As you can see from my comments under this earlier blog entry, the data of Thomas Richardson (1848. Beiträge zur chemischen Kenntnis der Vegetabilien. Annalen der Chemie und Pharmacie LXVII Bd. 3.)* may well be this first source that has caused this widespread opinion. Soon after 1848, educational publications started to spread the myth. For example, an encyclopaedia published by Brockhaus (1852. Die Gegenwart. Eine encyklopädische Darstellung der neuesten Zeitgeschichte für alle Stände. Siebenter Band. Leipzig: F. A. Brockhaus) stated at page 172: "Weiße Rüben enthalten nur eine geringe Menge von Eisen, Spinat dagegen viel." [White turnips contain only a little iron, spinach however a lot. My translation.] One year later, Aaron Bernstein published a popularisation of scientific findings in a work called "Aus dem Reiche der Naturwissenschaft: ein Buch für Jedermann aus dem Volke" (Berlin: Franz Duncker, 1853). At pages 157-158, Bernstein praised spinach as an iron rich, organic alternative to medicine for pale children.
*[this publication hangs in a digital limbo, because it has been attached to the end of the preceding article by C. List (1848. Ueber das sogenannte Terpentinölhydrat. Justus Liebigs Annalen der Chemie 67(3): 362-376.]
Richardson gave the values of various chemical compounds of various vegetables as percent values in relation to the raw ash and as percent values in relation to the pure ash (raw ash minus carbonic acid, charcoal and sand). The iron content, in particular, was given as the percent values of iron(III) phosphate. Richardson's data sheet also provides the percent values of the ashes in in relation to the fresh matter.
As I have argued elsewhere, it is false to calculate the percentage of, say, iron phosphate in the fresh matter by simply multiplying the percentage of iron phosphate in relation to ash with the percentage of ash in relation to fresh matter. The reason why this leads to false values is, because the ashes gain mass during combustion. In particular, gaseous oxygen combines with the burning matter and while some products of combustion end up gaseous themselves (e.g. carbon dioxide) others end up as solid ash (e.g. magnesium oxide). That is, the ashes gained mass through combustion that was not part of the fresh matter.
Nevertheless, Bunge (1892) has committed this particular mistake in manipulating data from Wolff (1871) as shown here. Therefore it seems likely that others have also simply multiplied the percent values of Richardson's data and drawn false conclusions. Or, anyway, it seems interesting to reconstruct what conclusions contemporaries of Richardson might have drawn from such a data manipulation.
David Morrison from The Genealogical World of Phylogenetic Networks has done that for the percent values of iron phosphate in relation to the pure ash times the percent values of ash in relation to the fresh matter and sent me the results. I added the same calculation for the raw ash. As you can see from the table below, spinach comes out second after radish herbage. Assuming that the herbage of radish was usually not eaten, however, spinach would be the edible item with the highest iron content in Richardson's data set. Hence Richardson (1848) may well be the first source from which the widespread opinion sprang that spinach was a good source for dietary iron in turn.
|Item||% iron phosphate in raw ash times % ash in fresh matter||% iron phosphate in pure ash times % ash in fresh matter|
|Ananas, ganze Frucht|
|Feige, ganze Frucht|
|Brocoli (Kohl), Herz||2,02||2,14|
|Kastanie, ganze Frucht||1,77||1,93|
|Erdbeere, ganze Frucht||3,69||4,56|
|(Kidney Beans) Bohnen||3,09||3,56|
|Pflaumen (greengages), ganze Frucht||1,80||2,42|
|Orleans-Pflaumen, Haut der Frucht||5,39||6,63|
|Orleans-Pflaumen, Fleisch derselben||0,95||1,49|
|Kirschen, ganze Frucht||1,47||1,61|
|ditto Stiel derselben||4,74||5,57|
|Birne, ganze Frucht||0,66||0,80|