Characterisation and application of fruit by-products as novel ingredients in gluten-free products

Abstract

Literature has revealed that “waste” left from the processing of fruit can still contain a substantial quantity of macro and minor nutrients. The aim of this thesis was to ascertain the nutritional and structural properties and potential uses of two fruit by-products [apple pomace (Malus domestica Cv. “Karmijn de Sonnaville”) and orange pomace (Citrus sinensis L. Cv. “Valencia”)] in glutenfree bread and extruded snack formulations. The physicochemical and nutritional properties of the fruit by-products were initially studied. Apple pomace contained a high level of fibre and pectin. The isolated pectin was demonstrated to have a high level of methylation which developed viscous pastes. Orange pomace also had high levels of fibre and pectin, and it was an abundant source of minerals such as potassium and magnesium. Orange pomace had a poor gelling ability. The flour obtained after milling dried orange pomace was used in the formulation of gluten-free bread with the aid of a response surface design. Due to the fibrous properties of orange pomace flour, proofing and water addition were also studied. When added at levels greater than 6%, the loaf volume decreased. The number of cells per slice also decreased with increasing orange pomace addition. Inclusion of orange pomace at levels of up to 4% increased crumb softness. An optimised formulation and proofing time was derived using the optimisation tool; these consisted of 5.5% orange pomace, 94.6% water inclusion and with 49 minutes proofing. These optimised parameters doubled the total dietary fibre content of the bread compared to the original control. The pasting properties, rheology, microstructure and sensory characteristics of the optimised formulation (batter and bread) were investigated. Pasting results showed how orange pomace inclusions reduced the final viscosity of the batter, hence reducing the occurrence of starch gelatinisation. Rheological properties such as the storage modulus (G') and complex modulus (G*) increased in the orange pomace batter compared to the control batter. This demonstrates how the orange pomace as an ingredient improved the robustness of the formulation. Sensory panellists scored the orange pomace bread comparably to the control bread. Milled apple pomace was studied as a potential novel ingredient in an extruded snack. As extrusion requires the trialling of a number of extruder parameters, a response surface design was again used to develop an optimised snack. The parameters studied were apple pomace addition, die head temperature and screw speed. Screw speed had the most significant impact on extrudate characteristics. As screw speed increased the favourable extrudate characteristics such as radical expansion ratio, porosity and specific volume decreased. The inclusion of apple pomace had a negative effect on extrudate characteristics at levels greater than 8% addition. Including apple pomace reduced the hardness and increased the crispiness of the snack. Using the optimisation tool, the optimised and validated formulation and extrusion process contained the following parameters: 7.7% apple pomace, 150oC die head temperature and a screw speed of 69 rpm.