Opportunities and challenges with upcycled products

Upcycled fruit and vegetable powders are made from material that otherwise would not have gone to human consumption, such as carrot and onion peels or apple pomace that are left over after juicing. These by-products are desirable ingredients because they contain valuable nutritional and functional components as well as provide an opportunity to help reduce food waste. In recent years, there has been a proliferation of healthy, clean label and sustainable foods containing added fruit and vegetable powders.

All fruit and vegetable by-products are rich sources of complex carbohydrates including various types of fibre, starch, and pectin. Apple pomace, potato peels, tomato pomace, onion peel and carrot peel all have between 45 to 60 per cent fibre content (dry basis). Apple pomace also contains pectin (15 per cent). Potato peel has starch (20 per cent). The fibre and complex carbohydrate content of these by-products can enhance water-holding capacity, thickening properties and extend shelf-life of diverse foods. It could also have applications as prebiotics. Several fruit and vegetable by-products such as potato peels, tomato seed meal and cucumber peels are sources of plant-based proteins. Antioxidants and natural colours are other valuable components found in fruit and vegetable by-products. Antioxidants exhibit both health-promoting and food formulation benefits including enhanced shelf-life and natural colour. For example, onion skin
powder can be added to yogurt to produce a stable red colour (Mourtzinos et al. 2018)¹. Tomato powder can be used to add red colour (lycopene) while decreasing lipid oxidation in meat patties and sausages (Dominguez et al. 2020)².

While fruit and vegetable powders offer many benefits, there are challenges and sensory considerations to address to incorporate them as functional ingredients into foods. One of these sensory challenges is the potential of the powders to contribute a gritty mouthfeel to smooth products. Our research team at Vineland Research and Innovation Centre (Vineland), Vineland Station, Ont., has completed a study with apple pomace powder to investigate how to overcome this limitation.

Study

Apple pomace is a by-product of juice and cider processing consisting of skin, flesh, seeds, and stems and can be dried and ground into a powder. Currently, there are several commercially available apple pomace powders including some that are marketed as apple fibre. Apple pomace has been used in a range of food products to increase water-holding capacity, thickening and nutritional fibre. It is also used as a filler to replace a portion of flour in formulations. However, apple pomace powder is generally limited to applications with inherent coarse textures, such as bran muffins or sausages, due to its ability to impart a gritty or floury mouthfeel. We conducted a sensory study at Vineland to determine if the applicability of apple pomace powder could be extended to smooth-textured products, like yogurt, by reducing particle size and determining the required size to fall below the human threshold of grittiness perception.

For our study, apple pomace samples were received from an Ontario commercial cider manufacturer and then dried and milled by Guelph Food and Innovation Centre, Guelph, Ont. Apple pomace powders were prepared by adjusting the refining time to create powders with seven different particle sizes (166 μm, 208 μm, 320 μm, 363 μm, 471 μm, 569 μm, and 642 μm). Particle size was measured after 24 hours of rehydration to mimic the size in a food sample at the time of consumption and is reported as Dv(90) (90 per cent of particles in the sample were at or below the reported size). After milling, apple pomace powder samples were added to fruit fillings in stirred yogurt products. Since apple pomace powder contains a high amount of pectin, one potential application is to use it to replace pectin as a thickener in fruit fillings. Yogurt samples were prepared with strawberry fillings that had five per cent apple pomace powder of each particle size (these were the target samples) or by omitting apple pomace powder and using 1.75 per cent pectin (these were the blank samples). Sensory threshold testing was completed with 71 consumers using blind tastings to compare target and blank samples to determine at which size the apple pomace powder would impart a gritty texture to the yogurt.

Fruit and vegetable powders offer many benefits but there are sensory challenges in incorporating them as functional ingredients into foods.

Findings

Results showed the particle size threshold for perception of grittiness was 259 μm, indicating that at sizes below 259 μm, apple pomace powder was not perceived as being gritty by the average participant. Commercial apple pomace powders are typically around 100 to 200 μm when dry, meaning they are far larger than the threshold size after rehydration. However, we found that milling to a smaller size could be an effective strategy for manufacturers to prevent grittiness in smooth products, thus extending apple pomace powder to a wider range of products.

We also found that participants had a wide range of sensitivity to particle size. Some participants could detect grittiness from the smallest particle sizes in our study, while others could not detect grittiness from the largest particles, which were nearly four times larger than the smallest particles. We identified three clusters of participants, those with high sensitivity (44 per cent of participants), medium sensitivity (31 per cent of participants) and low sensitivity (25 per cent of participants) to particles in yogurt and calculated thresholds for each group. This diversity in sensitivity presents a challenge for product developers.

It is possible to mill upcycled powders such as apple pomace to below the average threshold of perception for grittiness in smooth products (259 μm). However, there is a large range of sensitivity, so one must decide whether to reduce the particle size to below the threshold of grittiness perception for the average consumer or to below the threshold of the most sensitive consumers (151 μm) to satisfy all potential customers. 

The cost of refining the particle size to below the threshold of the most sensitive group must be weighed against the risk of alienating this segment of the population.

This study highlights the importance of consumer-focused research to ensure that when using fruit and vegetable powders, sensory properties, such as texture, align with consumer acceptance and liking. Future investigations will be key to optimizing all aspects of upcycled foods from functionality to sensory profiles.

Acknowledgments

This study was supported by the Canadian Agricultural Strategic Priorities Program (Agriculture and Agri-Food Canada), Martin’s Family Fruit Farms and the Canadian Agricultural Partnership (the Partnership), a five-year initiative. The study’s authors are grateful to their funders, without whom this research would not have been possible. 

References

¹ I. Mourtzinos, P. Prodromidis, S. Grigorakis, P.D. Makris, C.G. Biliaderis, and T. Moschakis (2018). Natural food colourants derived from onion wastes: Application in a yoghurt product. Electrophoresis, 39(15), 1975–1983. https://doi.org/10.1002/elps.201800073.

² R. Domínguez, P. Gullón, M. Pateiro, P.E.S. Munekata, W. Zhang, and J.M. Lorenzo (2020). Tomato as potential source of natural additives for meat industry. A review. Antioxidants, 9(73). https://doi.org/10.3390/antiox9010073.

Amy Blake, MSc., is senior research technician, Consumer Insights Vineland Research and Innovation Centre.