Thursday, May 1, 2014

Study Suggests: Frozen Veggies Worse Than Common Wisdom Says - Frozen Asparagus, Zucchini and Green Beans Lose More Antioxidants During Boiling

Green asparagus from the fridge and  from the market are not created equal - at least not when they finally end up on your plate after a short bath in hot water.
You just have to watch one of the consumer report shows on television to hear it: "Frozen veggies are way better than their reputation would suggest." Actually, here in Germany this sentence has been repeated to soften that I've even heard people say they'd buy the frozen broccoli because it contained "more vitamins and the other good stuff, you know." And you know what? For some veggies like spinach, for example, this may actually be the case. For others, like broccoli or peas, the nutrient status of the frozen and the raw uncooked vegetable appears to be more or less identical (Favell. 1998). But that's something you cannot say for the green asparagus stems, zucchini and green beans in a recent study from the Università degli Studi di Parma in Italy.
Warning: Don't take this article as an excuse and stop eating veggies completely. The frozen stuff may lose more vitamins, when you boil it, but (a) you can still blanch it and (b) even with significantly reduced antioxidant effects veggies are still among the healthiest things you can eat.
I am not an asparagus expert and can still tell that the cell structure of the Transverse  sections boiled (C - from raw | D - from frozen) is profoundly messed up compared to the raw (A) and blanched (B) variety | legend: c = collenchyma; vp = vascular bundle; p = parenchyma; f = fissure.
In the corresponding experiment, the Italian researchers bought Green asparagus stems (Asparagus officinalis L., var. Grande), zucchini (Cucurbita pepo L., va Quine) and green beans (Phaseolus vulgaris L., var. Giamaica) from a local producer and processed them within 24 hours from harvesting. For each of the veggies four samples were prepared: Raw/uncooked  (R), raw/boiled  (B), blanched (BL) and industrially frozen/boiled (FB)

The raw (ten kilograms of each vegetable), blanched (five kilograms of each vegetable) and industrially frozen  samples  (five  kilograms  of  each  vegetable) had been transported were  transported  to  the  University of  Parma laboratories  under  adequate  refrigerated conditions to avoid the exuberant nutrient loss that occurs upon inadequately slow (re-)freezing.
SuppVersity Suggested Read: " Conventional vs. Organic: It's Not About Getting More, But Getting Less For Your Money. Less Pesticides, Dioxins & Co" | read more if you want to know if the claim "organic is always better" is a similar misconcept as "frozen over fresh".
If you "freeze" your veggies in the freezer compartment of your fridge, this will make the cells blast, so that even before they are cooked, and the nutrients flow out. It is generally assume that the latter would not happen, if the veggies are shock-frosted.
Figure 1: Total antioxidant capacity of green asparagus, zucchini and green beans raw, blanched, boiled and frozen and boiled (Paciulli. 2014); as the data tells you frozen veggies with similar  icy grease on them like you see on the right may not really be a better source of antioxidants than fresh veggies from the farmers or even the supermarket.
If we look at the data in Figure 1, though, it would appear that the cells may have "cracked" already so that they are more susceptible to the subsequent heat assault and the frozen + boiled samples end up having consistently lower total antioxidant (Figure 2) and feric acid reducing capacity than their raw + boiled counterparts.

For a similar reason (nutrient retention), the blanched samples have been cooled immediately after blanching in an ice-water bath for 3 min before they have been transported to the laboratories, where their analysis shows that only the Zucchini lost a small, but significant amount of their total antioxidant activity.
Figure 2: It would be interesting to see if the negative effects of freezing and boiling occur in all vegetables. In view of the fact that previous studies compared raw vs. frozen, but nor raw + cooked vs. frozen + cooked, frozen Broccoli + cooked broccoli could be exactly as "bad" as asparagus, zucchini and green beans.
The thing that is of most practical relevance, tough, is the significant negative effect of freezing + boiling on both, the total antioxidant capacity (Figure 1) and the ferric reducing capacity (Figure 2) of all three vegetables.

The previously "cited" statement that you're better of with the "fresh" frozen veggies is thus probably only right, if you eat them raw. Compared to fresh veggies, the previously frozen asparagus, zucchini and green beans lost almost 11-30% of their antioxidant prowess during the cooking process - and the same may well happen to other veggies, including broccoli, which have been compared in previous studies only on a raw vs. frozen, but not on a cooked vs. frozen + cooked basis. Unless you're afraid that all the good veggies may limit your gains due to their potent anti-oxidant effects, it appears smart to stay away from their frozen varieties.
References:
  • Paciulli, Maria, et al. "Impact of the industrial freezing process on selected vegetables Part I. Structure, texture and antioxidant capacity." Food Research International (2014).