The vibrios are considered particularly sensitive to food processing, especially thermal treatment. However, in samples of sundried
Ulva lactuca cultivated in Turkey,
Vibrio spp. were reported in a number of <10 cfu/g
[17][46]. Using sensitive qPCR assays combined with microbial pre-enrichment, Barberi et al., 2020
[74][47] detected pathogenic
V. parahemolyticus in 78% of cultivated seaweed samples from North-East USA. Kudaka et al., 2008
[19][48] identified
V. parahemolyticus in 18.8% of samples of
Caulerpa lentillifera (Sea grape) cultivated in tanks. Although the thermostable hemolysin gene was not detected in any of the isolates, these findings led the authors to highlight the importance of a suitable sterilization process for
C. lentillifera to ensure food safety
[19][48].
V. alginolyticus was isolated from cultivated
A. esculenta in Scotland, but not
V. vulnificus,
V. parahemolyticus, or
V. cholera [8]. Conventional culturing methods failed to identify
Vibrio spp. in seaweeds collected in Ireland
[18][49] or Norway
[21][50].
2.3. Aeromonas sp.
The genus
Aeromonas belongs to the family Aeromonadaceae, and is a group of Gram-negative, rod-shaped, oxidase- and catalase-positive and facultatively anaerobic bacteria
[76,77][51][52]. Members of this genus are ubiquitous aquatic bacteria and thus common in environments such as fresh-, brackish and marine water, and also found as inhabitants of aquatic animals
[77][52].
Aeromonas spp. are potential foodborne pathogens and known to cause gastrointestinal as well as extra-intestinal infections in humans
[78][53]. Most studies have dealt with
A. hydrofila, which have been implicated in many seafood-borne outbreaks
[79][54]. The occurrence of
Aeromonas spp. has been frequently reported in water and food, including RTE seafood
[80,81,82][55][56][57]. Currently not much is known on the role of seaweeds as responsible food for infections. However, based on their indigenous aquatic prevalence,
Aeromonas spp. could be expected to colonize seaweeds and possibly follow the raw materials to processing. Furthermore, the ability of some
Aeromonas sp. to survive and even grow at chilled temperatures gives reason for concern for seaweed and other seafood products.
A. hydrophila was isolated from e.g.,
Ulva reticulata harvested in Malaysia
[83][58], and
Aeromonas spp., in concentrations up to log 5.9 cfu/g, from mauro prepared from
Chondrus crispus and
Chondracanthus teedii sold by fishmongers or from street stalls in Sicily, Italy
[25][59].
3. Processing and Factors That Control Microbial Growth in Seaweed
3.1. Drying
Drying may inhibit all microbial growth including yeast and mold by reducing the water activity (a
W) to 0.6 or below, while bacteria of relevance are inhibited at much higher a
W according to
Table 21. The optimal a
W for a food product is usually a compromise between several priorities. At a
W below 0.30, lipid oxidation will occur and Maillard reaction has an optimum at a
W = 0.65
[114][60] and high-temperature drying should therefore not be used down to this level. Seaweed processors will, in general, avoid drying to lower moisture content than needed for the preservation of the products as the weight loss and drying costs represent a direct economic loss. Determination of the optimal a
W and moisture content is therefore essential. To achieve this, the relationship between the moisture content of the seaweeds and a
W has to be determined but literature on this has not been found. Some correlations have been documented for other foods, e.g., algae and fish by the method of da Silva et al.
[115][61]. A more fundamental understanding of the relation of water content, a
W, and water structure in foods has been presented by Mathlouthi, 2001
[114][60] who proposed a method for determining the correlations and validated it for sugars.
The surface-to-volume ratio is very high for most seaweeds and the drying time is relatively short which makes it feasible to dry at low temperatures (<< 60 °C) without risking microbial growth during drying. Typical low-temperature drying methods are sun drying and drying with dehumidified air but may also be achieved by electromagnetic drying by microwaves or radio frequency. The latter may also be used for high-temperature drying alone or in combination with hot air drying, infrared drying, or alternatively by superheated steam drying. These high-temperature drying methods may be designed to inactivate both bacteria and spores of bacteria. This may be of interest when the dried seaweeds are intended for use as ingredients in moist foods intended to have a shelf life after the addition of the seaweeds.
3.2. Thermal Processing
Blanching and boiling of seaweeds are done for several purposes including the inactivation of microorganisms and inactivating inherent enzymes causing the breakdown of the product. Brown seaweeds commonly have an unacceptable high concentration of iodine which may be reduced by up to 94% by boiling for a few minutes. However, boiling causes loss of flavonoids and water-soluble nutrients which limits the prevalence
[116][62].
There are currently few thermally processed seaweed products in the market compared to dried seaweed, but they are found as ingredients in canned (e.g., mackerel in tomato sauce), pasteurized (e.g., fish burgers), fried and boiled (e.g., soup) products.
The edible seaweed laver (
Porphyra umbilicalis), commonly named nori, is cultivated and consumed in East Asia
[117][63] and is one of the most commonly used seaweeds for human consumption. It is manufactured as dried and/or processed products and is in great demand as side dishes and snacks. Dried laver may be a contamination source to kimbab and in rolled sushi
[118][64], but Choi et al., 2014
[89][65] showed that heat-processed laver (260 to 400 °C, 2 to 10 s) had reduced aerobic bacterial counts, and no non-spore-forming pathogens (coliforms,
L. monocytogenes,
S. aureus,
Salmonella spp. and
V. parahaemolyticus).
3.3. Fermentation
Successful fermentation stabilizes the raw seaweed biomass by producing lactic acid and quickly reducing the pH of the seaweeds to below 4.3, where most potentially pathogenic bacteria are inactivated at refrigeration temperatures (pH 3.7 for ambient temperatures, cf.
Table 21). Lactic acid fermentation of seaweed is a recent strategy and quite limited information is available on culture conditions
[122,123][66][67]. The absence of natural lactic acid bacteria (LAB) microflora and simple sugars in most seaweeds, as opposed to terrestrial plants, may have limited development of this technique in the former
[123][67]. Fermentation may be a preferred processing technique for seaweeds because several seaweed species are sensitive to both thermal treatment and freezing that often diminishes the sensorial properties, appearance, and nutritional value of the products. However, as shown by Uchida et al., 2007
[122][66], LAB fermentation of
Undaria pinnatifida is not straightforward due to the selective survival of potential pathogenic spore-forming
Bacillus spp. through the drying process that could not be effectively outcompeted by the LAB starter culture during fermentation. When cultivated seaweed was mixed with sauerkraut at a ratio of up to 1:1, LAB fermentation proved successful by resulting in sufficiently low pH and thus maintained acceptable microbial and sensorial quality up to 60 days post-inoculation
[123][67]. Heat treatment (95 °C for 15 min) followed by fermentation using a commercial
Lactobacillus plantarum starter culture led to a drop in pH and stabilization at pH 4.5 after 40 h in
Saccharina latissima [124][68], and although this is above the limit set at 4.3 in regards to the growth of
B. cereus (
Table 21), no colonies with the morphology of
B. cereus were observed
[124][68].
3.4. Freezing
During the freezing of seaweeds, most of the water content is immobilized around the freezing point of seawater which depends on the salt content of the actual seaweed, usually between 0 °C and −2 °C. Water bound to other molecules has shown a freezing depression in the range −12 °C to −25 °C before rinsing, but after proper rinsing and loss of salts, the freezing point is increased to 0 °C
[125][69]. This change in the freezing point is important for the availability of water to microorganisms.
There is surprisingly little literature available on the freezing of seaweeds, possibly due to the limited changes during long-time frozen storage. Del Olmo, Pico, and Nunez, 2019
[126][70] documented 72% retention of polyphenols and 79% retention of antioxidant capacity after 180 days of storage at −24 °C. While freezing to a temperature below −25 °C is an effective measure to protect against microbial growth during storage, the damage to the cell structure during freezing and thawing may make the plant more accessible to microorganisms after thawing. During thawing, the drip loss released from the seaweeds may provide a pathway for the microorganisms.
Rapid freezing and thawing are recommended to minimize the risk of microbial growth as well as to limit the drip loss as much as possible. This may be achieved by thin layer band freezers or in vertical plate freezers if the width of the blocks is limited to keep the freezing time below a few hours. Block freezing on racks without air circulation and other methods needing several days to freeze the product will be less effective than rapid freezing with respect to food safety.