Control of microbial hazards other techniques

Cooked chilled foods are usually prepared without preservatives or additives. The natural acidity of the product may be an additional barrier to bacterial growth. L. monocytogenes did not grow at 5°C or decreased in numbers at 21 °C in a range of commercial tomato products, and time to visible growth in a range of cooked puréed vegetables was related to the pH of vegetables, being longer in the vegetables with the lowest pH (Beuchat and Brackett, 1991). In the same way as for temperature, L. monocytogenes, C. botulinum and B. cereus have their own minimal pH for growth which are close to 4.6, 4.6 and 5.0, respectively (ICMSF, 1996).

Acidification of foods may be used in some specific applications to prolong shelf-life, or to give an additional safety factor to products potentially exposed to mild or severe temperature abuse. The organic acids that can be used are not

Table 11.3 Shortest time to visible growth in a nutrient broth, potato purée or broccoli purée inoculated with 107 spores per tube of non-proteolytic Clostridium botulinum following no heat treatment or heat treatment in a water bath at 80°C for 10min (adapted from Carlin et al., 2000a)

Incubation Shortest time to visible growth (days) in indicated substrate temperature (°C) following heating at 80°C for stated heating time (min)

Nutrient broth Potato Broccoli

Heating time Heating time Heating time

Nutrient broth Potato Broccoli

Heating time Heating time Heating time

0

10

0

10

0

10

20

2

3

3

6

3

11

10

4

7

16

>56

13

38

8

5

10

20

>56

15

>56

6

8

19

>56

>56

56

>56

4

>56

>56

>56

>56

>56

>56

specific to cooked chilled foods containing vegetables. However some applications in this context are, at least at the experimental stage, possible. Use of sorbic acid in association with citric acid or lactic acid to lower the pH to 5.0 was shown to reduce B. cereus growth at 8°C and 12°C in gnocchi, while sorbic acid alone was not efficient (Del Torre et al., 2001). Growth at 12°C of psychrotrophic strains of B. cereus in a carrot juice was inhibited for 60 days by acidification of the product to pH 5.0, whereas growth was possible at 5°C at the natural pH of the product (6.2) (Valero et al., 2000). According to the authors, the product was still acceptable to the consumer. Combinations of different organic acids (sorbic, ascorbic and citric acid) were also efficient in inhibiting C. botulinum growth in vacuum-packed potato. Some browning inhibitors (sulphite, mixtures of organic acids and antioxidants) inhibited (delayed) growth of L. monocytogenes at 4°C (15-28°C) in vacuum-packed potato (Juneja et al., 1998).

Addition of salt has a major disadvantage. Reduction in aw controls the development of pathogenic bacteria at only relatively high concentrations which are detrimental to the sensory quality of the food. For instance, in a mashed potato purée, addition of 2% sodium chloride was ineffective in controlling growth of

B. cereus at 30°C and delayed slightly growth at 10°C (Mahakarnchanakul and Beuchat, 1999). This salt concentration in foods is higher than is generally tolerated by consumers. Interactions between pH and aw in vacuum-packed potato was shown to result in a significant delay in growth of and toxin production by

C. botulinum (Dodds, 1989). However an efficient sodium chloride concentration was here also relatively high (about 2% and above).

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment