Maple sap collected from the trees doesn’t really taste sweet – it is about 98% water and 2% sugar solids. To make maple syrup requires the removal of a lot of water to concentrate the flavour, with the target for maple syrup being 66%-67% sugar.
The distinct flavour profiles in maple syrup arise from variations in sap chemistry (which vary by location and season), as well as the producer’s sap-to-syrup conversion process.
The sap-to-syrup conversion process has two goals:
1. Removal of water - to concentrate the sugar; and
2. Caramelisation of sugars - to add flavour and colour.
Traditionally, maple sap was boiled, which successfully achieves both goals. But boiling is a time, energy and cost intensive process. It takes about 40 litres of sap to makes 1 litre of syrup.
Modern processing uses a technique called reverse osmosis to reduce the water content in the first processing step, and then a smaller volume of liquid is heated to evaporate the rest of the water to the target density, and to caramelise the sugars. Finally, once maple syrup is produced, it is filtered to remove insoluble solids (mostly concentrated minerals from the sap) and a clear liquid is produced.
Reverse osmosis reduces water content…
Reverse osmosis uses pressure to force sap through a membrane that is only permeable to water. This means the sugar content, and other flavour components, are concentrated in the remaining sap. This technique increases efficiency through reduction in boiling time and fuel/energy costs.
But reverse osmosis has no impact on the composition, properties or flavour of the syrup produced. The process of sap to syrup conversion can’t be too efficient. Making maple syrup using reserve osmosis alone creates a product that is slightly yellow and tastes like corn syrup!
Heating is still necessary…
Maple syrup grades (Golden, Amber, Dark or Very Dark) are made up of two components: colour and flavour. And these characteristics are related: the darker the syrup, the stronger the maple flavour.
Time and heating are critical for the development of flavour, colour, and density to be achieved. Reverse osmosis is used to remove the majority of the original water content and increase sugar content to between 8% -16%, before a subsequent evaporation step.
Maple sap consists primarily of sucrose (i.e. common table sugar) and water. It also contains minerals, vitamins and organic compounds, such as organic acids, amino acids, proteins and phenol compounds. These extra features also help in flavour and colour development.
Heating breaks a small portion of the sucrose into its component molecules, fructose and glucose, which caramelise at a lower temperature. Flavour and colour develop through the caramelisation of these sugar compounds, as well as changes due to the Maillard reaction. This chemical reaction between amino acids and caramelising sugars gives browned food a distinct flavour – like toasted marshmallows, breads, cookies…and maple syrup.
The Maillard reaction involves glucose and fructose, but not sucrose. This means that the time of year the sap is collected influences the colour - lighter colours are produced earlier in the season (when the sap is higher in sucrose) and the darker colours are produced later in the season (when glucose and fructose are higher). Colour is also affected by the length of time spent boiling, pH and rate of heating.