Vegetables and fruits

There are several factors that potentially contribute to differences in nutrient levels between vegetables and fruits in the frozen format and those supplied as fresh or preserved by other processes. Any differences are likely to be in the loss and preservation of vitamins; it has been shown that compared with fresh vegetables, there are negligible differences between the mineral and fibre contents of equivalent frozen vegetables (Polo et al, 1992; Nyman, 1995).

15.3.1 Selection of cultivar and time of harvesting

Particular cultivars and harvest times are chosen to optimise sensory quality and these may differ between those selected for freezing and those that are consumed in fresh, canned or dried formats. The cultivar and harvest time may have some effects on nutritional value (Shewfelt, 1990); for example peas selected for

Fig. 15.1 Effects of storage and freezing on ascorbate retention in spinach: typical values for retention of ascorbate in spinach stored at either ambient or chill temperature (4°C) compared with blanched and frozen spinach. All samples were taken from the same field and time zero levels were obtained from freshly harvested spinach. Blanching and freezing were carried out in a commercial factory. (from Favell, 1998)

Time since harvest (days)

Fig. 15.1 Effects of storage and freezing on ascorbate retention in spinach: typical values for retention of ascorbate in spinach stored at either ambient or chill temperature (4°C) compared with blanched and frozen spinach. All samples were taken from the same field and time zero levels were obtained from freshly harvested spinach. Blanching and freezing were carried out in a commercial factory. (from Favell, 1998)

canning are usually harvested at a more mature stage than those selected for freezing and consequently have approximately 10% lower ascorbate concentration. The type of cultivar may also influence the amount of nutrient lost during processing, reflecting differences between culitvars in morphology and mechanical strength.

15.3.2 Storage after harvest

Many vegetables, and to a lesser extent fruits, are relatively unstable after harvesting and undergo rapid chemical changes that result in significantly reduced levels of some nutrients. For example, concentrations of ascorbate in spinach may fall to 50% of their initial, pre-harvest level, after two days of storage as shown in Fig. 15.1 (Favell, 1998). The magnitude of nutrient losses during storage prior to freezing is highly variable and depends on the crop, the method of harvesting and the duration and conditions of storage. To preserve the nutritional value of fresh vegetables and fruits it is clearly desirable to minimise the time in blanching and freezing and to cause minimal mechanical damage.

15.3.3 Washing and blanching

The need for washing of vegetables and fruits may cause some loss of water-soluble nutrients, particularly from cut surfaces. As noted above, oxidation is a key factor influencing stability in the frozen state and this is particularly a concern with vegetables and fruits because they contain many enzyme systems that give rise to reactive oxygen species. It is to prevent enzyme-mediated oxidation reactions that most vegetables and fruits are blanched before freezing. Another reason is to ensure microbiological safety but this can be achieved by other means. The advantages of blanching can be illustrated with reference to cauliflower and spinach. If they are frozen without blanching they become unpalatable after only a few months due to the development of 'off' flavours and odours caused primarily by oxidation of membrane lipids. If these vegetables are blanched before freezing they have a storage life of 18-24 months. Commercial blanching conditions typically involve heating in water or steam at 95-100°C for 3-10 minutes, depending on the type and size of material to be blanched. The conditions are chosen so as to ensure inactivation of the enzymes responsible for oxidation. During blanching, nutrients may be lost by leaching and by chemical degradation. A great deal of information has been published on losses of labile nutrients during blanching (for review see Clydesdale et al, 1991). Ascorbate is often used as an indicator of potential nutrient loss because of its high solubility, sensitivity to heat and ease of measurement. Typical losses of ascorbate from vegetables during blanching are of the order 5-40% (Favell, 1998; Bender, 1993). In general, it may be concluded that nutrient losses are minimised if the raw material is as little damaged as possible during handling and if processing conditions are chosen that keep the temperature, duration of heat exposure and product to water ratio as low as is consistent with denaturing the enzymes responsible for oxidative spoilage.

15.3.4 Frozen storage

Bender (1993) has summarised the contradictory results of published studies designed to estimate the magnitude of vitamin loss during frozen storage of vegetables and fruits. Even for a particular vegetable, processed and stored under apparently similar conditions, the extent of ascorbate loss has been reported as negligible, or up to 40% after a year of frozen storage (Bender, 1993). As Bender comments, there are many possible sources of experimental variation that may lead to these different conclusions, most notably incomplete denaturation of oxidative enzymes during blanching. Since the review by Bender no large scale systematic study addressing this issue has been published. It may be concluded that if vegetables and fruits are adequately blanched and stored at conventional freezer temperatures without undue temperature fluctuations they will still possess valuable levels of potentially labile nutrients for a period of at least 12-18 months.

15.3.5 Cooking

When comparing the nutritional value of different processing methods it is also necessary to consider the ways in which consumers handle these different products. Cooking methods may have important effects on the quantity of nutrients within a food. Because frozen vegetables have already been blanched, they require less cooking time than fresh vegetables to reach the same levels of palat-ability. This means that while frozen vegetables may have lost some nutrients during blanching they will probably suffer reduced losses during cooking.

It is increasingly recognised that regular consumption of vegetables and fruits significantly reduces the risk of some cancers and of cardiovascular disease. Although it is by no means certain, it appears likely that these beneficial effects are not just a consequence of consuming the recognised nutrients found in vegetables and fruits. Alarge number of potentially beneficial compounds, the so called phyto-nutrients, or non-nutrient phyto-chemicals are found in vegetables and fruits. It is not yet clear which particular compounds, or even which group of phy-tochemicals may be responsible for the health benefits, but if and when the protective agents are identified it will be necessary to ascertain the effects of freezing and associated processes on their retention in frozen vegetables and fruits.

Get Juiced

Get Juiced

This book will guide you through the processes of juicing your way to better health. Learn all the savvy tips and tricks to maintain your health and good body for a bright future ahead. This includes tips on diet, exercise, sleeping habits and etc.

Get My Free Ebook


Post a comment