Blanching is the brief boiling or steaming of whole foods like fruits and vegetables to kill the enzymes that would otherwise cause unwanted changes to the food during preservation and storage. These changes include loss of color, flavor, texture and nutrient density. Vegetables should always be blanched before freezing or drying them. While blanching dramatically reduces the rate of nutrient loss from food storage and preservation, it does itself cause some nutrient loss, particularly a reduction in water soluble nutrients.
Blanching may decrease the amount of carbohydrate, fat and water-soluble protein in certain vegetables. Early investigation in the 1970s and '80s found these nutrients decreased in several vegetables, such as one 1976 study that found that blanching decreased the albumen – a kind of protein – and amino acids – the building blocks of protein – in green peas. More recently, a 2003 study in the "Journal of the Science of Food and Agriculture" on the effect of blanching and freezing on the nutrition of over 20 vegetables found that blanching had either no effect on the dietary fiber content of vegetables, or in some cases, slightly increased available amounts.
All vitamins are either fat-soluble or water-soluble. Vitamins A, D, E and K are all fat-soluble while vitamins C and the B complex are water-soluble. Blanching helps to protect fat-soluble nutrients from breaking down, but in the process, some water-soluble nutrients are lost. Ascorbic acid and thiamine are particularly sensitive to thermal treatments, such as blanching, and break down easily under exposure to heat. The previously-mentioned 2003 study found folic acid particularly sensitive to blanching and carotenoids particularly resilient.
According to a 2007 research review published in the "Journal of the Science of Food and Agriculture," the mineral and dietary fiber content of vegetables tend to be more resilient against loss from processing than vitamins. The University of California-Davis researchers said that approximately 78 to 91 percent of minerals are retained after blanching. They further explain that when blanching in hard water, the uptake of calcium, potassium and sodium from the water far exceeds the potential mineral loss from the processing.
Antioxidants and Phytosterols
In a related report by the same UC-Davis team, blanching appeared to prevent the degradation of phenolic antioxidants from oxidation during storage and increase the amount of those antioxidants available to the human body. The researchers did, however, cite the 2003 "Journal of the Science of Food and Agriculture" study as finding a 20 to 30 percent loss of phenolic antioxidants after blanching and freezing, explaining this discrepancy by saying that water-soluble phenols may be leached into water during blanching and all phenolic antioxidants may oxidize somewhat from exposure to air. This same study also found that blanching produced no degradation of plant sterols.
Blanching Types and Procedures
Vegetables may be blanched in boiling water or steam, but fewer nutrients are lost in steam blanching. If blanching in water, heat 1 gallon of water to a rolling boil and then add only 1 pound of vegetables at a time to circulate water around each piece thoroughly. Place a lid on the pot and start timing the blanching process when the water returns to a boil. Blanching times vary for each type of vegetable. Steam blanching takes longer to bring the food up to the adequate temperatures. The interior of vegetables must achieve a temperature of 180 to 190 degrees F to kill the targeted enzymes.
Chill the vegetables in ice water immediately after blanching to halt the processes produced by the heat. To further minimize the reduction in nutrient content, regardless of the blanching method employed, only blanch vegetables for the length of time recommended in your blanching recipe or instructions. While blanching too little will fail to serve its intended purpose, overblanching is unnecessary and will only succeed in destroying more nutrients.
- "Nahrung"; Effect of Blanching Freezing and Sterilization on Protein and Amino Acid Content of Green Peas; C. Kulesza, et al.; 1976
- "Journal of the Science of Food and Agriculture"; Nutritional Comparison of Fresh, Frozen, and Canned Fruits and Vegetables. Part 1. Vitamins C and B and Phenolic Compounds; Joy C. Rickman, et al.; Mar 2007
- University of Illinois at Urbana-Champaign College of Architecture; "Drying Food"; Judy Troftgruben; Apr 1984