Partial replacement of whey proteins by rapeseed proteins in heat-induced gelled systems: Effect of pH
Graphical abstract
Introduction
Proteins play an important role in our foods as nutritional and techno-functional ingredients. Animal proteins, such as milk proteins and gelatin, are the most commonly used proteins in food applications. However, the increasing cost of and demand for proteins has led to an increased interest in utilizing plant proteins as partial replacers of animal proteins. This is a challenging task in product development, because replacing one protein with another leads to products with different sensorial properties, such as texture, compared to the original formulations. Therefore, before replacing animal proteins by plant proteins it is essential to investigate and understand the consequences for the rheological and structural properties in specific protein mixtures.
Presently soy proteins (SP) dominate the plant protein market and over the past two decades have been widely studied for their gelation properties, in either acid- or heat-induced gelled systems, in combination with animal proteins, such as gelatin (Ersch, ter Laak, van der Linden, Venema, & Martin, 2015), whey proteins (WP) (Comfort and Howell, 2002, Jose et al., 2016, Roesch and Corredig, 2005), caseins (Beliciu and Moraru, 2011, Martin et al., 2016b, Nguyen et al., 2017), milk concentrate (Chronakis & Kasapis, 1993) and egg white (Su et al., 2014). Typically, the main aim of these studies has been to enable the design of products with novel rheological and mechanical properties.
Only a very limited amount of research has been performed on mixing animal proteins with other seed proteins in gelled systems apart from soy. Pea protein has been investigated in a single study (Wong, Vasanthan, & Ozimek, 2013) as has the replacement of milk protein by sesame protein in fresh cheese (Lu, Schmitt, & Chen, 2010). In the present study we have used proteins derived from rapeseed or canola and combined them with whey proteins in heat-induced gelled systems under different pH conditions. To our knowledge, heat-induced gelation of rapeseed proteins (RP) has only been investigated in single systems (Kim, Varankovich, & Nickerson, 2016a; Schwenke et al., 1998, Tan et al., 2014, Withana-Gamage et al., 2014, Yang et al., 2014) and in mixtures with polysaccharides (Uruakpa and Arntfield, 2004, Uruakpa and Arntfield, 2005).
Most previous research on protein mixtures have focused on the relationship between gel microstructure and rheological properties. In most cases, the authors compare the final properties of the mixed gelled systems to the proteins in isolation at total protein concentration. Only a few studies have compared the properties of mixed systems to the single systems at the same concentration as they have been added in the mixtures. Ersch et al. (2015) tested the mechanical properties of gelatin mixed with SP and reported that the mixtures closely followed the fracture stress of pure gelatin at the corresponding concentration without being much influenced by the SP:gelatin mixing ratio. Jose et al. (2016) compared the storage modulus (G′) of WP:SP gels to the sum of the individual contributions of WP and SP to G′ at the given concentrations as in the mixture. They observed a synergistic enhancement in the mixed gels, which depended on the mixing ratio.
In this study we compared the measured parameters of the mixed systems to the single systems at the corresponding concentration at which they were present in the mixture as well as at the same total protein concentration. The objective of the study was to understand the influence of incorporating RP into WP on the network and rheological properties of heat-induced gels using differential scanning calorimetry (DSC), confocal laser scanning microscopy (CLSM), image analysis and small deformation rheology.
Section snippets
Materials
Whey protein isolate (Lacprodan DI-9224) (88% protein, 0.2% lactose, 0.2% fat, 4.5% ash and 6% moisture; specified by the manufacturer) was provided by Arla Foods Ingredients Group P/S (Viby J, Denmark). Lacprodan DI-9224 contains approximately 50% β-lactoglobulin (BLG) and 20% α-lactalbumin (ALA), according to the supplier. Demineralized rapeseed protein isolate (Isolexx, manufacturing Lot # BIOEXXI20130225) (92% protein, 0.6% fat, 1% ash and 6% moisture; specified by the manufacturer) was
Phase behavior of protein dispersions prior to heating
After sample preparation single protein dispersions and mixtures were examined visually. Single WP systems at pH 7 and pH 3 were transparent to opaque, while at pH 5 (close to the isoelectric point of whey proteins, pI ≈ 5 (Zydney, 1998)) WP dispersions were more turbid and cloudy. More specifically, turbidity of WP samples as a function of pH increased in the order: pH 3 < pH 7 < pH 5. At pH 5 precipitation could be observed over time, especially at low protein concentration (2.5% w/w),
Conclusion
WP and RP were able to form synergetic gels under conditions where RP modified the microstructural and/or denaturation properties of WP upon mixing. The extent of the synergistic enhancement was dependent on the disparity of the modification exerted by RP. The disparity of the modification was influenced by the protein mixing ratio and was generally more pronounced at high levels of RP. Synergistic gelation was evidently achieved at neutral pH conditions where both proteins were able to form
Funding
This research was supported by The Danish Council for Strategic Research (Grant No. 1308-00019B).
Acknowledgements
The authors would like to thank Luis Rozenszain and Samah Garringer from Teutexx Proteins for kindly donating the rapeseed protein isolate as well as Mikka Stenholdt Hansen from Arla Foods Amba for supplying to us the whey protein isolate.
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