Adding apple pomace as a functional ingredient in stirred-type yogurt and yogurt drinks

Highlights

• Apple pomace (3%) reduced the syneresis of stirred yogurt by half that of the control.

• AP addition (2–3%) resulted in a firmer, more viscous and cohesive stirred yogurt.

• AP contributed total phenolics and dietary fibre to stirred yogurt.

• AP (4–6%) could stabilise yogurt drinks with low MSNF (<5%).

Abstract

Apple pomace (AP) was processed into a freeze-dried powder to add into stirred-type yogurt to create 1, 2, and 3% (w/w) AP-fortified yogurt. Both texture analysis and rheological testing were used to evaluate the effect of AP on the structure and texture of yogurt during cold storage. The results indicated that apple pomace altered the structure of stirred yogurt, making it firmer and more cohesive, increased its viscosity and significantly reduced whey release during cold storage. Stronger influence on the parameters was observed at higher AP concentrations. Apple pomace added to a diluted yogurt system has the potential to stabilise the acid drink and reduce the sedimentation of protein aggregates. Addition of AP to already fermented milk gel allowed a higher concentration of AP (up to 6% w/w) to be incorporated, which made the stirred yogurt and yogurt drinks a vehicle for delivering a higher level of dietary fibre and phytochemicals (as compared to AP added to milk prior to fermentation). This study demonstrated that apple pomace has a potential as a natural stabiliser as well as a dietary fibre source in stirred type yogurts and yogurt drinks.

Introduction

Apple pomace derived from apple cider production has the potential to be transformed into an edible ingredient that can supply dietary fibre and phytochemicals when incorporated into food products (Shalini & Gupta, 2010). In addition to contributing valuable compounds, the functional properties of apple pomace, such as water holding capacity and gelling-texturizing capability have been used to improve the structure of food products (O'Shea, Arendt, & Gallagher, 2012). Previous studies have shown that AP powder exhibited good water retention capacity, solubility and swelling, as well as milk viscosifying capacity (Wang, Kristo, & LaPointe, 2019). Apple pomace contains mainly high-methoxyl (HM) pectin, which forms gels at acidic pH (Endress & Christensen, 2009; O'Shea et al., 2015). Therefore, apple pomace has the potential of stabilizing acidic dairy products such as stirred yogurt and yogurt drinks.

Stirred-type yogurt is produced from non-concentrated milk, with fruit pieces and flavours commonly added after fermentation. After completion of gel formation, gentle shearing is applied to the gel to obtain a consistent and viscous but still pourable semi-solid. Yogurt drinks are produced by diluting yogurt with fruit juice, sugar, and a dispersion of pectin in water to achieve the desired flavor, colour or consistency (Walstra, Wouters, & Geurts, 2005). However, both types of yogurts are often associated with “whey-off” issues, which negatively affect product quality and consumer acceptance (Loveday, Sarkar, & Singh, 2013). “Whey-off” is whey separation or syneresis defined as surface whey expelled from a gel during gelation and subsequent storage. Syneresis is caused by rearrangement of casein aggregates that favours escape of the serum from the three-dimensional gel network (Lucey, 2001). Stirring breaks down the gel structure, resulting in rearrangements of casein aggregates. Therefore, stabilisers are required to reduce syneresis and improve the texture of yogurt. Stabilisers, including gelatin, starch, pectin, alginate, carrageenan, derivatives of methylcellulose, gum arabic, tragacanth, karaya, locust bean gum or guar, are commonly used to achieve gelling, thickening and water binding functions in yogurt (Everett & McLeod, 2005).

However, stabilisers sometimes bring unfavourable flavour and texture to the yogurt. In addition, consumers demand “natural” products containing less or no stabilisers, or ingredients they can recognize (Loveday et al., 2013). Bimbo et al. (2017) investigated factors that affect consumer acceptance and preference for nutrition-modified dairy products and found that products with “natural” claims in terms of both carriers and ingredients showed the highest level of acceptance among consumers. Thus, novel and natural stabilisers are high in demand to meet the increasing “clean label” trend.

Some fruit and vegetable by-products have been considered as potential yogurt stabilizing agents due to their desirable functional attributes, such as water binding, gelling and thickening ability. The addition of yam soluble fibre to stirred type yogurt increased the viscosity, reduced the syneresis of yogurt and produced an acceptable mouthfeel (Ramirez-Santiago et al., 2010). In addition, passion fruit by-products, carrot cell wall particles, and pineapple peel and pomace powders have also been demonstrated to improve the structure and reduce the syneresis of yogurts (Espirito-Santo et al., 2013; McCann, Fabre, & Day, 2011; Sah, Vasiljevic, McKechnie, & Donkor, 2016). Apple pomace extract was utilized to create a fibre-enriched acidophilus yogurt, which showed acceptable sensory qualities (Issar, Sharma, & Gupta, 2017). A more recent study showed that incorporating water extract from apple pomace into yogurt formulation enhanced the antioxidant properties of the final product (Fernandes et al., 2019). However, to the best of our knowledge, research on fortifying yogurt with minimally processed apple by-products is scarce, which presents an opportunity to investigate.

The aim of present work was to investigate the potential of apple pomace powder as a functional ingredient for stirred-type yogurt and a yogurt drink.