Blanching, Canning, and Freezing

Keeping Food Fresher, Longer

In the modern age, the movement from a self-sustaining life on the farm to a lifestyle reliant on supermarkets for food has changed the way people shop, eat, and live. One of the challenges of this lifestyle is maintaining the nutritive value and freshness of products as they are transported from producer to consumer. Several methods have been devised to remedy this: canning, blanching, and freezing, allowing supermarkets the freedom of selling products that are not in season, like apples in the winter. This maintains a constant food supply for consumers around the world, and also gives them flexibility with the food they eat.


The first food processing method to be discussed, blanching, is a common precursor to canning and freezing.  It utilizes high temperatures at carefully timed intervals to preserve the nutritional, aesthetic, and textural properties of produce. Heat denatures enzymes that might otherwise spoil the food and kills many of the spoilage-inducing microbes. The exact temperature is carefully controlled, as under-blanching could actually shorten the longevity of the food through stimulation of the enzymes, while over-blanching is akin to boiling and alters many of the properties that blanching is supposed to preserve. The high heat of blanching also kills most, if not all, of the spoilage-inducing microbes.

There are several methods of blanching, including water, steam, microwave, and gas blanching, the first being the most commonly used technique. This involves an apparatus called a blanching basket, which submerges the fruits or vegetables in boiling water. Regardless of the initial step, the produce is always shocked in ice-water after heating, using about one pound of ice per pound of vegetables. This is very important as it immediately halts the cooking process.

Blanching Basket

The effects of blanching can be seen both qualitatively and quantitatively. Qualitatively, blanched food appears fresher, more vivid, and more appetizing than unblanched preserved foods. Blanched vegetables are also softer than unblanched ones, helping to reduce the likelihood of freezer burns. Because blanched foods are softer, they are easier to pack and consume. In addition, when blanching  is done correctly, it can prevent total loss of a nutrient, such as ascorbic acid (Vitamin C) in immature sweet peas blanched for 1 minute at 96ºC.


Freezing foodstuffs in order to preserve them has been a commonly practiced procedure for centuries. Even in ancient times, people were aware of the effect of temperature on the activity of bacteria and enzymes, which break down and spoil food. People created cool, dark cellars beneath the ground in order to lower the temperature of food and keep foods as fresh as possible. The ice industry skyrocketed in the 19th century as the use of ice to preserve food increased. People realized that ice floating in the ocean could be captured and sold for use in food preservation. Soon, every household had an icebox where people would place ice bought from a vendor and store their food in order to prevent it from spoiling.


A review by the Food Processing Center, part of University of Nebraska’s Food, Science, and Technology Department, showed that foodstuffs kept at temperatures below -18ºC are able to last a year without spoilage, foods kept at -10ºC can last 8 months, and foods kept at -2ºC spoil in less than two months. This information has led to the conclusion that -18ºC is an upper limit for food preservation using freezing. Any lower temperatures would prove to be more harmful than helpful for the following reasons:

Freezing in produce with a high water content causes the water to form large ice crystals that puncture the cell walls and destroy the cellular structure of the food. When food that has undergone this phenomenon is thawed out, it is limp and unpalatable. In order to solve this problem, Clarence Birdseye created a process called quick freezing, similar to the fish preservation method of the Eskimos. In quick freezing, food is frozen quickly, preventing large ice crystals from forming and damaging the cells of the food.

Quick Freezing vs. Slow Freezing


Canning is a process that is well-established as a method for food preservation. The process that frequently precedes it, as mentioned before, is blanching. A common misconception is that canned fruits and vegetables are not nearly as healthy as fresh ones. However, as seen with blanching and freezing, further processing does not necessarily mean further loss of nutrients. In fact, canning does a good job of preserving many of the nutrients in the foodstuff. However, the marked increase in shelf-life might not be outweighed by the potential loss of benefits of the product.

Canned Foods

One study examined the presence of various nutrients after canning. Different nutrients behave differently when subject to canning. Vitamin C, generally found in fruits as well as spinach and asparagus, is mostly retained in heat treatment. In fact, the amount of this vitamin is relatively stable even after storage for two years. The same holds true for Vitamin A, which is not water soluble and thus less likely to escape from the produce. Lycopene, a specific type of Vitamin A found in tomatoes, seems to have better anti-prostate cancer effects when consumed in canned form. Other nutrients that withstand canning well are potassium, dietary fiber, and protein. Canning not only preserves vitamins and nutrients – it also kills potentially harmful microbes. Due to the process’s excellent benefits, canned fruits and vegetables are excellent alternatives to their fresh counterparts.


The three methods of blanching, freezing, and canning go hand-in-hand to prolong the freshness, nutritive value, and natural appearance of food. Blanching before canning prevents degradation during its storage, while blanching before freezing prevents re-growth of microbes after thawing.  At the end of the day, no matter how elaborate the process, food will always wilt, wither, and decay. However, using the techniques of canning and freezing in coordination with blanching, nutritious foods can survive the long journey from the fields of farmers to the mouths of hungry consumers.


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