Microstructure and chemorheological behavior of whipped cream as affected by rice bran protein addition

 

The effect of rice bran protein (RBP) isolate addition on the rheological and structural properties of commercial whipped cream with 25% and 35% fat was investigated. Results showed that increasing the fat content from 25% to 35% leads to an increase in the elastic modulus. Furthermore, by increasing the amount of RBP from 1% to 3% in both creams, significant increase occurred in the complex modulus. As the fat content increased from 25% to 35%, the slope of flow behavior was increased, which revealed more thinning behavior and pseudoplasticity index of cream. The cream containing 35% fat and 3% RBP had also shown the low index (n = 0.298) which confirmed the firmer structure of the cream. The maximum consistency index (k) obtained was 9.41 for the cream with 35% fat and 3% RBP, which approved its strong foam structure. In general, according to our results it is obvious that whipped cream with the highest amount of fat and the lowest value of protein can lead to maximum stability of the whipping cream. Among the samples, the lowest stiffness was observed in cream of 35% fat, containing 3% rice bran protein. However, cream containing 35% fat and 1% RBP had convenient overrun and good stability. The microstructural results showed that the cream structure has relatively large globular aggregates in network and develops large pores, which permit to retain sufficient water/air. By increasing the fat content of cream from 25% to 35%, the voids and spaces in the cream were significantly decreased and the pores become less which improve the foam structure. Therefore, it can be concluded the cream with more fat has the more overrun and stability. In general, it is possible to improve the foam structure of cream by substituting fat by RBP.

Keywords: dairy, microstructure, rheology, rice bran protein, stability, whipping cream

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1. INTRODUCTION

Whipped cream is one of the most delicious dairy products used in dessert, pastries, cakes, and ice cream. It is a complex foam structure in which partially coalesced fat droplets stabilize air bubbles at the air–water interface. Air bubble introduction into whipping cream by mechanical agitation forms a more rigid structure, which is an interesting field of study in the aspect of viscoelastic behavior. Whipped cream's quality attributes are mainly affected by the rheological and structural properties. Since measuring the viscoelasticity of whipped cream is difficult, finding a correlation between the rheological properties and microstructure of the cream can be useful, and therefore, the microstructure of cream was investigated by cryo‐scanning electron microcopy methods. Noda & Shiinoki (¹⁹⁸⁶) have studied on this, and they have found a relation between the rheological properties and microstructure of whipped cream (Noda & Shiinoki, ¹⁹⁸⁶). Several factors such as fat content, processing conditions, and stabilizer/emulsifier addition can change the structural properties of whipped cream (Bruhn & Bruhn, ¹⁹⁸⁸). The minimum fat content of cream must be 30% to have a rigid foam structure, and the fat globule size would be in the range of 15–20 μm (Graf & Muller, ¹⁹⁶⁵). In order to promote partial coalescence and improve rigidity to the air bubble interface, at least 40% of the fat should be crystalline (Darling, ¹⁹⁸²).

Stability and rigidity of whipped cream are essential parameters in many food industries such as confectionary products. Addition of low molecular weight emulsifiers or stabilizers promotes the adsorption of partially coalesced fat at the air interface through a lowering of interfacial tension (Anderson & Brooker, ¹⁹⁸⁸; Paquin & Dickinson, ¹⁹⁹¹). However, the viscosity of the aqueous phase can be increased by the addition of large molecule polysaccharide stabilizers. It has been reported that carrageenan can interact with casein micelles and forms glycomacropeptide complex which would afford cohesion between membranes and the serum adding overall structural integrity to the foam. Therefore, several studies have been performed on the cream stabilizers including Aertex cream stabilizer (Smith, Goff, & Kakuda, ²⁰⁰⁰), locust bean gum and κ‐carrageenan (Camacho, Martínes‐Navarrete, & Chiralt, ²⁰⁰⁵), sodium caseinate, whey proteins, hydroxypropyl methylcellulose, and xanthan gum on the stability of whipped cream (Zhao, Zhao, Yang, & Cui, ²⁰⁰⁸, ²⁰⁰⁹).

Due to the functional and technological properties of whey, it has been found many applications in dairy products such as whipped cream and dairy foams. Indeed, the proteins can stabilize emulsions and foams (Nicorescu et al., ²⁰⁰⁸; Rullier, Axelos, Langevin, & Novales, ²⁰¹⁰). Proteins stabilize foams by strongly adsorbing to the air–water interfaces, forming viscoelastic adsorbed layers, and leading to a protein network with high viscosity (Rullier et al., ²⁰¹⁰). Several studies have been performed on the foaming properties and stability of whipped cream by whey proteins (Rullier, Novales, & Axelos, ²⁰⁰⁸; Zhu & Domodaran, ¹⁹⁹⁴). In comparison with whey, modified whey proteins have indicated more stability of whipped creams (Sajedi, Nasirpour, Keramat, & Desobry, ²⁰¹⁴).

Rice bran protein (RBP) has been considered as a suitable plant protein which has unique nutritional characteristics including reasonable protein efficiency ratio (2.0–2.5), high lysine content, more complete amino acid profile, and high digestibility (>90%) in comparison with other whole grain cereals or legumes (Wang et al., 1999; Juliano, 1994). It has also fascinating functional properties including foaming and emulsifying properties, which attracts researchers to get better understanding on their properties. For example, functional properties of rice bran protein isolate at different pH levels have been investigated (Esmaeili, Rafe, Shahidi, & Ghorbani Hasan‐Saraei, ²⁰¹⁵). It has found that RBP had higher surface hydrophobicity than that of casein and ovalbumin which can be utilized in the air–water interface systems. Moreover, as pH approaches to alkaline conditions, the protein solubility, and emulsifying and foaming properties of RBP have been improved and resulted in more overrun, which implies its excellent quality in making stable emulsions. In another work, the rheological behavior of RBP has been compared with some hydrocolloids such as xanthan, guar, and locust bean gum (Rafe, Mousavi, & Shahidi, ²⁰¹⁴). Results have shown that RBP had a weak network and non‐gelling ability which can be combined with gelling biopolymers such as whey protein. Therefore, the protein–protein interactions between RBP and whey were the issue of another survey to determine the compatibility of binary mixture of RBP and whey (Rafe, Vahedi, & Ghorbani Hasan‐Sarei, ²⁰¹⁶). It has been understood the elasticity of binary mixed system of RBP and whey protein concentrate was more than that of single biopolymer. However, the mixed system has shown thermodynamic compatibility and application feasibility in dairy formula and desserts, but adding fibrils of RBP to whey proteins induced more syneresis and less water‐holding capacity. In order to improve the people's health and decrease the cost of product, developing a low‐fat whipped cream is interesting. Therefore, the aim of the current work was to evaluate the rheological and structural properties of whipped cream influenced by adding RBP at varying levels to creams with different fat content to reduce the fat and produce a low‐fat whipped cream.

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2. MATERIALS AND METHODS

2.1. Materials

The commercially dried rough rice of Tarom cultivar was kindly provided by Rice‐lands of Falah, Inc. (Sari, Mazandaran, Iran). All the ingredients were of analytical grade and purchased from Sigma‐Aldrich (St. Louis, MO, USA).

2.2. Preparation of RBP

Rough rice (12% moisture content [wb]) was dehulled by a rice husker and debranned by a McGill No. 2 mill for 30 s. Then, the bran was immediately defatted to prevent lipid oxidation. Rice bran protein was extracted as previously described (Rafe et al., ²⁰¹⁴). The obtained RBP was freeze‐dried (Freeze dryer FDU‐8624, Operon, Gimpo city, Korea) and stored at −5°C for further experiments. The protein content was measured by the Kjeldahl method, and the other chemical composition of RBP such as crude fiber was determined based on our previous work (Rafe et al., ²⁰¹⁴). The content of moisture (925.10), ash (923.03), fiber (920.86), crude fat (920.39), and crude protein (920.87) was determined by the Association of Official Analytical Chemists methods (AOAC, 2002), and the content of carbohydrate was calculated by subtracting the amount of other compounds from 100. The chemical composition of Tarom RBP on the basis of weight of the protein showed it contains protein, fiber, carbohydrate, ash, and moisture as 77.61%, 14.20%, 4.65%, 1.78%, and 3.33%, respectively.

2.3. Rheological assay

2.3.1. Flow behavior

Whipped cream with 25% (low‐fat) and 35% (high‐fat) at varying concentrations of RBP from 1% to 3% was used. The samples were conditioned at 10°C for 2 min, and then, the shear rate was increased from 0.1 to 1,000 s⁻¹. Then, the experimental flow curve was plotted in shear stress (τ) versus shear rate (γ). The apparent shear viscosity (η _a) was determined as a function of increasing shear rate in the ramp‐up mode. The rheological behavior of cream with varying amount of RBP was fitted by using different models such as power‐law (Equation (1)), Bingham (Equation (2)), and Casson (Equation (3)) as follows: