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Structural Design Approaches for Creation of Reduced Fat Products

In awake of the health issues related with high-calorie diet, there was a strong focus in the development of reduced-fat products in the food industry. However, fat plays an important role in determining the quality attributes of food products such as texture, appearance, flavor and stability, therefore there has been limited success for reduced-fat products. This thesis project thus targeted liquid and semi-solid products such as dressings, sauces, and aimed to address problems associated with low-fat by utilizing structural designed approach. The first part of this project focused on using controlled aggregation of protein-stabilized lipid droplets to regulate the microstructure and physicochemical properties of a model system (i.e. mixed food dispersion) containing 2 wt% whey protein isolate-stabilized lipid droplets and 4 wt% modified starch (hydroxypropyl distarch phosphate). There were three mechanisms to induce aggregation: 1) when the pH was around the isoelectric point (pI) of whey protein (e.g., pH 5), lipid droplets became relatively neutral and aggregated due to reduced electrostatic repulsion; 2) when the pH > pI, aggregation can be induced by adding cations (preferably multivalent cations such as Ca2+), due to ion binding, charge neutralization and salt bridge formation; 3) when the pH < pI, aggregation can be induced by adding negatively charged polysaccharides such as xanthan gum, due to bridge flocculation. After the samples were heated (90 °C, 5 min), starch granule swelled, and the aggregation had a significant impact on the microstructure and rheological properties of the system. Extensive aggregation was achieved at pH = pI, or at high calcium (pH > pI) or xanthan concentrations (pH < pI). In the contrast, the lipid droplets appeared to coat around the swollen starch granules and formed three-dimensional network that trapped starch. Consequently, in spite of the low fat content, high viscosity, yield stress and shear modulus were obtained due to these unique aggregated microstructures. This phenomenon was ascribed to the increased effective volume fraction caused by aggregation. Advantageously, the rheological properties can be carefully regulated by controlling degree of aggregation which in turn was manipulated by the pH, ionic strength and/or polysaccharide type or content. However, the aggregation method required starch acting as inactive filler for this approach to work. In addition, the hydrolysis of starch granule by salivary a-amylase was known for a melting mouthfeel. Therefore, inspired by the functionalities of swollen starch granules, the second part of the thesis project focused on developing fat/starch mimetics using hydrogel particles formed by electrostatic complexation of gelatin with polysaccharides such as pectin. Hydrogel particles were formed by mixing gelatin with pectin at pH values above the pI of the gelatin (pH 10), followed by acidification to pH 5 (< pI of gelatin). The biopolymer mixture can thus spontaneously form micron-sized particles due to electrostatic attraction of cationic gelatin and anionic pectin. Because electrostatic complexation is essentially a charge neutralization process and follows nucleation and growth mechanism, the particle dimension was largely affected by the gelatin-to-pectin ratio, ionic strength, shear rate, and acidification rate. Due to the thermo-reversible gelation property of gelatin, the particle morphology was also regulated by the temperature annealing process (i.e., formation temperature, annealing/reheating temperature and incubation time). By carefully controlling the processing parameters, spherical hydrogel particles with similar dimensions to swollen starch granules were formed. The resulting hydrogel particles were harvested by centrifugation as hydrogel paste. This gelatin-pectin particle paste provided high yield stress and shear viscosity thus can be used to provide similar texture properties as swollen starch granules. Meanwhile, these hydrogel particles can melt at close to body temperature, thus has a great potential to mimic the melting mouth-feel of fat crystals or starch granules (a-amylase hydrolysis). In addition to pectin, hydrogel particles can also be formed between gelatin and other negatively charged polysaccharide. For example, mixing gelatin with octenyl succinic anhydride (OSA) starch led to formation of spindle to oval shaped particles. In all, this thesis project may have important implication on designing food materials with various microstructures and physicochemical properties for improving the quality of reduced-fat foods, and thereby help tackle health problems associated with high calorie diet.
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