Industrial processing involved a decrease of total folate concentration of 38% for spinach and 30% for green beans.
3.2. Impact of individual processing steps on total folate content
The total folates contents for individual processing steps, determined by HPLC with fluorimetric detection after precolumn derivatisation, are presented in Table 2 for spinach and in Table 3 for green beans.
The average folates concentrations in green beans were 0.56 mg/kg fw (6.0 mg/kg dw) and 0.58 mg/kg fw (5.9 mg/kg dw), before and after blanching respectively. For spinach, the average concentrations were 1.09 mg/kg of fw (18 mg/kg dw) before blanching and 0.96 mg/kg fw (15 mg/kg dw) after blanching. Mass yields observed during the process, between the input and the output at the blanching step is between 96% and 99% for green beans and between 70% and 97% for spinach. There is therefore no water uptake by plants during blanching. Blanching had no statistically significant effect for either spinach or green beans.
For spinach, the folate content was 1.45 mg/kg fw before the washing step and 1.09 mg/kg fw after the washing step. This represents a significant folate decrease of 25% during the washing step. At the same time, dry matter decreased from 0.085 to 0.059 g/g, i.e. by 30%: the decrease in concentration of folate appeared concomitant with the loss and/or dilution of other soluble molecules. In the green beans canning process, the washing step had no significant effect on folate content in green beans: the averages before and after washing (which correspond to raw material and before blanching sampling points) were 0.52 and 0.56 mg/kg fw, respectively, corresponding to 5.53 and 6.04 mg/kg dw.
The freezing step brought no statistically significant decrease in folate concentration for spinach (from 0.96 mg/kg fw before freezing to 0.91 mg/kg fw after freezing). Though neither blanching nor freezing had a statistically significant effect, taken individually, there was a statistically significant decrease (15%) of folate concentration between the washed raw spinach and the frozen quoits.
For green beans, a statistically significant decrease occurred after sterilisation: the average concentrations before and after this step, were 0.57 and 0.37 mg/kg fw. At the same time the dry matter decreased from 0.098 to 0.087 g/g, so that concentrations relative to dry matter only decreased from 5.61 to 4.25 mg/kg. The folate average concentration in the covering liquid was 0.16 mg/l. The concentration in the covering liquid was 0.117 mg/l in small cans (212 g net weight; size 1/4) and 0.223 mg/l in large cans (850 g net weight; size 4/4). The amounts of folates present in a can, as calculated by adding up those present in the green beans and those present in the covering liquid, were related to the weight of vegetables: the can contained the equivalent of 90% of the concentration of folates in the raw green beans.
3.3. Distribution of individual folate derivatives
The proportions of folates derivatives determined by stable isotope dilution assay are presented in Fig. 4 for spinach and Fig. 5 for green beans.