step is the lubrication, hydration, and dissolution due to the enzymatic action of the saliva, the small broken piece of food is then converted to bolus and swallowed through the esophagus into the stomach. Food industries are facing the problem to make foods healthier and at the same time not diminish sensory quality [5,6]. Moreover, nutrient delivery systems, microencapsulation, and protection of active ingredients and as consequence dispersant agents are to be taken into account during structural design [7∗]. Manufactured foods commonly exist in the colloidal state as emulsions, foams, gels, and dispersions. The food colloidal science investigates the influence of ingredient composition and formulation conditions on structure, stability, and mechanical properties [8∗]. The texture concern is mainly the flow behavior for dispersions, while breaking force, breaking strain, and the size distribution of the newly formed particles are the main physical parameters that can be correlated to hardness, brittleness, crispness, crunchiness, and crumbliness for solid-like foods. The mouthfeel of food emulsions is strongly influenced by the type, concentration, and interactions of the particles and macromolecules present. The perceived fattiness, creaminess, and thickness of oil-in-water emulsions have been found to increase as the droplet concertation increases [9]. The creaminess was also found to depend on droplet size and on the emulsifier type, which might be due to droplet flocculation and emulsion viscosity. During the consumption of some food emulsions, there is a cooling sensation associated with melting of emulsified fat in the mouth due to the endothermic enthalpy change associated with fat crystal melting [10,11].

To obtain proper texture and to stabilize the food products, several emulsifiers, solubilizers, and dispersing agents are adopted. The focus of this review will be the protein–polysaccharide combinations in conjugates or complex structures and their multiple applications in food industries. The protein–polysaccharide combinations are particularly interesting because they are able to change product shelf life by varying food texture, that is, rheological properties of food colloids [12, 13, ∗∗14], for such reason they have been the object of intense research [15, ∗∗16, ∗∗17, 18, 19, 20, 21]. Since then there are several interesting reviews on the matter [4,∗7, ∗8,∗∗14, ∗∗17,21,22∗] along with a vast literature. This review should not be considered fully comprehensive, while the intent is to provide a general overview of the topic along with the most recent findings.

Food proteins can avoid flocculation of emulsion droplets because they have a strong tendency to adsorb onto hydrophobic–hydrophilic interfaces and subsequently unfold (or partially unfold) forming relatively thin adsorbed surface layers (∼2–6 nm) that generates electrostatic and steric stabilization [23]. The van der Waals attraction force between colloidal particles overcome the electrostatic repulsion, at the same time the protein charge is the main reason for which colloidal stability can be lost. At the isoelectric point (pI) the negative and positive charges are balanced, reducing repulsive electrostatic forces and causing aggregation and precipitation [24,25]. Moreover, several studies on proteins demonstrated how pH, ionic strength, concentration, and heat treatment influence the pI [26,27].

The protein and polysaccharide combinations allow designing an amphiphilic conjugate to be strongly anchored to the oil–water interface via the protein's hydrophobic regions, leading to a viscoelastic layer, with the nonadsorbing polysaccharide (copolymer) region to provide enhanced steric stabilization [16∗∗] that can lead to gelling behavior.

There are two main kinds of interactions between polysaccharides and proteins: covalent or noncovalent bonds. The covalent bond is obtained through a Maillard-type reaction that it is leading to protein–polysaccharide conjugates with elevated heat stability. However, the reaction conditions, such as pH and temperature, should be properly settled to obtain the desired reaction. The next paragraph will address the reaction condition needed for a successful reaction. The driving forces for the noncovalent bonds are electrostatic, hydrophobic, H bonding, and Van der Waals interactions, such forces can generate coacervates that are useful tools to change food texture and encapsulate active compounds. One relevant topic of this review is the combination of the Maillard reaction with the electrostatically driven aggregates such as coacervates.