What biomasses are generated in horticultural production?
Saleable yields of vegetables, berries and fruits include the parts that people usually eat. Vegetable yields can, for example, consist of leaves, tubers, bulbs, roots or fruits. In addition to saleable yields, horticultural production generates various edible and inedible side streams. Edible side streams include crops that remain in the field or stock due to market disruptions and that cannot be sold. Inedible side streams include crops that are ruined by the weather or pests, and plant parts other than those produced for food, such as tops and other green biomasses.
The nature of biomass left in the field or greenhouse after harvesting varies significantly depending on the type of plant. However, typical biomasses include tops and stems. In the production of tomatoes and cucumbers in greenhouses, leaves are removed on a weekly basis during the harvest season, and the remaining plant parts are removed at the end of the season.
Berries and fruits are perennial plants, from which berries and fruits and usually harvested. Tree and bush shoots are removed by cutting. Similarly, strawberry plants are pruned after harvesting. Strawberries and raspberries are also produced annually in tunnels using large crops that can already produce a high yield in the year in which they are planted. All plants are removed from the tunnels after the harvest season.
The aim is to market as much of the yield as possible. Some products may end up as sorted waste due to insufficient quality or wastage in shops, storage or further processing. Sorted waste is used as feed for domestic animals and game, or it is composted and returned to the field as soil improvement material. In places, by-products from plants are also delivered to biogas plants.
Biomasses remaining in fields are not usually removed, apart from shoots of perennial plants. Some biomasses, such as cabbage residues, have a high nutrient content, and these nutrients are released for use by other plants. Crop residues also play an important part in maintaining organic matter in soil.
Utilising side streams will be even more important in the future. New applications include the use of side streams from horticultural production as fresh feed in insect production. These could also be used increasingly as feed, especially if their useful life can be increased, for example, by means of preservation or drying.
In particular, connecting greenhouse production with other energy and food production is part of the circular economy of the future, where it will be possible to recycle nutrients and energy from side streams, for example, via biogas plants.
Side streams contain valuable chemical components, such as fibre, aroma compounds, colouring agents and compounds that improve health. For example, greenhouses produce high volumes of side streams from the leaves and shoots of tomatoes and cucumbers. It would be possible to separate small molecules of valuable components from these for food and cosmetic products, and for industrial uses. In addition, fibres from shoots can be used. Research and development in this field is currently underway, also in Finland. The separation and commercial use of valuable fractions still requires new enterprises in the field.
Properties of horticultural biomasses
The dry matter content in harvested garden plants is usually very low, ranging from 3–4 per cent in cucumbers and lettuce to 12–15 per cent in onions. The dry matter content in other plant parts, such as leaves and stems, is slightly higher, and its composition is very different. All green plant parts contain pigments – chlorophylls and carotenoids – that are needed in photosynthesis. In addition to these, they contain phenolic compounds that could be separated, for example, from greenhouse biomasses. Shoots may also contain compounds that are hazardous to health (such as alkaloids in tomatoes) that need to be taken into account when planning the use of side streams in the food industry.
Cutting residues from perennial plants are treelike, and they can be processed similarly to other treelike side streams. Some garden plants and certain treelike plant parts contain aromatic oils or plant oils.
Fresh biomasses rot fairly quickly. Due to their short useful life and high water content, biomasses should be processed fairly close to their point of origin. To increase the useful life, various preservation methods have been tested, for example, for biomasses suitable for use as feed.
A significant part of the nutrients in harvested crops transfer to side streams. Composted matter made from side streams of plants can be used as soil improvement material. It can also be used as phosphorus and potassium fertiliser. Its effectiveness as nitrogen fertiliser is lower, and it should be supplemented using other nitrogen fertilisers. When using composted matter, any regulations on pest control must be taken into account.
Estimating the volume of side streams
Side streams are produced round the year, for example, in greenhouse production and when packaging and further processing stock products. However, the highest side streams are produced during the harvest season or soon afterwards. These include plant parts left in the field and biomasses generated during industrial preservation or freezing.
In the outdoor production of vegetables, the tops of carrots and onions account for approximately one sixth of their total field biomasses. For iceberg lettuce, this proportion is higher, as its lower leaves left in the field comprise as much as half of its total biomass. For strawberry, runners make up one fifth of its total biomasses (Joensuu et al. 2016).
In the greenhouse production of tomatoes and cucumbers, stems and leaves cover roughly 20 per cent of total fresh material, being 25 per cent of yield material in relation to the weight of the yield. This is affected by the duration of the cultivation period, the amount of light and the total yield. As the dry matter content in stems and leaves is higher, approximately 30–40 per cent of dry matter is in stems and leaves.
An average of one fourth of the parts of carrots cultivated for use as food ends up in side streams. For onions, cabbages, lettuce, peas and strawberries, this proportion ranges from 13 to 19 per cent. With regard to the yield of greenhouse vegetables, only 1 per cent ends up in side streams. (Franke et al. 2016).
In further processing, such as in peeling root vegetables, wet side streams that rot quickly are produced. Side streams comprise up to 40–60 per cent of all material in the peeling process (Lehto et al. 2018).
Picture on top of the page: Terhi Suojala-Ahlfors, Luke