Mapping of property of type java.util.Map as 'serializable' still generates join table

Trying to persist property of type java.util.Map as Hibernate's 'serializable' like this:

class Domain {
String name
String ser
Map<String, String> map

static mapping =

}

Property 'ser' is mapped correctly to DB like 'varbinary(255)' when using HSQLDialect. I would expect that property 'map' should be mapped to DB similar to 'ser' as some variant of 'varbinary'. But it is mapped using join table domain_map.

create table domain (id bigint generated by default as identity (start with 1), version bigint not null, ser varbinary(255) not null, name varchar(255) not null, primary key (id));
create table domain_map (map bigint, map_idx varchar(255), map_elt varbinary(-1) not null);

How can I persist instances of java.util.Map as hibernate's 'serializable'?