Spinach is among the commonest components of a pastry filling. Both its nutritional value and its kitchen behaviour are governed by the same compound: oxalate.
What is spinach's nutrition profile?
A hundred grams of raw spinach carries 23 calories.
Protein 2.9 grams, carbohydrate 3.6 grams, fibre 2.2 grams.
Fat is negligible.
Water is 91 percent; that is the real issue in the kitchen.
It is strong on vitamins: 469 micrograms of vitamin K per hundred grams, 469 micrograms of vitamin A equivalent and 194 micrograms of folate.
The vitamin K figure is more than three times the daily requirement.
On minerals, magnesium 79 milligrams and potassium 558 milligrams.
Iron 2.7 milligrams; the figure spinach is famous for.
But that number misleads, and the reason is oxalate.
Why is the iron not fully absorbed?
Spinach contains oxalic acid: 750 to 970 milligrams per hundred grams.
Oxalate binds iron and calcium.
Bound minerals cannot be absorbed in the gut.
The absorption rate of iron from spinach is around 2 to 5 percent.
From red meat it is 15 to 25 percent.
So of the 2.7 milligrams of iron in a hundred grams of spinach, only about 0.1 milligrams is absorbed.
The situation is similar for calcium; most of the 99 milligrams per hundred grams is bound.
That does not make spinach worthless; vitamin K, folate and magnesium are unaffected.
But positioning it as a source of iron is inaccurate.
How is absorption improved?
Three methods help.
The first is vitamin C. Ascorbic acid reduces iron to an absorbable form.
Lemon juice, tomato or pepper eaten in the same meal raises absorption two- or threefold.
Adding lemon zest to a pastry filling both performs that job and contributes flavour.
The second is blanching. Some of the oxalate dissolves in water; discarding the blanching water reduces oxalate by 30 to 50 percent.
But it also reduces the water-soluble vitamins.
The third is separating it from calcium. Where spinach and dairy are eaten together, the oxalate binds to the milk calcium and the spinach's iron is left relatively free.
That is an unexpected advantage of spinach and cheese pastry.
| Nutrient (100 g raw) | Amount | Note |
|---|---|---|
| Calories | 23 kcal | Very low |
| Water | 91% | The main problem in fillings |
| Protein | 2.9 g | Moderate |
| Fibre | 2.2 g | Good |
| Vitamin K | 469 µg | Above the daily requirement |
| Folate | 194 µg | High |
| Iron | 2.7 mg | 2-5% absorbed |
| Oxalate | 750-970 mg | Blocks absorption |
Why is water a problem in the filling?
Spinach is 91 percent water, and that water is released during cooking.
In the oven the filling inside the pastry heats, the cell walls break down and the water comes out.
That water soaks the pastry.
The result is a pastry with a doughy base and layers stuck together.
A kilogram of raw spinach can release around 700 to 750 millilitres of water.
That water cannot be allowed to stay in the filling.
The solution has two stages: first remove the water, then bind what remains.
How is it wilted and drained?
The order matters.
Step one: wash the spinach and chop it roughly.
Step two: salting. Salt is scattered over and left fifteen minutes; osmotic pressure draws water out.
That step shortens the cooking time.
Step three: wilting. Three to five minutes in a wide pan over high heat.
A wide pan is essential; in a small pan the spinach does not wilt but boils in its own water.
High heat is essential too; over low heat the water accumulates instead of evaporating.
Step four: draining and squeezing. The wilted spinach goes into a colander and is squeezed by hand once cool.
The squeezing must be done seriously; no moisture should remain at the surface.
A kilogram of raw spinach yields 250 to 300 grams of drained filling.
When should salt be added?
This is a frequent mistake.
Salt goes in twice, at the beginning and at the end of preparing the filling.
The first salting is to draw water out; most of that salt leaves with the drained liquid.
The second is for seasoning and goes in once the filling is complete.
But care is needed where the filling contains cheese; white cheese is already salty.
The saltiness of the cheese should be judged by tasting and the added salt set accordingly.
A filling made with salty cheese generally needs no extra salt.
The filling should be tasted once made; the perception of salt drops slightly on baking, and that belongs in the calculation.
What cheese ratio?
Cheese performs three functions in the filling.
The first is flavour. The second is binding; cheese proteins hold the filling together.
The third is moisture management; cheese binds some of the remaining water.
The ratio is 40 to 60 percent of the drained spinach weight.
So 120 to 180 grams of cheese to 300 grams of drained spinach.
Too little cheese lets the filling fall apart; too much overwhelms the spinach and weeps fat during baking.
The type of cheese affects the result too.
Curd cheese is the driest; it binds moisture but adds little flavour.
White cheese is balanced, giving flavour and binding, but it is salty.
Kaşar melts and makes the filling fluid; it should not be used alone.
A blend gives the best result: half curd, half white cheese.
What changes when spinach pastry is frozen?
Spinach pastry is among the hardest products in frozen production.
The reason is water again.
During freezing the water in the cells forms ice crystals and tears the cell walls.
On thawing that water is released and the filling turns wet.
Spinach should therefore be drained further for frozen production; aim for 10 to 15 percent less moisture than in fresh production.
The second precaution is adding a binder. A tablespoon of breadcrumbs or semolina holds the remaining moisture.
The third is blast freezing. In rapid freezing the crystals stay small and cell damage falls.
Frozen pastry should be baked without thawing; in a thawed product the water passes into the pastry.
The baking time extends by five to eight minutes.
Where frozen spinach is used it should be thawed and squeezed thoroughly; frozen spinach carries more water than fresh.
The shelf life of a raw frozen product is three months; beyond that the filling loses texture.
